rustc_hir_analysis/hir_ty_lowering/

mod.rs

//! HIR ty lowering: Lowers type-system entities[^1] from the [HIR][hir] to
//! the [`rustc_middle::ty`] representation.
//!
//! Not to be confused with *AST lowering* which lowers AST constructs to HIR ones
//! or with *THIR* / *MIR* *lowering* / *building* which lowers HIR *bodies*
//! (i.e., “executable code”) to THIR / MIR.
//!
//! Most lowering routines are defined on [`dyn HirTyLowerer`](HirTyLowerer) directly,
//! like the main routine of this module, `lower_ty`.
//!
//! This module used to be called `astconv`.
//!
//! [^1]: This includes types, lifetimes / regions, constants in type positions,
//! trait references and bounds.

mod bounds;
mod cmse;
mod dyn_trait;
pub mod errors;
pub mod generics;

use std::{assert_matches, slice};

use rustc_abi::FIRST_VARIANT;
use rustc_ast::LitKind;
use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
use rustc_errors::codes::*;
use rustc_errors::{
    Applicability, Diag, DiagCtxtHandle, Diagnostic, ErrorGuaranteed, FatalError, Level,
    struct_span_code_err,
};
use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::{self as hir, AnonConst, GenericArg, GenericArgs, HirId};
use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
use rustc_infer::traits::DynCompatibilityViolation;
use rustc_macros::{TypeFoldable, TypeVisitable};
use rustc_middle::middle::stability::AllowUnstable;
use rustc_middle::ty::print::PrintPolyTraitRefExt as _;
use rustc_middle::ty::{
    self, Const, GenericArgKind, GenericArgsRef, GenericParamDefKind, LitToConstInput, Ty, TyCtxt,
    TypeSuperFoldable, TypeVisitableExt, TypingMode, Upcast, const_lit_matches_ty, fold_regions,
};
use rustc_middle::{bug, span_bug};
use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
use rustc_session::parse::feature_err;
use rustc_span::{DUMMY_SP, Ident, Span, kw, sym};
use rustc_trait_selection::infer::InferCtxtExt;
use rustc_trait_selection::traits::wf::object_region_bounds;
use rustc_trait_selection::traits::{self, FulfillmentError};
use tracing::{debug, instrument};

use crate::check::check_abi;
use crate::errors::{AmbiguousLifetimeBound, BadReturnTypeNotation, NoFieldOnType};
use crate::hir_ty_lowering::errors::{GenericsArgsErrExtend, prohibit_assoc_item_constraint};
use crate::hir_ty_lowering::generics::{check_generic_arg_count, lower_generic_args};
use crate::middle::resolve_bound_vars as rbv;
use crate::{NoVariantNamed, check_c_variadic_abi};

/// The context in which an implied bound is being added to a item being lowered (i.e. a sizedness
/// trait or a default trait)
#[derive(Clone, Copy)]
pub(crate) enum ImpliedBoundsContext<'tcx> {
    /// An implied bound is added to a trait definition (i.e. a new supertrait), used when adding
    /// a default `MetaSized` supertrait
    TraitDef(LocalDefId),
    /// An implied bound is added to a type parameter
    TyParam(LocalDefId, &'tcx [hir::WherePredicate<'tcx>]),
    /// An implied bound being added in any other context
    AssociatedTypeOrImplTrait,
}

/// A path segment that is semantically allowed to have generic arguments.
#[derive(Debug)]
pub struct GenericPathSegment(pub DefId, pub usize);

#[derive(Copy, Clone, Debug)]
pub enum PredicateFilter {
    /// All predicates may be implied by the trait.
    All,

    /// Only traits that reference `Self: ..` are implied by the trait.
    SelfOnly,

    /// Only traits that reference `Self: ..` and define an associated type
    /// with the given ident are implied by the trait. This mode exists to
    /// side-step query cycles when lowering associated types.
    SelfTraitThatDefines(Ident),

    /// Only traits that reference `Self: ..` and their associated type bounds.
    /// For example, given `Self: Tr<A: B>`, this would expand to `Self: Tr`
    /// and `<Self as Tr>::A: B`.
    SelfAndAssociatedTypeBounds,

    /// Filter only the `[const]` bounds, which are lowered into `HostEffect` clauses.
    ConstIfConst,

    /// Filter only the `[const]` bounds which are *also* in the supertrait position.
    SelfConstIfConst,
}

#[derive(Debug)]
pub enum RegionInferReason<'a> {
    /// Lifetime on a trait object that is spelled explicitly, e.g. `+ 'a` or `+ '_`.
    ExplicitObjectLifetime,
    /// A trait object's lifetime when it is elided, e.g. `dyn Any`.
    ObjectLifetimeDefault(Span),
    /// Generic lifetime parameter
    Param(&'a ty::GenericParamDef),
    RegionPredicate,
    Reference,
    OutlivesBound,
}

#[derive(Copy, Clone, TypeFoldable, TypeVisitable, Debug)]
pub struct InherentAssocCandidate {
    pub impl_: DefId,
    pub assoc_item: DefId,
    pub scope: DefId,
}

/// A context which can lower type-system entities from the [HIR][hir] to
/// the [`rustc_middle::ty`] representation.
///
/// This trait used to be called `AstConv`.
pub trait HirTyLowerer<'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx>;

    fn dcx(&self) -> DiagCtxtHandle<'_>;

    /// Returns the [`LocalDefId`] of the overarching item whose constituents get lowered.
    fn item_def_id(&self) -> LocalDefId;

    /// Returns the region to use when a lifetime is omitted (and not elided).
    fn re_infer(&self, span: Span, reason: RegionInferReason<'_>) -> ty::Region<'tcx>;

    /// Returns the type to use when a type is omitted.
    fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx>;

    /// Returns the const to use when a const is omitted.
    fn ct_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Const<'tcx>;

    fn register_trait_ascription_bounds(
        &self,
        bounds: Vec<(ty::Clause<'tcx>, Span)>,
        hir_id: HirId,
        span: Span,
    );

    /// Probe bounds in scope where the bounded type coincides with the given type parameter.
    ///
    /// Rephrased, this returns bounds of the form `T: Trait`, where `T` is a type parameter
    /// with the given `def_id`. This is a subset of the full set of bounds.
    ///
    /// This method may use the given `assoc_name` to disregard bounds whose trait reference
    /// doesn't define an associated item with the provided name.
    ///
    /// This is used for one specific purpose: Resolving “short-hand” associated type references
    /// like `T::Item` where `T` is a type parameter. In principle, we would do that by first
    /// getting the full set of predicates in scope and then filtering down to find those that
    /// apply to `T`, but this can lead to cycle errors. The problem is that we have to do this
    /// resolution *in order to create the predicates in the first place*.
    /// Hence, we have this “special pass”.
    fn probe_ty_param_bounds(
        &self,
        span: Span,
        def_id: LocalDefId,
        assoc_ident: Ident,
    ) -> ty::EarlyBinder<'tcx, &'tcx [(ty::Clause<'tcx>, Span)]>;

    fn select_inherent_assoc_candidates(
        &self,
        span: Span,
        self_ty: Ty<'tcx>,
        candidates: Vec<InherentAssocCandidate>,
    ) -> (Vec<InherentAssocCandidate>, Vec<FulfillmentError<'tcx>>);

    /// Lower a path to an associated item (of a trait) to a projection.
    ///
    /// This method has to be defined by the concrete lowering context because
    /// dealing with higher-ranked trait references depends on its capabilities:
    ///
    /// If the context can make use of type inference, it can simply instantiate
    /// any late-bound vars bound by the trait reference with inference variables.
    /// If it doesn't support type inference, there is nothing reasonable it can
    /// do except reject the associated type.
    ///
    /// The canonical example of this is associated type `T::P` where `T` is a type
    /// param constrained by `T: for<'a> Trait<'a>` and where `Trait` defines `P`.
    fn lower_assoc_item_path(
        &self,
        span: Span,
        item_def_id: DefId,
        item_segment: &hir::PathSegment<'tcx>,
        poly_trait_ref: ty::PolyTraitRef<'tcx>,
    ) -> Result<(DefId, GenericArgsRef<'tcx>), ErrorGuaranteed>;

    fn lower_fn_sig(
        &self,
        decl: &hir::FnDecl<'tcx>,
        generics: Option<&hir::Generics<'_>>,
        hir_id: HirId,
        hir_ty: Option<&hir::Ty<'_>>,
    ) -> (Vec<Ty<'tcx>>, Ty<'tcx>);

    /// Returns `AdtDef` if `ty` is an ADT.
    ///
    /// Note that `ty` might be a alias type that needs normalization.
    /// This used to get the enum variants in scope of the type.
    /// For example, `Self::A` could refer to an associated type
    /// or to an enum variant depending on the result of this function.
    fn probe_adt(&self, span: Span, ty: Ty<'tcx>) -> Option<ty::AdtDef<'tcx>>;

    /// Record the lowered type of a HIR node in this context.
    fn record_ty(&self, hir_id: HirId, ty: Ty<'tcx>, span: Span);

    /// The inference context of the lowering context if applicable.
    fn infcx(&self) -> Option<&InferCtxt<'tcx>>;

    /// Convenience method for coercing the lowering context into a trait object type.
    ///
    /// Most lowering routines are defined on the trait object type directly
    /// necessitating a coercion step from the concrete lowering context.
    fn lowerer(&self) -> &dyn HirTyLowerer<'tcx>
    where
        Self: Sized,
    {
        self
    }

    /// Performs minimalistic dyn compat checks outside of bodies, but full within bodies.
    /// Outside of bodies we could end up in cycles, so we delay most checks to later phases.
    fn dyn_compatibility_violations(&self, trait_def_id: DefId) -> Vec<DynCompatibilityViolation>;
}

/// The "qualified self" of an associated item path.
///
/// For diagnostic purposes only.
enum AssocItemQSelf {
    Trait(DefId),
    TyParam(LocalDefId, Span),
    SelfTyAlias,
}

impl AssocItemQSelf {
    fn to_string(&self, tcx: TyCtxt<'_>) -> String {
        match *self {
            Self::Trait(def_id) => tcx.def_path_str(def_id),
            Self::TyParam(def_id, _) => tcx.hir_ty_param_name(def_id).to_string(),
            Self::SelfTyAlias => kw::SelfUpper.to_string(),
        }
    }
}

#[derive(Debug, Clone, Copy)]
enum LowerTypeRelativePathMode {
    Type(PermitVariants),
    Const,
}

impl LowerTypeRelativePathMode {
    fn assoc_tag(self) -> ty::AssocTag {
        match self {
            Self::Type(_) => ty::AssocTag::Type,
            Self::Const => ty::AssocTag::Const,
        }
    }

    ///NOTE: use `assoc_tag` for any important logic
    fn def_kind_for_diagnostics(self) -> DefKind {
        match self {
            Self::Type(_) => DefKind::AssocTy,
            Self::Const => DefKind::AssocConst { is_type_const: false },
        }
    }

    fn permit_variants(self) -> PermitVariants {
        match self {
            Self::Type(permit_variants) => permit_variants,
            // FIXME(mgca): Support paths like `Option::<T>::None` or `Option::<T>::Some` which
            // resolve to const ctors/fn items respectively.
            Self::Const => PermitVariants::No,
        }
    }
}

/// Whether to permit a path to resolve to an enum variant.
#[derive(Debug, Clone, Copy)]
pub enum PermitVariants {
    Yes,
    No,
}

#[derive(Debug, Clone, Copy)]
enum TypeRelativePath<'tcx> {
    AssocItem(DefId, GenericArgsRef<'tcx>),
    Variant { adt: Ty<'tcx>, variant_did: DefId },
    Ctor { ctor_def_id: DefId, args: GenericArgsRef<'tcx> },
}

/// New-typed boolean indicating whether explicit late-bound lifetimes
/// are present in a set of generic arguments.
///
/// For example if we have some method `fn f<'a>(&'a self)` implemented
/// for some type `T`, although `f` is generic in the lifetime `'a`, `'a`
/// is late-bound so should not be provided explicitly. Thus, if `f` is
/// instantiated with some generic arguments providing `'a` explicitly,
/// we taint those arguments with `ExplicitLateBound::Yes` so that we
/// can provide an appropriate diagnostic later.
#[derive(Copy, Clone, PartialEq, Debug)]
pub enum ExplicitLateBound {
    Yes,
    No,
}

#[derive(Copy, Clone, PartialEq)]
pub enum IsMethodCall {
    Yes,
    No,
}

/// Denotes the "position" of a generic argument, indicating if it is a generic type,
/// generic function or generic method call.
#[derive(Debug, Copy, Clone, PartialEq)]
pub(crate) enum GenericArgPosition {
    Type,
    Value, // e.g., functions
    MethodCall,
}

/// Whether to allow duplicate associated iten constraints in a trait ref, e.g.
/// `Trait<Assoc = Ty, Assoc = Ty>`. This is forbidden in `dyn Trait<...>`
/// but allowed everywhere else.
#[derive(Clone, Copy, Debug, PartialEq)]
pub(crate) enum OverlappingAsssocItemConstraints {
    Allowed,
    Forbidden,
}

/// A marker denoting that the generic arguments that were
/// provided did not match the respective generic parameters.
#[derive(Clone, Debug)]
pub struct GenericArgCountMismatch {
    pub reported: ErrorGuaranteed,
    /// A list of indices of arguments provided that were not valid.
    pub invalid_args: Vec<usize>,
}

/// Decorates the result of a generic argument count mismatch
/// check with whether explicit late bounds were provided.
#[derive(Clone, Debug)]
pub struct GenericArgCountResult {
    pub explicit_late_bound: ExplicitLateBound,
    pub correct: Result<(), GenericArgCountMismatch>,
}

/// A context which can lower HIR's [`GenericArg`] to `rustc_middle`'s [`ty::GenericArg`].
///
/// Its only consumer is [`generics::lower_generic_args`].
/// Read its documentation to learn more.
pub trait GenericArgsLowerer<'a, 'tcx> {
    fn args_for_def_id(&mut self, def_id: DefId) -> (Option<&'a GenericArgs<'tcx>>, bool);

    fn provided_kind(
        &mut self,
        preceding_args: &[ty::GenericArg<'tcx>],
        param: &ty::GenericParamDef,
        arg: &GenericArg<'tcx>,
    ) -> ty::GenericArg<'tcx>;

    fn inferred_kind(
        &mut self,
        preceding_args: &[ty::GenericArg<'tcx>],
        param: &ty::GenericParamDef,
        infer_args: bool,
    ) -> ty::GenericArg<'tcx>;
}

struct ForbidMCGParamUsesFolder<'tcx> {
    tcx: TyCtxt<'tcx>,
    anon_const_def_id: LocalDefId,
    span: Span,
    is_self_alias: bool,
}

impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {
    fn error(&self) -> ErrorGuaranteed {
        let msg = if self.is_self_alias {
            "generic `Self` types are currently not permitted in anonymous constants"
        } else if self.tcx.features().opaque_generic_const_args() {
            "generic parameters in const blocks are only allowed as the direct value of a `type const`"
        } else {
            "generic parameters may not be used in const operations"
        };
        let mut diag = self.tcx.dcx().struct_span_err(self.span, msg);
        if self.is_self_alias {
            let anon_const_hir_id: HirId = HirId::make_owner(self.anon_const_def_id);
            let parent_impl = self.tcx.hir_parent_owner_iter(anon_const_hir_id).find_map(
                |(_, node)| match node {
                    hir::OwnerNode::Item(hir::Item {
                        kind: hir::ItemKind::Impl(impl_), ..
                    }) => Some(impl_),
                    _ => None,
                },
            );
            if let Some(impl_) = parent_impl {
                diag.span_note(impl_.self_ty.span, "not a concrete type");
            }
        }
        if self.tcx.features().min_generic_const_args() {
            if !self.tcx.features().opaque_generic_const_args() {
                diag.help("add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items");
            } else {
                diag.help("consider factoring the expression into a `type const` item and use it as the const argument instead");
            }
        };
        diag.emit()
    }
}

impl<'tcx> ty::TypeFolder<TyCtxt<'tcx>> for ForbidMCGParamUsesFolder<'tcx> {
    fn cx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
        if matches!(t.kind(), ty::Param(..)) {
            return Ty::new_error(self.tcx, self.error());
        }
        t.super_fold_with(self)
    }

    fn fold_const(&mut self, c: Const<'tcx>) -> Const<'tcx> {
        if matches!(c.kind(), ty::ConstKind::Param(..)) {
            return Const::new_error(self.tcx, self.error());
        }
        c.super_fold_with(self)
    }

    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
        if matches!(r.kind(), ty::RegionKind::ReEarlyParam(..) | ty::RegionKind::ReLateParam(..)) {
            return ty::Region::new_error(self.tcx, self.error());
        }
        r
    }
}

impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
    /// See `check_param_uses_if_mcg`.
    ///
    /// FIXME(mgca): this is pub only for instantiate_value_path and would be nice to avoid altogether
    pub fn check_param_res_if_mcg_for_instantiate_value_path(
        &self,
        res: Res,
        span: Span,
    ) -> Result<(), ErrorGuaranteed> {
        let tcx = self.tcx();
        let parent_def_id = self.item_def_id();
        if let Res::Def(DefKind::ConstParam, _) = res
            && tcx.def_kind(parent_def_id) == DefKind::AnonConst
            && let ty::AnonConstKind::MCG = tcx.anon_const_kind(parent_def_id)
        {
            let folder = ForbidMCGParamUsesFolder {
                tcx,
                anon_const_def_id: parent_def_id,
                span,
                is_self_alias: false,
            };
            return Err(folder.error());
        }
        Ok(())
    }

    /// Check for uses of generic parameters that are not in scope due to this being
    /// in a non-generic anon const context.
    #[must_use = "need to use transformed output"]
    fn check_param_uses_if_mcg<T>(&self, term: T, span: Span, is_self_alias: bool) -> T
    where
        T: ty::TypeFoldable<TyCtxt<'tcx>>,
    {
        let tcx = self.tcx();
        let parent_def_id = self.item_def_id();
        if tcx.def_kind(parent_def_id) == DefKind::AnonConst
            && let ty::AnonConstKind::MCG = tcx.anon_const_kind(parent_def_id)
            // Fast path if contains no params/escaping bound vars.
            && (term.has_param() || term.has_escaping_bound_vars())
        {
            let mut folder = ForbidMCGParamUsesFolder {
                tcx,
                anon_const_def_id: parent_def_id,
                span,
                is_self_alias,
            };
            term.fold_with(&mut folder)
        } else {
            term
        }
    }

    /// Lower a lifetime from the HIR to our internal notion of a lifetime called a *region*.
    #[instrument(level = "debug", skip(self), ret)]
    pub fn lower_lifetime(
        &self,
        lifetime: &hir::Lifetime,
        reason: RegionInferReason<'_>,
    ) -> ty::Region<'tcx> {
        if let Some(resolved) = self.tcx().named_bound_var(lifetime.hir_id) {
            let region = self.lower_resolved_lifetime(resolved);
            self.check_param_uses_if_mcg(region, lifetime.ident.span, false)
        } else {
            self.re_infer(lifetime.ident.span, reason)
        }
    }

    /// Lower a lifetime from the HIR to our internal notion of a lifetime called a *region*.
    #[instrument(level = "debug", skip(self), ret)]
    fn lower_resolved_lifetime(&self, resolved: rbv::ResolvedArg) -> ty::Region<'tcx> {
        let tcx = self.tcx();

        match resolved {
            rbv::ResolvedArg::StaticLifetime => tcx.lifetimes.re_static,

            rbv::ResolvedArg::LateBound(debruijn, index, def_id) => {
                let br = ty::BoundRegion {
                    var: ty::BoundVar::from_u32(index),
                    kind: ty::BoundRegionKind::Named(def_id.to_def_id()),
                };
                ty::Region::new_bound(tcx, debruijn, br)
            }

            rbv::ResolvedArg::EarlyBound(def_id) => {
                let name = tcx.hir_ty_param_name(def_id);
                let item_def_id = tcx.hir_ty_param_owner(def_id);
                let generics = tcx.generics_of(item_def_id);
                let index = generics.param_def_id_to_index[&def_id.to_def_id()];
                ty::Region::new_early_param(tcx, ty::EarlyParamRegion { index, name })
            }

            rbv::ResolvedArg::Free(scope, id) => {
                ty::Region::new_late_param(
                    tcx,
                    scope.to_def_id(),
                    ty::LateParamRegionKind::Named(id.to_def_id()),
                )

                // (*) -- not late-bound, won't change
            }

            rbv::ResolvedArg::Error(guar) => ty::Region::new_error(tcx, guar),
        }
    }

    pub fn lower_generic_args_of_path_segment(
        &self,
        span: Span,
        def_id: DefId,
        item_segment: &hir::PathSegment<'tcx>,
    ) -> GenericArgsRef<'tcx> {
        let (args, _) = self.lower_generic_args_of_path(
            span,
            def_id,
            &[],
            item_segment,
            None,
            GenericArgPosition::Type,
        );
        if let Some(c) = item_segment.args().constraints.first() {
            prohibit_assoc_item_constraint(self, c, Some((def_id, item_segment, span)));
        }
        args
    }

    /// Lower the generic arguments provided to some path.
    ///
    /// If this is a trait reference, you also need to pass the self type `self_ty`.
    /// The lowering process may involve applying defaulted type parameters.
    ///
    /// Associated item constraints are not handled here! They are either lowered via
    /// `lower_assoc_item_constraint` or rejected via `prohibit_assoc_item_constraint`.
    ///
    /// ### Example
    ///
    /// ```ignore (illustrative)
    ///    T: std::ops::Index<usize, Output = u32>
    /// // ^1 ^^^^^^^^^^^^^^2 ^^^^3  ^^^^^^^^^^^4
    /// ```
    ///
    /// 1. The `self_ty` here would refer to the type `T`.
    /// 2. The path in question is the path to the trait `std::ops::Index`,
    ///    which will have been resolved to a `def_id`
    /// 3. The `generic_args` contains info on the `<...>` contents. The `usize` type
    ///    parameters are returned in the `GenericArgsRef`
    /// 4. Associated item constraints like `Output = u32` are contained in `generic_args.constraints`.
    ///
    /// Note that the type listing given here is *exactly* what the user provided.
    ///
    /// For (generic) associated types
    ///
    /// ```ignore (illustrative)
    /// <Vec<u8> as Iterable<u8>>::Iter::<'a>
    /// ```
    ///
    /// We have the parent args are the args for the parent trait:
    /// `[Vec<u8>, u8]` and `generic_args` are the arguments for the associated
    /// type itself: `['a]`. The returned `GenericArgsRef` concatenates these two
    /// lists: `[Vec<u8>, u8, 'a]`.
    #[instrument(level = "debug", skip(self, span), ret)]
    pub(crate) fn lower_generic_args_of_path(
        &self,
        span: Span,
        def_id: DefId,
        parent_args: &[ty::GenericArg<'tcx>],
        segment: &hir::PathSegment<'tcx>,
        self_ty: Option<Ty<'tcx>>,
        pos: GenericArgPosition,
    ) -> (GenericArgsRef<'tcx>, GenericArgCountResult) {
        // If the type is parameterized by this region, then replace this
        // region with the current anon region binding (in other words,
        // whatever & would get replaced with).

        let tcx = self.tcx();
        let generics = tcx.generics_of(def_id);
        debug!(?generics);

        if generics.has_self {
            if generics.parent.is_some() {
                // The parent is a trait so it should have at least one
                // generic parameter for the `Self` type.
                assert!(!parent_args.is_empty())
            } else {
                // This item (presumably a trait) needs a self-type.
                assert!(self_ty.is_some());
            }
        } else {
            assert!(self_ty.is_none());
        }

        let arg_count =
            check_generic_arg_count(self, def_id, segment, generics, pos, self_ty.is_some());

        // Skip processing if type has no generic parameters.
        // Traits always have `Self` as a generic parameter, which means they will not return early
        // here and so associated item constraints will be handled regardless of whether there are
        // any non-`Self` generic parameters.
        if generics.is_own_empty() {
            return (tcx.mk_args(parent_args), arg_count);
        }

        struct GenericArgsCtxt<'a, 'tcx> {
            lowerer: &'a dyn HirTyLowerer<'tcx>,
            def_id: DefId,
            generic_args: &'a GenericArgs<'tcx>,
            span: Span,
            infer_args: bool,
            incorrect_args: &'a Result<(), GenericArgCountMismatch>,
        }

        impl<'a, 'tcx> GenericArgsLowerer<'a, 'tcx> for GenericArgsCtxt<'a, 'tcx> {
            fn args_for_def_id(&mut self, did: DefId) -> (Option<&'a GenericArgs<'tcx>>, bool) {
                if did == self.def_id {
                    (Some(self.generic_args), self.infer_args)
                } else {
                    // The last component of this tuple is unimportant.
                    (None, false)
                }
            }

            fn provided_kind(
                &mut self,
                preceding_args: &[ty::GenericArg<'tcx>],
                param: &ty::GenericParamDef,
                arg: &GenericArg<'tcx>,
            ) -> ty::GenericArg<'tcx> {
                let tcx = self.lowerer.tcx();

                if let Err(incorrect) = self.incorrect_args {
                    if incorrect.invalid_args.contains(&(param.index as usize)) {
                        return param.to_error(tcx);
                    }
                }

                let handle_ty_args = |has_default, ty: &hir::Ty<'tcx>| {
                    if has_default {
                        tcx.check_optional_stability(
                            param.def_id,
                            Some(arg.hir_id()),
                            arg.span(),
                            None,
                            AllowUnstable::No,
                            |_, _| {
                                // Default generic parameters may not be marked
                                // with stability attributes, i.e. when the
                                // default parameter was defined at the same time
                                // as the rest of the type. As such, we ignore missing
                                // stability attributes.
                            },
                        );
                    }
                    self.lowerer.lower_ty(ty).into()
                };

                match (&param.kind, arg) {
                    (GenericParamDefKind::Lifetime, GenericArg::Lifetime(lt)) => {
                        self.lowerer.lower_lifetime(lt, RegionInferReason::Param(param)).into()
                    }
                    (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Type(ty)) => {
                        // We handle the other parts of `Ty` in the match arm below
                        handle_ty_args(has_default, ty.as_unambig_ty())
                    }
                    (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Infer(inf)) => {
                        handle_ty_args(has_default, &inf.to_ty())
                    }
                    (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => self
                        .lowerer
                        // Ambig portions of `ConstArg` are handled in the match arm below
                        .lower_const_arg(
                            ct.as_unambig_ct(),
                            tcx.type_of(param.def_id).instantiate(tcx, preceding_args),
                        )
                        .into(),
                    (&GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
                        self.lowerer.ct_infer(Some(param), inf.span).into()
                    }
                    (kind, arg) => span_bug!(
                        self.span,
                        "mismatched path argument for kind {kind:?}: found arg {arg:?}"
                    ),
                }
            }

            fn inferred_kind(
                &mut self,
                preceding_args: &[ty::GenericArg<'tcx>],
                param: &ty::GenericParamDef,
                infer_args: bool,
            ) -> ty::GenericArg<'tcx> {
                let tcx = self.lowerer.tcx();

                if let Err(incorrect) = self.incorrect_args {
                    if incorrect.invalid_args.contains(&(param.index as usize)) {
                        return param.to_error(tcx);
                    }
                }
                match param.kind {
                    GenericParamDefKind::Lifetime => {
                        self.lowerer.re_infer(self.span, RegionInferReason::Param(param)).into()
                    }
                    GenericParamDefKind::Type { has_default, .. } => {
                        if !infer_args && has_default {
                            // No type parameter provided, but a default exists.
                            if let Some(prev) =
                                preceding_args.iter().find_map(|arg| match arg.kind() {
                                    GenericArgKind::Type(ty) => ty.error_reported().err(),
                                    _ => None,
                                })
                            {
                                // Avoid ICE #86756 when type error recovery goes awry.
                                return Ty::new_error(tcx, prev).into();
                            }
                            tcx.at(self.span)
                                .type_of(param.def_id)
                                .instantiate(tcx, preceding_args)
                                .into()
                        } else if infer_args {
                            self.lowerer.ty_infer(Some(param), self.span).into()
                        } else {
                            // We've already errored above about the mismatch.
                            Ty::new_misc_error(tcx).into()
                        }
                    }
                    GenericParamDefKind::Const { has_default, .. } => {
                        let ty = tcx
                            .at(self.span)
                            .type_of(param.def_id)
                            .instantiate(tcx, preceding_args);
                        if let Err(guar) = ty.error_reported() {
                            return ty::Const::new_error(tcx, guar).into();
                        }
                        if !infer_args && has_default {
                            tcx.const_param_default(param.def_id)
                                .instantiate(tcx, preceding_args)
                                .into()
                        } else if infer_args {
                            self.lowerer.ct_infer(Some(param), self.span).into()
                        } else {
                            // We've already errored above about the mismatch.
                            ty::Const::new_misc_error(tcx).into()
                        }
                    }
                }
            }
        }

        let mut args_ctx = GenericArgsCtxt {
            lowerer: self,
            def_id,
            span,
            generic_args: segment.args(),
            infer_args: segment.infer_args,
            incorrect_args: &arg_count.correct,
        };

        let args = lower_generic_args(
            self,
            def_id,
            parent_args,
            self_ty.is_some(),
            self_ty,
            &arg_count,
            &mut args_ctx,
        );

        (args, arg_count)
    }

    #[instrument(level = "debug", skip(self))]
    pub fn lower_generic_args_of_assoc_item(
        &self,
        span: Span,
        item_def_id: DefId,
        item_segment: &hir::PathSegment<'tcx>,
        parent_args: GenericArgsRef<'tcx>,
    ) -> GenericArgsRef<'tcx> {
        let (args, _) = self.lower_generic_args_of_path(
            span,
            item_def_id,
            parent_args,
            item_segment,
            None,
            GenericArgPosition::Type,
        );
        if let Some(c) = item_segment.args().constraints.first() {
            prohibit_assoc_item_constraint(self, c, Some((item_def_id, item_segment, span)));
        }
        args
    }

    /// Lower a trait reference as found in an impl header as the implementee.
    ///
    /// The self type `self_ty` is the implementer of the trait.
    pub fn lower_impl_trait_ref(
        &self,
        trait_ref: &hir::TraitRef<'tcx>,
        self_ty: Ty<'tcx>,
    ) -> ty::TraitRef<'tcx> {
        let [leading_segments @ .., segment] = trait_ref.path.segments else { bug!() };

        let _ = self.prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);

        self.lower_mono_trait_ref(
            trait_ref.path.span,
            trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
            self_ty,
            segment,
            true,
        )
    }

    /// Lower a polymorphic trait reference given a self type into `bounds`.
    ///
    /// *Polymorphic* in the sense that it may bind late-bound vars.
    ///
    /// This may generate auxiliary bounds iff the trait reference contains associated item constraints.
    ///
    /// ### Example
    ///
    /// Given the trait ref `Iterator<Item = u32>` and the self type `Ty`, this will add the
    ///
    /// 1. *trait predicate* `<Ty as Iterator>` (known as `Ty: Iterator` in the surface syntax) and the
    /// 2. *projection predicate* `<Ty as Iterator>::Item = u32`
    ///
    /// to `bounds`.
    ///
    /// ### A Note on Binders
    ///
    /// Against our usual convention, there is an implied binder around the `self_ty` and the
    /// `trait_ref` here. So they may reference late-bound vars.
    ///
    /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
    /// where `'a` is a bound region at depth 0. Similarly, the `trait_ref` would be `Bar<'a>`.
    /// The lowered poly-trait-ref will track this binder explicitly, however.
    #[instrument(level = "debug", skip(self, bounds))]
    pub(crate) fn lower_poly_trait_ref(
        &self,
        &hir::PolyTraitRef {
            bound_generic_params,
            modifiers: hir::TraitBoundModifiers { constness, polarity },
            trait_ref,
            span,
        }: &hir::PolyTraitRef<'tcx>,
        self_ty: Ty<'tcx>,
        bounds: &mut Vec<(ty::Clause<'tcx>, Span)>,
        predicate_filter: PredicateFilter,
        overlapping_assoc_item_constraints: OverlappingAsssocItemConstraints,
    ) -> GenericArgCountResult {
        let tcx = self.tcx();

        // We use the *resolved* bound vars later instead of the HIR ones since the former
        // also include the bound vars of the overarching predicate if applicable.
        let _ = bound_generic_params;

        let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());

        // Relaxed bounds `?Trait` and `PointeeSized` bounds aren't represented in the middle::ty IR
        // as they denote the *absence* of a default bound. However, we can't bail out early here since
        // we still need to perform several validation steps (see below). Instead, simply "pour" all
        // resulting bounds "down the drain", i.e., into a new `Vec` that just gets dropped at the end.
        let transient = match polarity {
            hir::BoundPolarity::Positive => {
                // To elaborate on the comment directly above, regarding `PointeeSized` specifically,
                // we don't "reify" such bounds to avoid trait system limitations -- namely,
                // non-global where-clauses being preferred over item bounds (where `PointeeSized`
                // bounds would be proven) -- which can result in errors when a `PointeeSized`
                // supertrait / bound / predicate is added to some items.
                tcx.is_lang_item(trait_def_id, hir::LangItem::PointeeSized)
            }
            hir::BoundPolarity::Negative(_) => false,
            hir::BoundPolarity::Maybe(_) => {
                self.require_bound_to_relax_default_trait(trait_ref, span);
                true
            }
        };
        let bounds = if transient { &mut Vec::new() } else { bounds };

        let polarity = match polarity {
            hir::BoundPolarity::Positive | hir::BoundPolarity::Maybe(_) => {
                ty::PredicatePolarity::Positive
            }
            hir::BoundPolarity::Negative(_) => ty::PredicatePolarity::Negative,
        };

        let [leading_segments @ .., segment] = trait_ref.path.segments else { bug!() };

        let _ = self.prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
        self.report_internal_fn_trait(span, trait_def_id, segment, false);

        let (generic_args, arg_count) = self.lower_generic_args_of_path(
            trait_ref.path.span,
            trait_def_id,
            &[],
            segment,
            Some(self_ty),
            GenericArgPosition::Type,
        );

        let constraints = segment.args().constraints;

        if transient && (!generic_args[1..].is_empty() || !constraints.is_empty()) {
            // Since the bound won't be present in the middle::ty IR as established above, any
            // arguments or constraints won't be checked for well-formedness in later passes.
            //
            // This is only an issue if the trait ref is otherwise valid which can only happen if
            // the corresponding default trait has generic parameters or associated items. Such a
            // trait would be degenerate. We delay a bug to detect and guard us against these.
            //
            // E.g: Given `/*default*/ trait Bound<'a: 'static, T, const N: usize> {}`,
            // `?Bound<Vec<str>, { panic!() }>` won't be wfchecked.
            self.dcx()
                .span_delayed_bug(span, "transient bound should not have args or constraints");
        }

        let bound_vars = tcx.late_bound_vars(trait_ref.hir_ref_id);
        debug!(?bound_vars);

        let poly_trait_ref = ty::Binder::bind_with_vars(
            ty::TraitRef::new_from_args(tcx, trait_def_id, generic_args),
            bound_vars,
        );

        debug!(?poly_trait_ref);

        // We deal with const conditions later.
        match predicate_filter {
            PredicateFilter::All
            | PredicateFilter::SelfOnly
            | PredicateFilter::SelfTraitThatDefines(..)
            | PredicateFilter::SelfAndAssociatedTypeBounds => {
                let bound = poly_trait_ref.map_bound(|trait_ref| {
                    ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity })
                });
                let bound = (bound.upcast(tcx), span);
                // FIXME(-Znext-solver): We can likely remove this hack once the
                // new trait solver lands. This fixed an overflow in the old solver.
                // This may have performance implications, so please check perf when
                // removing it.
                // This was added in <https://github.com/rust-lang/rust/pull/123302>.
                if tcx.is_lang_item(trait_def_id, rustc_hir::LangItem::Sized) {
                    bounds.insert(0, bound);
                } else {
                    bounds.push(bound);
                }
            }
            PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {}
        }

        if let hir::BoundConstness::Always(span) | hir::BoundConstness::Maybe(span) = constness
            && !tcx.is_const_trait(trait_def_id)
        {
            let (def_span, suggestion, suggestion_pre) =
                match (trait_def_id.as_local(), tcx.sess.is_nightly_build()) {
                    (Some(trait_def_id), true) => {
                        let span = tcx.hir_expect_item(trait_def_id).vis_span;
                        let span = tcx.sess.source_map().span_extend_while_whitespace(span);

                        (
                            None,
                            Some(span.shrink_to_hi()),
                            if self.tcx().features().const_trait_impl() {
                                ""
                            } else {
                                "enable `#![feature(const_trait_impl)]` in your crate and "
                            },
                        )
                    }
                    (None, _) | (_, false) => (Some(tcx.def_span(trait_def_id)), None, ""),
                };
            self.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
                span,
                modifier: constness.as_str(),
                def_span,
                trait_name: tcx.def_path_str(trait_def_id),
                suggestion,
                suggestion_pre,
            });
        } else {
            match predicate_filter {
                // This is only concerned with trait predicates.
                PredicateFilter::SelfTraitThatDefines(..) => {}
                PredicateFilter::All
                | PredicateFilter::SelfOnly
                | PredicateFilter::SelfAndAssociatedTypeBounds => {
                    match constness {
                        hir::BoundConstness::Always(_) => {
                            if polarity == ty::PredicatePolarity::Positive {
                                bounds.push((
                                    poly_trait_ref
                                        .to_host_effect_clause(tcx, ty::BoundConstness::Const),
                                    span,
                                ));
                            }
                        }
                        hir::BoundConstness::Maybe(_) => {
                            // We don't emit a const bound here, since that would mean that we
                            // unconditionally need to prove a `HostEffect` predicate, even when
                            // the predicates are being instantiated in a non-const context. This
                            // is instead handled in the `const_conditions` query.
                        }
                        hir::BoundConstness::Never => {}
                    }
                }
                // On the flip side, when filtering `ConstIfConst` bounds, we only need to convert
                // `[const]` bounds. All other predicates are handled in their respective queries.
                //
                // Note that like `PredicateFilter::SelfOnly`, we don't need to do any filtering
                // here because we only call this on self bounds, and deal with the recursive case
                // in `lower_assoc_item_constraint`.
                PredicateFilter::ConstIfConst | PredicateFilter::SelfConstIfConst => {
                    match constness {
                        hir::BoundConstness::Maybe(_) => {
                            if polarity == ty::PredicatePolarity::Positive {
                                bounds.push((
                                    poly_trait_ref
                                        .to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
                                    span,
                                ));
                            }
                        }
                        hir::BoundConstness::Always(_) | hir::BoundConstness::Never => {}
                    }
                }
            }
        }

        let mut dup_constraints = (overlapping_assoc_item_constraints
            == OverlappingAsssocItemConstraints::Forbidden)
            .then_some(FxIndexMap::default());

        for constraint in constraints {
            // Don't register any associated item constraints for negative bounds,
            // since we should have emitted an error for them earlier, and they
            // would not be well-formed!
            if polarity == ty::PredicatePolarity::Negative {
                self.dcx().span_delayed_bug(
                    constraint.span,
                    "negative trait bounds should not have assoc item constraints",
                );
                break;
            }

            // Specify type to assert that error was already reported in `Err` case.
            let _: Result<_, ErrorGuaranteed> = self.lower_assoc_item_constraint(
                trait_ref.hir_ref_id,
                poly_trait_ref,
                constraint,
                bounds,
                dup_constraints.as_mut(),
                constraint.span,
                predicate_filter,
            );
            // Okay to ignore `Err` because of `ErrorGuaranteed` (see above).
        }

        arg_count
    }

    /// Lower a monomorphic trait reference given a self type while prohibiting associated item bindings.
    ///
    /// *Monomorphic* in the sense that it doesn't bind any late-bound vars.
    fn lower_mono_trait_ref(
        &self,
        span: Span,
        trait_def_id: DefId,
        self_ty: Ty<'tcx>,
        trait_segment: &hir::PathSegment<'tcx>,
        is_impl: bool,
    ) -> ty::TraitRef<'tcx> {
        self.report_internal_fn_trait(span, trait_def_id, trait_segment, is_impl);

        let (generic_args, _) = self.lower_generic_args_of_path(
            span,
            trait_def_id,
            &[],
            trait_segment,
            Some(self_ty),
            GenericArgPosition::Type,
        );
        if let Some(c) = trait_segment.args().constraints.first() {
            prohibit_assoc_item_constraint(self, c, Some((trait_def_id, trait_segment, span)));
        }
        ty::TraitRef::new_from_args(self.tcx(), trait_def_id, generic_args)
    }

    fn probe_trait_that_defines_assoc_item(
        &self,
        trait_def_id: DefId,
        assoc_tag: ty::AssocTag,
        assoc_ident: Ident,
    ) -> bool {
        self.tcx()
            .associated_items(trait_def_id)
            .find_by_ident_and_kind(self.tcx(), assoc_ident, assoc_tag, trait_def_id)
            .is_some()
    }

    fn lower_path_segment(
        &self,
        span: Span,
        did: DefId,
        item_segment: &hir::PathSegment<'tcx>,
    ) -> Ty<'tcx> {
        let tcx = self.tcx();
        let args = self.lower_generic_args_of_path_segment(span, did, item_segment);

        if let DefKind::TyAlias = tcx.def_kind(did)
            && tcx.type_alias_is_lazy(did)
        {
            // Type aliases defined in crates that have the
            // feature `lazy_type_alias` enabled get encoded as a type alias that normalization will
            // then actually instantiate the where bounds of.
            let alias_ty = ty::AliasTy::new_from_args(tcx, did, args);
            Ty::new_alias(tcx, ty::Free, alias_ty)
        } else {
            tcx.at(span).type_of(did).instantiate(tcx, args)
        }
    }

    /// Search for a trait bound on a type parameter whose trait defines the associated item
    /// given by `assoc_ident` and `kind`.
    ///
    /// This fails if there is no such bound in the list of candidates or if there are multiple
    /// candidates in which case it reports ambiguity.
    ///
    /// `ty_param_def_id` is the `LocalDefId` of the type parameter.
    #[instrument(level = "debug", skip_all, ret)]
    fn probe_single_ty_param_bound_for_assoc_item(
        &self,
        ty_param_def_id: LocalDefId,
        ty_param_span: Span,
        assoc_tag: ty::AssocTag,
        assoc_ident: Ident,
        span: Span,
    ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
        debug!(?ty_param_def_id, ?assoc_ident, ?span);
        let tcx = self.tcx();

        let predicates = &self.probe_ty_param_bounds(span, ty_param_def_id, assoc_ident);
        debug!("predicates={:#?}", predicates);

        self.probe_single_bound_for_assoc_item(
            || {
                let trait_refs = predicates
                    .iter_identity_copied()
                    .filter_map(|(p, _)| Some(p.as_trait_clause()?.map_bound(|t| t.trait_ref)));
                traits::transitive_bounds_that_define_assoc_item(tcx, trait_refs, assoc_ident)
            },
            AssocItemQSelf::TyParam(ty_param_def_id, ty_param_span),
            assoc_tag,
            assoc_ident,
            span,
            None,
        )
    }

    /// Search for a single trait bound whose trait defines the associated item given by
    /// `assoc_ident`.
    ///
    /// This fails if there is no such bound in the list of candidates or if there are multiple
    /// candidates in which case it reports ambiguity.
    #[instrument(level = "debug", skip(self, all_candidates, qself, constraint), ret)]
    fn probe_single_bound_for_assoc_item<I>(
        &self,
        all_candidates: impl Fn() -> I,
        qself: AssocItemQSelf,
        assoc_tag: ty::AssocTag,
        assoc_ident: Ident,
        span: Span,
        constraint: Option<&hir::AssocItemConstraint<'tcx>>,
    ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed>
    where
        I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
    {
        let tcx = self.tcx();

        let mut matching_candidates = all_candidates().filter(|r| {
            self.probe_trait_that_defines_assoc_item(r.def_id(), assoc_tag, assoc_ident)
        });

        let Some(bound) = matching_candidates.next() else {
            return Err(self.report_unresolved_assoc_item(
                all_candidates,
                qself,
                assoc_tag,
                assoc_ident,
                span,
                constraint,
            ));
        };
        debug!(?bound);

        if let Some(bound2) = matching_candidates.next() {
            debug!(?bound2);

            let assoc_kind_str = errors::assoc_tag_str(assoc_tag);
            let qself_str = qself.to_string(tcx);
            let mut err = self.dcx().create_err(crate::errors::AmbiguousAssocItem {
                span,
                assoc_kind: assoc_kind_str,
                assoc_ident,
                qself: &qself_str,
            });
            // Provide a more specific error code index entry for equality bindings.
            err.code(
                if let Some(constraint) = constraint
                    && let hir::AssocItemConstraintKind::Equality { .. } = constraint.kind
                {
                    E0222
                } else {
                    E0221
                },
            );

            // FIXME(#97583): Print associated item bindings properly (i.e., not as equality
            // predicates!).
            // FIXME: Turn this into a structured, translatable & more actionable suggestion.
            let mut where_bounds = vec![];
            for bound in [bound, bound2].into_iter().chain(matching_candidates) {
                let bound_id = bound.def_id();
                let assoc_item = tcx.associated_items(bound_id).find_by_ident_and_kind(
                    tcx,
                    assoc_ident,
                    assoc_tag,
                    bound_id,
                );
                let bound_span = assoc_item.and_then(|item| tcx.hir_span_if_local(item.def_id));

                if let Some(bound_span) = bound_span {
                    err.span_label(
                        bound_span,
                        format!("ambiguous `{assoc_ident}` from `{}`", bound.print_trait_sugared(),),
                    );
                    if let Some(constraint) = constraint {
                        match constraint.kind {
                            hir::AssocItemConstraintKind::Equality { term } => {
                                let term: ty::Term<'_> = match term {
                                    hir::Term::Ty(ty) => self.lower_ty(ty).into(),
                                    hir::Term::Const(ct) => {
                                        let assoc_item =
                                            assoc_item.expect("assoc_item should be present");
                                        let projection_term = bound.map_bound(|trait_ref| {
                                            let item_segment = hir::PathSegment {
                                                ident: constraint.ident,
                                                hir_id: constraint.hir_id,
                                                res: Res::Err,
                                                args: Some(constraint.gen_args),
                                                infer_args: false,
                                            };

                                            let alias_args = self.lower_generic_args_of_assoc_item(
                                                constraint.ident.span,
                                                assoc_item.def_id,
                                                &item_segment,
                                                trait_ref.args,
                                            );
                                            ty::AliasTerm::new_from_args(
                                                tcx,
                                                assoc_item.def_id,
                                                alias_args,
                                            )
                                        });

                                        // FIXME(mgca): code duplication with other places we lower
                                        // the rhs' of associated const bindings
                                        let ty = projection_term.map_bound(|alias| {
                                            tcx.type_of(alias.def_id).instantiate(tcx, alias.args)
                                        });
                                        let ty = bounds::check_assoc_const_binding_type(
                                            self,
                                            constraint.ident,
                                            ty,
                                            constraint.hir_id,
                                        );

                                        self.lower_const_arg(ct, ty).into()
                                    }
                                };
                                if term.references_error() {
                                    continue;
                                }
                                // FIXME(#97583): This isn't syntactically well-formed!
                                where_bounds.push(format!(
                                    "        T: {trait}::{assoc_ident} = {term}",
                                    trait = bound.print_only_trait_path(),
                                ));
                            }
                            // FIXME: Provide a suggestion.
                            hir::AssocItemConstraintKind::Bound { bounds: _ } => {}
                        }
                    } else {
                        err.span_suggestion_verbose(
                            span.with_hi(assoc_ident.span.lo()),
                            "use fully-qualified syntax to disambiguate",
                            format!("<{qself_str} as {}>::", bound.print_only_trait_path()),
                            Applicability::MaybeIncorrect,
                        );
                    }
                } else {
                    let trait_ =
                        tcx.short_string(bound.print_only_trait_path(), err.long_ty_path());
                    err.note(format!(
                        "associated {assoc_kind_str} `{assoc_ident}` could derive from `{trait_}`",
                    ));
                }
            }
            if !where_bounds.is_empty() {
                err.help(format!(
                    "consider introducing a new type parameter `T` and adding `where` constraints:\
                     \n    where\n        T: {qself_str},\n{}",
                    where_bounds.join(",\n"),
                ));
                let reported = err.emit();
                return Err(reported);
            }
            err.emit();
        }

        Ok(bound)
    }

    /// Lower a [type-relative](hir::QPath::TypeRelative) path in type position to a type.
    ///
    /// If the path refers to an enum variant and `permit_variants` holds,
    /// the returned type is simply the provided self type `qself_ty`.
    ///
    /// A path like `A::B::C::D` is understood as `<A::B::C>::D`. I.e.,
    /// `qself_ty` / `qself` is `A::B::C` and `assoc_segment` is `D`.
    /// We return the lowered type and the `DefId` for the whole path.
    ///
    /// We only support associated type paths whose self type is a type parameter or a `Self`
    /// type alias (in a trait impl) like `T::Ty` (where `T` is a ty param) or `Self::Ty`.
    /// We **don't** support paths whose self type is an arbitrary type like `Struct::Ty` where
    /// struct `Struct` impls an in-scope trait that defines an associated type called `Ty`.
    /// For the latter case, we report ambiguity.
    /// While desirable to support, the implementation would be non-trivial. Tracked in [#22519].
    ///
    /// At the time of writing, *inherent associated types* are also resolved here. This however
    /// is [problematic][iat]. A proper implementation would be as non-trivial as the one
    /// described in the previous paragraph and their modeling of projections would likely be
    /// very similar in nature.
    ///
    /// [#22519]: https://github.com/rust-lang/rust/issues/22519
    /// [iat]: https://github.com/rust-lang/rust/issues/8995#issuecomment-1569208403
    //
    // NOTE: When this function starts resolving `Trait::AssocTy` successfully
    // it should also start reporting the `BARE_TRAIT_OBJECTS` lint.
    #[instrument(level = "debug", skip_all, ret)]
    pub fn lower_type_relative_ty_path(
        &self,
        self_ty: Ty<'tcx>,
        hir_self_ty: &'tcx hir::Ty<'tcx>,
        segment: &'tcx hir::PathSegment<'tcx>,
        qpath_hir_id: HirId,
        span: Span,
        permit_variants: PermitVariants,
    ) -> Result<(Ty<'tcx>, DefKind, DefId), ErrorGuaranteed> {
        let tcx = self.tcx();
        match self.lower_type_relative_path(
            self_ty,
            hir_self_ty,
            segment,
            qpath_hir_id,
            span,
            LowerTypeRelativePathMode::Type(permit_variants),
        )? {
            TypeRelativePath::AssocItem(def_id, args) => {
                let alias_ty = ty::AliasTy::new_from_args(tcx, def_id, args);
                let ty = Ty::new_alias(tcx, alias_ty.kind(tcx), alias_ty);
                let ty = self.check_param_uses_if_mcg(ty, span, false);
                Ok((ty, tcx.def_kind(def_id), def_id))
            }
            TypeRelativePath::Variant { adt, variant_did } => {
                let adt = self.check_param_uses_if_mcg(adt, span, false);
                Ok((adt, DefKind::Variant, variant_did))
            }
            TypeRelativePath::Ctor { .. } => {
                let e = tcx.dcx().span_err(span, "expected type, found tuple constructor");
                Err(e)
            }
        }
    }

    /// Lower a [type-relative][hir::QPath::TypeRelative] path to a (type-level) constant.
    #[instrument(level = "debug", skip_all, ret)]
    fn lower_type_relative_const_path(
        &self,
        self_ty: Ty<'tcx>,
        hir_self_ty: &'tcx hir::Ty<'tcx>,
        segment: &'tcx hir::PathSegment<'tcx>,
        qpath_hir_id: HirId,
        span: Span,
    ) -> Result<Const<'tcx>, ErrorGuaranteed> {
        let tcx = self.tcx();
        match self.lower_type_relative_path(
            self_ty,
            hir_self_ty,
            segment,
            qpath_hir_id,
            span,
            LowerTypeRelativePathMode::Const,
        )? {
            TypeRelativePath::AssocItem(def_id, args) => {
                self.require_type_const_attribute(def_id, span)?;
                let ct = Const::new_unevaluated(tcx, ty::UnevaluatedConst::new(def_id, args));
                let ct = self.check_param_uses_if_mcg(ct, span, false);
                Ok(ct)
            }
            TypeRelativePath::Ctor { ctor_def_id, args } => match tcx.def_kind(ctor_def_id) {
                DefKind::Ctor(_, CtorKind::Fn) => {
                    Ok(ty::Const::zero_sized(tcx, Ty::new_fn_def(tcx, ctor_def_id, args)))
                }
                DefKind::Ctor(ctor_of, CtorKind::Const) => {
                    Ok(self.construct_const_ctor_value(ctor_def_id, ctor_of, args))
                }
                _ => unreachable!(),
            },
            // FIXME(mgca): implement support for this once ready to support all adt ctor expressions,
            // not just const ctors
            TypeRelativePath::Variant { .. } => {
                span_bug!(span, "unexpected variant res for type associated const path")
            }
        }
    }

    /// Lower a [type-relative][hir::QPath::TypeRelative] (and type-level) path.
    #[instrument(level = "debug", skip_all, ret)]
    fn lower_type_relative_path(
        &self,
        self_ty: Ty<'tcx>,
        hir_self_ty: &'tcx hir::Ty<'tcx>,
        segment: &'tcx hir::PathSegment<'tcx>,
        qpath_hir_id: HirId,
        span: Span,
        mode: LowerTypeRelativePathMode,
    ) -> Result<TypeRelativePath<'tcx>, ErrorGuaranteed> {
        struct AmbiguousAssocItem<'tcx> {
            variant_def_id: DefId,
            item_def_id: DefId,
            span: Span,
            segment_ident: Ident,
            bound_def_id: DefId,
            self_ty: Ty<'tcx>,
            tcx: TyCtxt<'tcx>,
            mode: LowerTypeRelativePathMode,
        }

        impl<'a, 'tcx> Diagnostic<'a, ()> for AmbiguousAssocItem<'tcx> {
            fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
                let Self {
                    variant_def_id,
                    item_def_id,
                    span,
                    segment_ident,
                    bound_def_id,
                    self_ty,
                    tcx,
                    mode,
                } = self;
                let mut lint = Diag::new(dcx, level, "ambiguous associated item");

                let mut could_refer_to = |kind: DefKind, def_id, also| {
                    let note_msg = format!(
                        "`{}` could{} refer to the {} defined here",
                        segment_ident,
                        also,
                        tcx.def_kind_descr(kind, def_id)
                    );
                    lint.span_note(tcx.def_span(def_id), note_msg);
                };

                could_refer_to(DefKind::Variant, variant_def_id, "");
                could_refer_to(mode.def_kind_for_diagnostics(), item_def_id, " also");

                lint.span_suggestion(
                    span,
                    "use fully-qualified syntax",
                    format!("<{} as {}>::{}", self_ty, tcx.item_name(bound_def_id), segment_ident),
                    Applicability::MachineApplicable,
                );
                lint
            }
        }

        debug!(%self_ty, ?segment.ident);
        let tcx = self.tcx();

        // Check if we have an enum variant or an inherent associated type.
        let mut variant_def_id = None;
        if let Some(adt_def) = self.probe_adt(span, self_ty) {
            if adt_def.is_enum() {
                let variant_def = adt_def
                    .variants()
                    .iter()
                    .find(|vd| tcx.hygienic_eq(segment.ident, vd.ident(tcx), adt_def.did()));
                if let Some(variant_def) = variant_def {
                    // FIXME(mgca): do we want constructor resolutions to take priority over
                    // other possible resolutions?
                    if matches!(mode, LowerTypeRelativePathMode::Const)
                        && let Some((_, ctor_def_id)) = variant_def.ctor
                    {
                        tcx.check_stability(variant_def.def_id, Some(qpath_hir_id), span, None);
                        let _ = self.prohibit_generic_args(
                            slice::from_ref(segment).iter(),
                            GenericsArgsErrExtend::EnumVariant {
                                qself: hir_self_ty,
                                assoc_segment: segment,
                                adt_def,
                            },
                        );
                        let ty::Adt(_, enum_args) = self_ty.kind() else { unreachable!() };
                        return Ok(TypeRelativePath::Ctor { ctor_def_id, args: enum_args });
                    }
                    if let PermitVariants::Yes = mode.permit_variants() {
                        tcx.check_stability(variant_def.def_id, Some(qpath_hir_id), span, None);
                        let _ = self.prohibit_generic_args(
                            slice::from_ref(segment).iter(),
                            GenericsArgsErrExtend::EnumVariant {
                                qself: hir_self_ty,
                                assoc_segment: segment,
                                adt_def,
                            },
                        );
                        return Ok(TypeRelativePath::Variant {
                            adt: self_ty,
                            variant_did: variant_def.def_id,
                        });
                    } else {
                        variant_def_id = Some(variant_def.def_id);
                    }
                }
            }

            // FIXME(inherent_associated_types, #106719): Support self types other than ADTs.
            if let Some((did, args)) = self.probe_inherent_assoc_item(
                segment,
                adt_def.did(),
                self_ty,
                qpath_hir_id,
                span,
                mode.assoc_tag(),
            )? {
                return Ok(TypeRelativePath::AssocItem(did, args));
            }
        }

        let (item_def_id, bound) = self.resolve_type_relative_path(
            self_ty,
            hir_self_ty,
            mode.assoc_tag(),
            segment,
            qpath_hir_id,
            span,
            variant_def_id,
        )?;

        let (item_def_id, args) = self.lower_assoc_item_path(span, item_def_id, segment, bound)?;

        if let Some(variant_def_id) = variant_def_id {
            tcx.emit_node_span_lint(
                AMBIGUOUS_ASSOCIATED_ITEMS,
                qpath_hir_id,
                span,
                AmbiguousAssocItem {
                    variant_def_id,
                    item_def_id,
                    span,
                    segment_ident: segment.ident,
                    bound_def_id: bound.def_id(),
                    self_ty,
                    tcx,
                    mode,
                },
            );
        }

        Ok(TypeRelativePath::AssocItem(item_def_id, args))
    }

    /// Resolve a [type-relative](hir::QPath::TypeRelative) (and type-level) path.
    fn resolve_type_relative_path(
        &self,
        self_ty: Ty<'tcx>,
        hir_self_ty: &'tcx hir::Ty<'tcx>,
        assoc_tag: ty::AssocTag,
        segment: &'tcx hir::PathSegment<'tcx>,
        qpath_hir_id: HirId,
        span: Span,
        variant_def_id: Option<DefId>,
    ) -> Result<(DefId, ty::PolyTraitRef<'tcx>), ErrorGuaranteed> {
        let tcx = self.tcx();

        let self_ty_res = match hir_self_ty.kind {
            hir::TyKind::Path(hir::QPath::Resolved(_, path)) => path.res,
            _ => Res::Err,
        };

        // Find the type of the assoc item, and the trait where the associated item is declared.
        let bound = match (self_ty.kind(), self_ty_res) {
            (_, Res::SelfTyAlias { alias_to: impl_def_id, is_trait_impl: true, .. }) => {
                // `Self` in an impl of a trait -- we have a concrete self type and a
                // trait reference.
                let trait_ref = tcx.impl_trait_ref(impl_def_id);

                self.probe_single_bound_for_assoc_item(
                    || {
                        let trait_ref = ty::Binder::dummy(trait_ref.instantiate_identity());
                        traits::supertraits(tcx, trait_ref)
                    },
                    AssocItemQSelf::SelfTyAlias,
                    assoc_tag,
                    segment.ident,
                    span,
                    None,
                )?
            }
            (
                &ty::Param(_),
                Res::SelfTyParam { trait_: param_did } | Res::Def(DefKind::TyParam, param_did),
            ) => self.probe_single_ty_param_bound_for_assoc_item(
                param_did.expect_local(),
                hir_self_ty.span,
                assoc_tag,
                segment.ident,
                span,
            )?,
            _ => {
                return Err(self.report_unresolved_type_relative_path(
                    self_ty,
                    hir_self_ty,
                    assoc_tag,
                    segment.ident,
                    qpath_hir_id,
                    span,
                    variant_def_id,
                ));
            }
        };

        let assoc_item = self
            .probe_assoc_item(segment.ident, assoc_tag, qpath_hir_id, span, bound.def_id())
            .expect("failed to find associated item");

        Ok((assoc_item.def_id, bound))
    }

    /// Search for inherent associated items for use at the type level.
    fn probe_inherent_assoc_item(
        &self,
        segment: &hir::PathSegment<'tcx>,
        adt_did: DefId,
        self_ty: Ty<'tcx>,
        block: HirId,
        span: Span,
        assoc_tag: ty::AssocTag,
    ) -> Result<Option<(DefId, GenericArgsRef<'tcx>)>, ErrorGuaranteed> {
        let tcx = self.tcx();

        if !tcx.features().inherent_associated_types() {
            match assoc_tag {
                // Don't attempt to look up inherent associated types when the feature is not
                // enabled. Theoretically it'd be fine to do so since we feature-gate their
                // definition site. However, the current implementation of inherent associated
                // items is somewhat brittle, so let's not run it by default.
                ty::AssocTag::Type => return Ok(None),
                ty::AssocTag::Const => {
                    // We also gate the mgca codepath for type-level uses of inherent consts
                    // with the inherent_associated_types feature gate since it relies on the
                    // same machinery and has similar rough edges.
                    return Err(feature_err(
                        &tcx.sess,
                        sym::inherent_associated_types,
                        span,
                        "inherent associated types are unstable",
                    )
                    .emit());
                }
                ty::AssocTag::Fn => unreachable!(),
            }
        }

        let name = segment.ident;
        let candidates: Vec<_> = tcx
            .inherent_impls(adt_did)
            .iter()
            .filter_map(|&impl_| {
                let (item, scope) =
                    self.probe_assoc_item_unchecked(name, assoc_tag, block, impl_)?;
                Some(InherentAssocCandidate { impl_, assoc_item: item.def_id, scope })
            })
            .collect();

        // At the moment, we actually bail out with a hard error if the selection of an inherent
        // associated item fails (see below). This means we never consider trait associated items
        // as potential fallback candidates (#142006). To temporarily mask that issue, let's not
        // select at all if there are no early inherent candidates.
        if candidates.is_empty() {
            return Ok(None);
        }

        let (applicable_candidates, fulfillment_errors) =
            self.select_inherent_assoc_candidates(span, self_ty, candidates.clone());

        // FIXME(#142006): Don't eagerly error here, there might be applicable trait candidates.
        let InherentAssocCandidate { impl_, assoc_item, scope: def_scope } =
            match &applicable_candidates[..] {
                &[] => Err(self.report_unresolved_inherent_assoc_item(
                    name,
                    self_ty,
                    candidates,
                    fulfillment_errors,
                    span,
                    assoc_tag,
                )),

                &[applicable_candidate] => Ok(applicable_candidate),

                &[_, ..] => Err(self.report_ambiguous_inherent_assoc_item(
                    name,
                    candidates.into_iter().map(|cand| cand.assoc_item).collect(),
                    span,
                )),
            }?;

        // FIXME(#142006): Don't eagerly validate here, there might be trait candidates that are
        // accessible (visible and stable) contrary to the inherent candidate.
        self.check_assoc_item(assoc_item, name, def_scope, block, span);

        // FIXME(fmease): Currently creating throwaway `parent_args` to please
        // `lower_generic_args_of_assoc_item`. Modify the latter instead (or sth. similar) to
        // not require the parent args logic.
        let parent_args = ty::GenericArgs::identity_for_item(tcx, impl_);
        let args = self.lower_generic_args_of_assoc_item(span, assoc_item, segment, parent_args);
        let args = tcx.mk_args_from_iter(
            std::iter::once(ty::GenericArg::from(self_ty))
                .chain(args.into_iter().skip(parent_args.len())),
        );

        Ok(Some((assoc_item, args)))
    }

    /// Given name and kind search for the assoc item in the provided scope and check if it's accessible[^1].
    ///
    /// [^1]: I.e., accessible in the provided scope wrt. visibility and stability.
    fn probe_assoc_item(
        &self,
        ident: Ident,
        assoc_tag: ty::AssocTag,
        block: HirId,
        span: Span,
        scope: DefId,
    ) -> Option<ty::AssocItem> {
        let (item, scope) = self.probe_assoc_item_unchecked(ident, assoc_tag, block, scope)?;
        self.check_assoc_item(item.def_id, ident, scope, block, span);
        Some(item)
    }

    /// Given name and kind search for the assoc item in the provided scope
    /// *without* checking if it's accessible[^1].
    ///
    /// [^1]: I.e., accessible in the provided scope wrt. visibility and stability.
    fn probe_assoc_item_unchecked(
        &self,
        ident: Ident,
        assoc_tag: ty::AssocTag,
        block: HirId,
        scope: DefId,
    ) -> Option<(ty::AssocItem, /*scope*/ DefId)> {
        let tcx = self.tcx();

        let (ident, def_scope) = tcx.adjust_ident_and_get_scope(ident, scope, block);
        // We have already adjusted the item name above, so compare with `.normalize_to_macros_2_0()`
        // instead of calling `filter_by_name_and_kind` which would needlessly normalize the
        // `ident` again and again.
        let item = tcx
            .associated_items(scope)
            .filter_by_name_unhygienic(ident.name)
            .find(|i| i.tag() == assoc_tag && i.ident(tcx).normalize_to_macros_2_0() == ident)?;

        Some((*item, def_scope))
    }

    /// Check if the given assoc item is accessible in the provided scope wrt. visibility and stability.
    fn check_assoc_item(
        &self,
        item_def_id: DefId,
        ident: Ident,
        scope: DefId,
        block: HirId,
        span: Span,
    ) {
        let tcx = self.tcx();

        if !tcx.visibility(item_def_id).is_accessible_from(scope, tcx) {
            self.dcx().emit_err(crate::errors::AssocItemIsPrivate {
                span,
                kind: tcx.def_descr(item_def_id),
                name: ident,
                defined_here_label: tcx.def_span(item_def_id),
            });
        }

        tcx.check_stability(item_def_id, Some(block), span, None);
    }

    fn probe_traits_that_match_assoc_ty(
        &self,
        qself_ty: Ty<'tcx>,
        assoc_ident: Ident,
    ) -> Vec<String> {
        let tcx = self.tcx();

        // In contexts that have no inference context, just make a new one.
        // We do need a local variable to store it, though.
        let infcx_;
        let infcx = if let Some(infcx) = self.infcx() {
            infcx
        } else {
            assert!(!qself_ty.has_infer());
            infcx_ = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
            &infcx_
        };

        tcx.all_traits_including_private()
            .filter(|trait_def_id| {
                // Consider only traits with the associated type
                tcx.associated_items(*trait_def_id)
                        .in_definition_order()
                        .any(|i| {
                            i.is_type()
                                && !i.is_impl_trait_in_trait()
                                && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident
                        })
                    // Consider only accessible traits
                    && tcx.visibility(*trait_def_id)
                        .is_accessible_from(self.item_def_id(), tcx)
                    && tcx.all_impls(*trait_def_id)
                        .any(|impl_def_id| {
                            let header = tcx.impl_trait_header(impl_def_id);
                            let trait_ref = header.trait_ref.instantiate(
                                tcx,
                                infcx.fresh_args_for_item(DUMMY_SP, impl_def_id),
                            );

                            let value = fold_regions(tcx, qself_ty, |_, _| tcx.lifetimes.re_erased);
                            // FIXME: Don't bother dealing with non-lifetime binders here...
                            if value.has_escaping_bound_vars() {
                                return false;
                            }
                            infcx
                                .can_eq(
                                    ty::ParamEnv::empty(),
                                    trait_ref.self_ty(),
                                    value,
                                ) && header.polarity != ty::ImplPolarity::Negative
                        })
            })
            .map(|trait_def_id| tcx.def_path_str(trait_def_id))
            .collect()
    }

    /// Lower a [resolved][hir::QPath::Resolved] associated type path to a projection.
    #[instrument(level = "debug", skip_all)]
    fn lower_resolved_assoc_ty_path(
        &self,
        span: Span,
        opt_self_ty: Option<Ty<'tcx>>,
        item_def_id: DefId,
        trait_segment: Option<&hir::PathSegment<'tcx>>,
        item_segment: &hir::PathSegment<'tcx>,
    ) -> Ty<'tcx> {
        match self.lower_resolved_assoc_item_path(
            span,
            opt_self_ty,
            item_def_id,
            trait_segment,
            item_segment,
            ty::AssocTag::Type,
        ) {
            Ok((item_def_id, item_args)) => {
                Ty::new_projection_from_args(self.tcx(), item_def_id, item_args)
            }
            Err(guar) => Ty::new_error(self.tcx(), guar),
        }
    }

    /// Lower a [resolved][hir::QPath::Resolved] associated const path to a (type-level) constant.
    #[instrument(level = "debug", skip_all)]
    fn lower_resolved_assoc_const_path(
        &self,
        span: Span,
        opt_self_ty: Option<Ty<'tcx>>,
        item_def_id: DefId,
        trait_segment: Option<&hir::PathSegment<'tcx>>,
        item_segment: &hir::PathSegment<'tcx>,
    ) -> Result<Const<'tcx>, ErrorGuaranteed> {
        let (item_def_id, item_args) = self.lower_resolved_assoc_item_path(
            span,
            opt_self_ty,
            item_def_id,
            trait_segment,
            item_segment,
            ty::AssocTag::Const,
        )?;
        self.require_type_const_attribute(item_def_id, span)?;
        let uv = ty::UnevaluatedConst::new(item_def_id, item_args);
        Ok(Const::new_unevaluated(self.tcx(), uv))
    }

    /// Lower a [resolved][hir::QPath::Resolved] (type-level) associated item path.
    #[instrument(level = "debug", skip_all)]
    fn lower_resolved_assoc_item_path(
        &self,
        span: Span,
        opt_self_ty: Option<Ty<'tcx>>,
        item_def_id: DefId,
        trait_segment: Option<&hir::PathSegment<'tcx>>,
        item_segment: &hir::PathSegment<'tcx>,
        assoc_tag: ty::AssocTag,
    ) -> Result<(DefId, GenericArgsRef<'tcx>), ErrorGuaranteed> {
        let tcx = self.tcx();

        let trait_def_id = tcx.parent(item_def_id);
        debug!(?trait_def_id);

        let Some(self_ty) = opt_self_ty else {
            return Err(self.report_missing_self_ty_for_resolved_path(
                trait_def_id,
                span,
                item_segment,
                assoc_tag,
            ));
        };
        debug!(?self_ty);

        let trait_ref =
            self.lower_mono_trait_ref(span, trait_def_id, self_ty, trait_segment.unwrap(), false);
        debug!(?trait_ref);

        let item_args =
            self.lower_generic_args_of_assoc_item(span, item_def_id, item_segment, trait_ref.args);

        Ok((item_def_id, item_args))
    }

    pub fn prohibit_generic_args<'a>(
        &self,
        segments: impl Iterator<Item = &'a hir::PathSegment<'a>> + Clone,
        err_extend: GenericsArgsErrExtend<'a>,
    ) -> Result<(), ErrorGuaranteed> {
        let args_visitors = segments.clone().flat_map(|segment| segment.args().args);
        let mut result = Ok(());
        if let Some(_) = args_visitors.clone().next() {
            result = Err(self.report_prohibited_generic_args(
                segments.clone(),
                args_visitors,
                err_extend,
            ));
        }

        for segment in segments {
            // Only emit the first error to avoid overloading the user with error messages.
            if let Some(c) = segment.args().constraints.first() {
                return Err(prohibit_assoc_item_constraint(self, c, None));
            }
        }

        result
    }

    /// Probe path segments that are semantically allowed to have generic arguments.
    ///
    /// ### Example
    ///
    /// ```ignore (illustrative)
    ///    Option::None::<()>
    /// //         ^^^^ permitted to have generic args
    ///
    /// // ==> [GenericPathSegment(Option_def_id, 1)]
    ///
    ///    Option::<()>::None
    /// // ^^^^^^        ^^^^ *not* permitted to have generic args
    /// // permitted to have generic args
    ///
    /// // ==> [GenericPathSegment(Option_def_id, 0)]
    /// ```
    // FIXME(eddyb, varkor) handle type paths here too, not just value ones.
    pub fn probe_generic_path_segments(
        &self,
        segments: &[hir::PathSegment<'_>],
        self_ty: Option<Ty<'tcx>>,
        kind: DefKind,
        def_id: DefId,
        span: Span,
    ) -> Vec<GenericPathSegment> {
        // We need to extract the generic arguments supplied by the user in
        // the path `path`. Due to the current setup, this is a bit of a
        // tricky process; the problem is that resolve only tells us the
        // end-point of the path resolution, and not the intermediate steps.
        // Luckily, we can (at least for now) deduce the intermediate steps
        // just from the end-point.
        //
        // There are basically five cases to consider:
        //
        // 1. Reference to a constructor of a struct:
        //
        //        struct Foo<T>(...)
        //
        //    In this case, the generic arguments are declared in the type space.
        //
        // 2. Reference to a constructor of an enum variant:
        //
        //        enum E<T> { Foo(...) }
        //
        //    In this case, the generic arguments are defined in the type space,
        //    but may be specified either on the type or the variant.
        //
        // 3. Reference to a free function or constant:
        //
        //        fn foo<T>() {}
        //
        //    In this case, the path will again always have the form
        //    `a::b::foo::<T>` where only the final segment should have generic
        //    arguments. However, in this case, those arguments are declared on
        //    a value, and hence are in the value space.
        //
        // 4. Reference to an associated function or constant:
        //
        //        impl<A> SomeStruct<A> {
        //            fn foo<B>(...) {}
        //        }
        //
        //    Here we can have a path like `a::b::SomeStruct::<A>::foo::<B>`,
        //    in which case generic arguments may appear in two places. The
        //    penultimate segment, `SomeStruct::<A>`, contains generic arguments
        //    in the type space, and the final segment, `foo::<B>` contains
        //    generic arguments in value space.
        //
        // The first step then is to categorize the segments appropriately.

        let tcx = self.tcx();

        assert!(!segments.is_empty());
        let last = segments.len() - 1;

        let mut generic_segments = vec![];

        match kind {
            // Case 1. Reference to a struct constructor.
            DefKind::Ctor(CtorOf::Struct, ..) => {
                // Everything but the final segment should have no
                // parameters at all.
                let generics = tcx.generics_of(def_id);
                // Variant and struct constructors use the
                // generics of their parent type definition.
                let generics_def_id = generics.parent.unwrap_or(def_id);
                generic_segments.push(GenericPathSegment(generics_def_id, last));
            }

            // Case 2. Reference to a variant constructor.
            DefKind::Ctor(CtorOf::Variant, ..) | DefKind::Variant => {
                let (generics_def_id, index) = if let Some(self_ty) = self_ty {
                    let adt_def = self.probe_adt(span, self_ty).unwrap();
                    debug_assert!(adt_def.is_enum());
                    (adt_def.did(), last)
                } else if last >= 1 && segments[last - 1].args.is_some() {
                    // Everything but the penultimate segment should have no
                    // parameters at all.
                    let mut def_id = def_id;

                    // `DefKind::Ctor` -> `DefKind::Variant`
                    if let DefKind::Ctor(..) = kind {
                        def_id = tcx.parent(def_id);
                    }

                    // `DefKind::Variant` -> `DefKind::Enum`
                    let enum_def_id = tcx.parent(def_id);
                    (enum_def_id, last - 1)
                } else {
                    // FIXME: lint here recommending `Enum::<...>::Variant` form
                    // instead of `Enum::Variant::<...>` form.

                    // Everything but the final segment should have no
                    // parameters at all.
                    let generics = tcx.generics_of(def_id);
                    // Variant and struct constructors use the
                    // generics of their parent type definition.
                    (generics.parent.unwrap_or(def_id), last)
                };
                generic_segments.push(GenericPathSegment(generics_def_id, index));
            }

            // Case 3. Reference to a top-level value.
            DefKind::Fn | DefKind::Const { .. } | DefKind::ConstParam | DefKind::Static { .. } => {
                generic_segments.push(GenericPathSegment(def_id, last));
            }

            // Case 4. Reference to a method or associated const.
            DefKind::AssocFn | DefKind::AssocConst { .. } => {
                if segments.len() >= 2 {
                    let generics = tcx.generics_of(def_id);
                    generic_segments.push(GenericPathSegment(generics.parent.unwrap(), last - 1));
                }
                generic_segments.push(GenericPathSegment(def_id, last));
            }

            kind => bug!("unexpected definition kind {:?} for {:?}", kind, def_id),
        }

        debug!(?generic_segments);

        generic_segments
    }

    /// Lower a [resolved][hir::QPath::Resolved] path to a type.
    #[instrument(level = "debug", skip_all)]
    pub fn lower_resolved_ty_path(
        &self,
        opt_self_ty: Option<Ty<'tcx>>,
        path: &hir::Path<'tcx>,
        hir_id: HirId,
        permit_variants: PermitVariants,
    ) -> Ty<'tcx> {
        debug!(?path.res, ?opt_self_ty, ?path.segments);
        let tcx = self.tcx();

        let span = path.span;
        match path.res {
            Res::Def(DefKind::OpaqueTy, did) => {
                // Check for desugared `impl Trait`.
                assert_matches!(tcx.opaque_ty_origin(did), hir::OpaqueTyOrigin::TyAlias { .. });
                let [leading_segments @ .., segment] = path.segments else { bug!() };
                let _ = self.prohibit_generic_args(
                    leading_segments.iter(),
                    GenericsArgsErrExtend::OpaqueTy,
                );
                let args = self.lower_generic_args_of_path_segment(span, did, segment);
                Ty::new_opaque(tcx, did, args)
            }
            Res::Def(
                DefKind::Enum
                | DefKind::TyAlias
                | DefKind::Struct
                | DefKind::Union
                | DefKind::ForeignTy,
                did,
            ) => {
                assert_eq!(opt_self_ty, None);
                let [leading_segments @ .., segment] = path.segments else { bug!() };
                let _ = self
                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
                self.lower_path_segment(span, did, segment)
            }
            Res::Def(kind @ DefKind::Variant, def_id)
                if let PermitVariants::Yes = permit_variants =>
            {
                // Lower "variant type" as if it were a real type.
                // The resulting `Ty` is type of the variant's enum for now.
                assert_eq!(opt_self_ty, None);

                let generic_segments =
                    self.probe_generic_path_segments(path.segments, None, kind, def_id, span);
                let indices: FxHashSet<_> =
                    generic_segments.iter().map(|GenericPathSegment(_, index)| index).collect();
                let _ = self.prohibit_generic_args(
                    path.segments.iter().enumerate().filter_map(|(index, seg)| {
                        if !indices.contains(&index) { Some(seg) } else { None }
                    }),
                    GenericsArgsErrExtend::DefVariant(&path.segments),
                );

                let &GenericPathSegment(def_id, index) = generic_segments.last().unwrap();
                self.lower_path_segment(span, def_id, &path.segments[index])
            }
            Res::Def(DefKind::TyParam, def_id) => {
                assert_eq!(opt_self_ty, None);
                let _ = self.prohibit_generic_args(
                    path.segments.iter(),
                    GenericsArgsErrExtend::Param(def_id),
                );
                self.lower_ty_param(hir_id)
            }
            Res::SelfTyParam { .. } => {
                // `Self` in trait or type alias.
                assert_eq!(opt_self_ty, None);
                let _ = self.prohibit_generic_args(
                    path.segments.iter(),
                    if let [hir::PathSegment { args: Some(args), ident, .. }] = &path.segments {
                        GenericsArgsErrExtend::SelfTyParam(
                            ident.span.shrink_to_hi().to(args.span_ext),
                        )
                    } else {
                        GenericsArgsErrExtend::None
                    },
                );
                self.check_param_uses_if_mcg(tcx.types.self_param, span, false)
            }
            Res::SelfTyAlias { alias_to: def_id, .. } => {
                // `Self` in impl (we know the concrete type).
                assert_eq!(opt_self_ty, None);
                // Try to evaluate any array length constants.
                let ty = tcx.at(span).type_of(def_id).instantiate_identity();
                let _ = self.prohibit_generic_args(
                    path.segments.iter(),
                    GenericsArgsErrExtend::SelfTyAlias { def_id, span },
                );
                self.check_param_uses_if_mcg(ty, span, true)
            }
            Res::Def(DefKind::AssocTy, def_id) => {
                let trait_segment = if let [modules @ .., trait_, _item] = path.segments {
                    let _ = self.prohibit_generic_args(modules.iter(), GenericsArgsErrExtend::None);
                    Some(trait_)
                } else {
                    None
                };
                self.lower_resolved_assoc_ty_path(
                    span,
                    opt_self_ty,
                    def_id,
                    trait_segment,
                    path.segments.last().unwrap(),
                )
            }
            Res::PrimTy(prim_ty) => {
                assert_eq!(opt_self_ty, None);
                let _ = self.prohibit_generic_args(
                    path.segments.iter(),
                    GenericsArgsErrExtend::PrimTy(prim_ty),
                );
                match prim_ty {
                    hir::PrimTy::Bool => tcx.types.bool,
                    hir::PrimTy::Char => tcx.types.char,
                    hir::PrimTy::Int(it) => Ty::new_int(tcx, it),
                    hir::PrimTy::Uint(uit) => Ty::new_uint(tcx, uit),
                    hir::PrimTy::Float(ft) => Ty::new_float(tcx, ft),
                    hir::PrimTy::Str => tcx.types.str_,
                }
            }
            Res::Err => {
                let e = self
                    .tcx()
                    .dcx()
                    .span_delayed_bug(path.span, "path with `Res::Err` but no error emitted");
                Ty::new_error(tcx, e)
            }
            Res::Def(..) => {
                assert_eq!(
                    path.segments.get(0).map(|seg| seg.ident.name),
                    Some(kw::SelfUpper),
                    "only expected incorrect resolution for `Self`"
                );
                Ty::new_error(
                    self.tcx(),
                    self.dcx().span_delayed_bug(span, "incorrect resolution for `Self`"),
                )
            }
            _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
        }
    }

    /// Lower a type parameter from the HIR to our internal notion of a type.
    ///
    /// Early-bound type parameters get lowered to [`ty::Param`]
    /// and late-bound ones to [`ty::Bound`].
    pub(crate) fn lower_ty_param(&self, hir_id: HirId) -> Ty<'tcx> {
        let tcx = self.tcx();

        let ty = match tcx.named_bound_var(hir_id) {
            Some(rbv::ResolvedArg::LateBound(debruijn, index, def_id)) => {
                let br = ty::BoundTy {
                    var: ty::BoundVar::from_u32(index),
                    kind: ty::BoundTyKind::Param(def_id.to_def_id()),
                };
                Ty::new_bound(tcx, debruijn, br)
            }
            Some(rbv::ResolvedArg::EarlyBound(def_id)) => {
                let item_def_id = tcx.hir_ty_param_owner(def_id);
                let generics = tcx.generics_of(item_def_id);
                let index = generics.param_def_id_to_index[&def_id.to_def_id()];
                Ty::new_param(tcx, index, tcx.hir_ty_param_name(def_id))
            }
            Some(rbv::ResolvedArg::Error(guar)) => Ty::new_error(tcx, guar),
            arg => bug!("unexpected bound var resolution for {hir_id:?}: {arg:?}"),
        };
        self.check_param_uses_if_mcg(ty, tcx.hir_span(hir_id), false)
    }

    /// Lower a const parameter from the HIR to our internal notion of a constant.
    ///
    /// Early-bound const parameters get lowered to [`ty::ConstKind::Param`]
    /// and late-bound ones to [`ty::ConstKind::Bound`].
    pub(crate) fn lower_const_param(&self, param_def_id: DefId, path_hir_id: HirId) -> Const<'tcx> {
        let tcx = self.tcx();

        let ct = match tcx.named_bound_var(path_hir_id) {
            Some(rbv::ResolvedArg::EarlyBound(_)) => {
                // Find the name and index of the const parameter by indexing the generics of
                // the parent item and construct a `ParamConst`.
                let item_def_id = tcx.parent(param_def_id);
                let generics = tcx.generics_of(item_def_id);
                let index = generics.param_def_id_to_index[&param_def_id];
                let name = tcx.item_name(param_def_id);
                ty::Const::new_param(tcx, ty::ParamConst::new(index, name))
            }
            Some(rbv::ResolvedArg::LateBound(debruijn, index, _)) => ty::Const::new_bound(
                tcx,
                debruijn,
                ty::BoundConst::new(ty::BoundVar::from_u32(index)),
            ),
            Some(rbv::ResolvedArg::Error(guar)) => ty::Const::new_error(tcx, guar),
            arg => bug!("unexpected bound var resolution for {:?}: {arg:?}", path_hir_id),
        };
        self.check_param_uses_if_mcg(ct, tcx.hir_span(path_hir_id), false)
    }

    /// Lower a [`hir::ConstArg`] to a (type-level) [`ty::Const`](Const).
    #[instrument(skip(self), level = "debug")]
    pub fn lower_const_arg(&self, const_arg: &hir::ConstArg<'tcx>, ty: Ty<'tcx>) -> Const<'tcx> {
        let tcx = self.tcx();

        if let hir::ConstArgKind::Anon(anon) = &const_arg.kind {
            // FIXME(generic_const_parameter_types): Ideally we remove these errors below when
            // we have the ability to intermix typeck of anon const const args with the parent
            // bodies typeck.

            // We also error if the type contains any regions as effectively any region will wind
            // up as a region variable in mir borrowck. It would also be somewhat concerning if
            // hir typeck was using equality but mir borrowck wound up using subtyping as that could
            // result in a non-infer in hir typeck but a region variable in borrowck.
            if tcx.features().generic_const_parameter_types()
                && (ty.has_free_regions() || ty.has_erased_regions())
            {
                let e = self.dcx().span_err(
                    const_arg.span,
                    "anonymous constants with lifetimes in their type are not yet supported",
                );
                tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
                return ty::Const::new_error(tcx, e);
            }
            // We must error if the instantiated type has any inference variables as we will
            // use this type to feed the `type_of` and query results must not contain inference
            // variables otherwise we will ICE.
            if ty.has_non_region_infer() {
                let e = self.dcx().span_err(
                    const_arg.span,
                    "anonymous constants with inferred types are not yet supported",
                );
                tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
                return ty::Const::new_error(tcx, e);
            }
            // We error when the type contains unsubstituted generics since we do not currently
            // give the anon const any of the generics from the parent.
            if ty.has_non_region_param() {
                let e = self.dcx().span_err(
                    const_arg.span,
                    "anonymous constants referencing generics are not yet supported",
                );
                tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(Ty::new_error(tcx, e)));
                return ty::Const::new_error(tcx, e);
            }

            tcx.feed_anon_const_type(anon.def_id, ty::EarlyBinder::bind(ty));
        }

        let hir_id = const_arg.hir_id;
        match const_arg.kind {
            hir::ConstArgKind::Tup(exprs) => self.lower_const_arg_tup(exprs, ty, const_arg.span),
            hir::ConstArgKind::Path(hir::QPath::Resolved(maybe_qself, path)) => {
                debug!(?maybe_qself, ?path);
                let opt_self_ty = maybe_qself.as_ref().map(|qself| self.lower_ty(qself));
                self.lower_resolved_const_path(opt_self_ty, path, hir_id)
            }
            hir::ConstArgKind::Path(hir::QPath::TypeRelative(hir_self_ty, segment)) => {
                debug!(?hir_self_ty, ?segment);
                let self_ty = self.lower_ty(hir_self_ty);
                self.lower_type_relative_const_path(
                    self_ty,
                    hir_self_ty,
                    segment,
                    hir_id,
                    const_arg.span,
                )
                .unwrap_or_else(|guar| Const::new_error(tcx, guar))
            }
            hir::ConstArgKind::Struct(qpath, inits) => {
                self.lower_const_arg_struct(hir_id, qpath, inits, const_arg.span)
            }
            hir::ConstArgKind::TupleCall(qpath, args) => {
                self.lower_const_arg_tuple_call(hir_id, qpath, args, const_arg.span)
            }
            hir::ConstArgKind::Array(array_expr) => self.lower_const_arg_array(array_expr, ty),
            hir::ConstArgKind::Anon(anon) => self.lower_const_arg_anon(anon),
            hir::ConstArgKind::Infer(()) => self.ct_infer(None, const_arg.span),
            hir::ConstArgKind::Error(e) => ty::Const::new_error(tcx, e),
            hir::ConstArgKind::Literal { lit, negated } => {
                self.lower_const_arg_literal(&lit, negated, ty, const_arg.span)
            }
        }
    }

    fn lower_const_arg_array(
        &self,
        array_expr: &'tcx hir::ConstArgArrayExpr<'tcx>,
        ty: Ty<'tcx>,
    ) -> Const<'tcx> {
        let tcx = self.tcx();

        let elem_ty = match ty.kind() {
            ty::Array(elem_ty, _) => elem_ty,
            ty::Error(e) => return Const::new_error(tcx, *e),
            _ => {
                let e = tcx
                    .dcx()
                    .span_err(array_expr.span, format!("expected `{}`, found const array", ty));
                return Const::new_error(tcx, e);
            }
        };

        let elems = array_expr
            .elems
            .iter()
            .map(|elem| self.lower_const_arg(elem, *elem_ty))
            .collect::<Vec<_>>();

        let valtree = ty::ValTree::from_branches(tcx, elems);

        ty::Const::new_value(tcx, valtree, ty)
    }

    fn lower_const_arg_tuple_call(
        &self,
        hir_id: HirId,
        qpath: hir::QPath<'tcx>,
        args: &'tcx [&'tcx hir::ConstArg<'tcx>],
        span: Span,
    ) -> Const<'tcx> {
        let tcx = self.tcx();

        let non_adt_or_variant_res = || {
            let e = tcx.dcx().span_err(span, "tuple constructor with invalid base path");
            ty::Const::new_error(tcx, e)
        };

        let ctor_const = match qpath {
            hir::QPath::Resolved(maybe_qself, path) => {
                let opt_self_ty = maybe_qself.as_ref().map(|qself| self.lower_ty(qself));
                self.lower_resolved_const_path(opt_self_ty, path, hir_id)
            }
            hir::QPath::TypeRelative(hir_self_ty, segment) => {
                let self_ty = self.lower_ty(hir_self_ty);
                match self.lower_type_relative_const_path(
                    self_ty,
                    hir_self_ty,
                    segment,
                    hir_id,
                    span,
                ) {
                    Ok(c) => c,
                    Err(_) => return non_adt_or_variant_res(),
                }
            }
        };

        let Some(value) = ctor_const.try_to_value() else {
            return non_adt_or_variant_res();
        };

        let (adt_def, adt_args, variant_did) = match value.ty.kind() {
            ty::FnDef(def_id, fn_args)
                if let DefKind::Ctor(CtorOf::Variant, _) = tcx.def_kind(*def_id) =>
            {
                let parent_did = tcx.parent(*def_id);
                let enum_did = tcx.parent(parent_did);
                (tcx.adt_def(enum_did), fn_args, parent_did)
            }
            ty::FnDef(def_id, fn_args)
                if let DefKind::Ctor(CtorOf::Struct, _) = tcx.def_kind(*def_id) =>
            {
                let parent_did = tcx.parent(*def_id);
                (tcx.adt_def(parent_did), fn_args, parent_did)
            }
            _ => {
                let e = self.dcx().span_err(
                    span,
                    "complex const arguments must be placed inside of a `const` block",
                );
                return Const::new_error(tcx, e);
            }
        };

        let variant_def = adt_def.variant_with_id(variant_did);
        let variant_idx = adt_def.variant_index_with_id(variant_did).as_u32();

        if args.len() != variant_def.fields.len() {
            let e = tcx.dcx().span_err(
                span,
                format!(
                    "tuple constructor has {} arguments but {} were provided",
                    variant_def.fields.len(),
                    args.len()
                ),
            );
            return ty::Const::new_error(tcx, e);
        }

        let fields = variant_def
            .fields
            .iter()
            .zip(args)
            .map(|(field_def, arg)| {
                self.lower_const_arg(arg, tcx.type_of(field_def.did).instantiate(tcx, adt_args))
            })
            .collect::<Vec<_>>();

        let opt_discr_const = if adt_def.is_enum() {
            let valtree = ty::ValTree::from_scalar_int(tcx, variant_idx.into());
            Some(ty::Const::new_value(tcx, valtree, tcx.types.u32))
        } else {
            None
        };

        let valtree = ty::ValTree::from_branches(tcx, opt_discr_const.into_iter().chain(fields));
        let adt_ty = Ty::new_adt(tcx, adt_def, adt_args);
        ty::Const::new_value(tcx, valtree, adt_ty)
    }

    fn lower_const_arg_tup(
        &self,
        exprs: &'tcx [&'tcx hir::ConstArg<'tcx>],
        ty: Ty<'tcx>,
        span: Span,
    ) -> Const<'tcx> {
        let tcx = self.tcx();

        let tys = match ty.kind() {
            ty::Tuple(tys) => tys,
            ty::Error(e) => return Const::new_error(tcx, *e),
            _ => {
                let e = tcx.dcx().span_err(span, format!("expected `{}`, found const tuple", ty));
                return Const::new_error(tcx, e);
            }
        };

        let exprs = exprs
            .iter()
            .zip(tys.iter())
            .map(|(expr, ty)| self.lower_const_arg(expr, ty))
            .collect::<Vec<_>>();

        let valtree = ty::ValTree::from_branches(tcx, exprs);
        ty::Const::new_value(tcx, valtree, ty)
    }

    fn lower_const_arg_struct(
        &self,
        hir_id: HirId,
        qpath: hir::QPath<'tcx>,
        inits: &'tcx [&'tcx hir::ConstArgExprField<'tcx>],
        span: Span,
    ) -> Const<'tcx> {
        // FIXME(mgca): try to deduplicate this function with
        // the equivalent HIR typeck logic.
        let tcx = self.tcx();

        let non_adt_or_variant_res = || {
            let e = tcx.dcx().span_err(span, "struct expression with invalid base path");
            ty::Const::new_error(tcx, e)
        };

        let (ty, variant_did) = match qpath {
            hir::QPath::Resolved(maybe_qself, path) => {
                debug!(?maybe_qself, ?path);
                let opt_self_ty = maybe_qself.as_ref().map(|qself| self.lower_ty(qself));
                let ty =
                    self.lower_resolved_ty_path(opt_self_ty, path, hir_id, PermitVariants::Yes);
                let variant_did = match path.res {
                    Res::Def(DefKind::Variant | DefKind::Struct, did) => did,
                    _ => return non_adt_or_variant_res(),
                };

                (ty, variant_did)
            }
            hir::QPath::TypeRelative(hir_self_ty, segment) => {
                debug!(?hir_self_ty, ?segment);
                let self_ty = self.lower_ty(hir_self_ty);
                let opt_res = self.lower_type_relative_ty_path(
                    self_ty,
                    hir_self_ty,
                    segment,
                    hir_id,
                    span,
                    PermitVariants::Yes,
                );

                let (ty, _, res_def_id) = match opt_res {
                    Ok(r @ (_, DefKind::Variant | DefKind::Struct, _)) => r,
                    Ok(_) => return non_adt_or_variant_res(),
                    Err(e) => return ty::Const::new_error(tcx, e),
                };

                (ty, res_def_id)
            }
        };

        let ty::Adt(adt_def, adt_args) = ty.kind() else { unreachable!() };

        let variant_def = adt_def.variant_with_id(variant_did);
        let variant_idx = adt_def.variant_index_with_id(variant_did).as_u32();

        let fields = variant_def
            .fields
            .iter()
            .map(|field_def| {
                // FIXME(mgca): we aren't really handling privacy, stability,
                // or macro hygeniene but we should.
                let mut init_expr =
                    inits.iter().filter(|init_expr| init_expr.field.name == field_def.name);

                match init_expr.next() {
                    Some(expr) => {
                        if let Some(expr) = init_expr.next() {
                            let e = tcx.dcx().span_err(
                                expr.span,
                                format!(
                                    "struct expression with multiple initialisers for `{}`",
                                    field_def.name,
                                ),
                            );
                            return ty::Const::new_error(tcx, e);
                        }

                        self.lower_const_arg(
                            expr.expr,
                            tcx.type_of(field_def.did).instantiate(tcx, adt_args),
                        )
                    }
                    None => {
                        let e = tcx.dcx().span_err(
                            span,
                            format!(
                                "struct expression with missing field initialiser for `{}`",
                                field_def.name
                            ),
                        );
                        ty::Const::new_error(tcx, e)
                    }
                }
            })
            .collect::<Vec<_>>();

        let opt_discr_const = if adt_def.is_enum() {
            let valtree = ty::ValTree::from_scalar_int(tcx, variant_idx.into());
            Some(ty::Const::new_value(tcx, valtree, tcx.types.u32))
        } else {
            None
        };

        let valtree = ty::ValTree::from_branches(tcx, opt_discr_const.into_iter().chain(fields));
        ty::Const::new_value(tcx, valtree, ty)
    }

    /// Lower a [resolved][hir::QPath::Resolved] path to a (type-level) constant.
    fn lower_resolved_const_path(
        &self,
        opt_self_ty: Option<Ty<'tcx>>,
        path: &hir::Path<'tcx>,
        hir_id: HirId,
    ) -> Const<'tcx> {
        let tcx = self.tcx();
        let span = path.span;
        let ct = match path.res {
            Res::Def(DefKind::ConstParam, def_id) => {
                assert_eq!(opt_self_ty, None);
                let _ = self.prohibit_generic_args(
                    path.segments.iter(),
                    GenericsArgsErrExtend::Param(def_id),
                );
                self.lower_const_param(def_id, hir_id)
            }
            Res::Def(DefKind::Const { .. }, did) => {
                if let Err(guar) = self.require_type_const_attribute(did, span) {
                    return Const::new_error(self.tcx(), guar);
                }

                assert_eq!(opt_self_ty, None);
                let [leading_segments @ .., segment] = path.segments else { bug!() };
                let _ = self
                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
                let args = self.lower_generic_args_of_path_segment(span, did, segment);
                ty::Const::new_unevaluated(tcx, ty::UnevaluatedConst::new(did, args))
            }
            Res::Def(DefKind::Ctor(ctor_of, CtorKind::Const), did) => {
                assert_eq!(opt_self_ty, None);
                let [leading_segments @ .., segment] = path.segments else { bug!() };
                let _ = self
                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);

                let parent_did = tcx.parent(did);
                let generics_did = match ctor_of {
                    CtorOf::Variant => tcx.parent(parent_did),
                    CtorOf::Struct => parent_did,
                };
                let args = self.lower_generic_args_of_path_segment(span, generics_did, segment);

                self.construct_const_ctor_value(did, ctor_of, args)
            }
            Res::Def(DefKind::Ctor(_, CtorKind::Fn), did) => {
                assert_eq!(opt_self_ty, None);
                let [leading_segments @ .., segment] = path.segments else { bug!() };
                let _ = self
                    .prohibit_generic_args(leading_segments.iter(), GenericsArgsErrExtend::None);
                let parent_did = tcx.parent(did);
                let generics_did = if let DefKind::Ctor(CtorOf::Variant, _) = tcx.def_kind(did) {
                    tcx.parent(parent_did)
                } else {
                    parent_did
                };
                let args = self.lower_generic_args_of_path_segment(span, generics_did, segment);
                ty::Const::zero_sized(tcx, Ty::new_fn_def(tcx, did, args))
            }
            Res::Def(DefKind::AssocConst { .. }, did) => {
                let trait_segment = if let [modules @ .., trait_, _item] = path.segments {
                    let _ = self.prohibit_generic_args(modules.iter(), GenericsArgsErrExtend::None);
                    Some(trait_)
                } else {
                    None
                };
                self.lower_resolved_assoc_const_path(
                    span,
                    opt_self_ty,
                    did,
                    trait_segment,
                    path.segments.last().unwrap(),
                )
                .unwrap_or_else(|guar| Const::new_error(tcx, guar))
            }
            Res::Def(DefKind::Static { .. }, _) => {
                span_bug!(span, "use of bare `static` ConstArgKind::Path's not yet supported")
            }
            // FIXME(const_generics): create real const to allow fn items as const paths
            Res::Def(DefKind::Fn | DefKind::AssocFn, did) => {
                self.dcx().span_delayed_bug(span, "function items cannot be used as const args");
                let args = self.lower_generic_args_of_path_segment(
                    span,
                    did,
                    path.segments.last().unwrap(),
                );
                ty::Const::zero_sized(tcx, Ty::new_fn_def(tcx, did, args))
            }

            // Exhaustive match to be clear about what exactly we're considering to be
            // an invalid Res for a const path.
            res @ (Res::Def(
                DefKind::Mod
                | DefKind::Enum
                | DefKind::Variant
                | DefKind::Struct
                | DefKind::OpaqueTy
                | DefKind::TyAlias
                | DefKind::TraitAlias
                | DefKind::AssocTy
                | DefKind::Union
                | DefKind::Trait
                | DefKind::ForeignTy
                | DefKind::TyParam
                | DefKind::Macro(_)
                | DefKind::LifetimeParam
                | DefKind::Use
                | DefKind::ForeignMod
                | DefKind::AnonConst
                | DefKind::InlineConst
                | DefKind::Field
                | DefKind::Impl { .. }
                | DefKind::Closure
                | DefKind::ExternCrate
                | DefKind::GlobalAsm
                | DefKind::SyntheticCoroutineBody,
                _,
            )
            | Res::PrimTy(_)
            | Res::SelfTyParam { .. }
            | Res::SelfTyAlias { .. }
            | Res::SelfCtor(_)
            | Res::Local(_)
            | Res::ToolMod
            | Res::NonMacroAttr(_)
            | Res::Err) => Const::new_error_with_message(
                tcx,
                span,
                format!("invalid Res {res:?} for const path"),
            ),
        };
        self.check_param_uses_if_mcg(ct, span, false)
    }

    /// Literals are eagerly converted to a constant, everything else becomes `Unevaluated`.
    #[instrument(skip(self), level = "debug")]
    fn lower_const_arg_anon(&self, anon: &AnonConst) -> Const<'tcx> {
        let tcx = self.tcx();

        let expr = &tcx.hir_body(anon.body).value;
        debug!(?expr);

        // FIXME(generic_const_parameter_types): We should use the proper generic args
        // here. It's only used as a hint for literals so doesn't matter too much to use the right
        // generic arguments, just weaker type inference.
        let ty = tcx.type_of(anon.def_id).instantiate_identity();

        match self.try_lower_anon_const_lit(ty, expr) {
            Some(v) => v,
            None => ty::Const::new_unevaluated(
                tcx,
                ty::UnevaluatedConst {
                    def: anon.def_id.to_def_id(),
                    args: ty::GenericArgs::identity_for_item(tcx, anon.def_id.to_def_id()),
                },
            ),
        }
    }

    #[instrument(skip(self), level = "debug")]
    fn lower_const_arg_literal(
        &self,
        kind: &LitKind,
        neg: bool,
        ty: Ty<'tcx>,
        span: Span,
    ) -> Const<'tcx> {
        let tcx = self.tcx();
        if let LitKind::Err(guar) = *kind {
            return ty::Const::new_error(tcx, guar);
        }
        let input = LitToConstInput { lit: *kind, ty, neg };
        match tcx.at(span).lit_to_const(input) {
            Some(value) => ty::Const::new_value(tcx, value.valtree, value.ty),
            None => {
                let e = tcx.dcx().span_err(span, "type annotations needed for the literal");
                ty::Const::new_error(tcx, e)
            }
        }
    }

    #[instrument(skip(self), level = "debug")]
    fn try_lower_anon_const_lit(
        &self,
        ty: Ty<'tcx>,
        expr: &'tcx hir::Expr<'tcx>,
    ) -> Option<Const<'tcx>> {
        let tcx = self.tcx();

        // Unwrap a block, so that e.g. `{ 1 }` is recognised as a literal. This makes the
        // performance optimisation of directly lowering anon consts occur more often.
        let expr = match &expr.kind {
            hir::ExprKind::Block(block, _) if block.stmts.is_empty() && block.expr.is_some() => {
                block.expr.as_ref().unwrap()
            }
            _ => expr,
        };

        let lit_input = match expr.kind {
            hir::ExprKind::Lit(lit) => Some(LitToConstInput { lit: lit.node, ty, neg: false }),
            hir::ExprKind::Unary(hir::UnOp::Neg, expr) => match expr.kind {
                hir::ExprKind::Lit(lit) => Some(LitToConstInput { lit: lit.node, ty, neg: true }),
                _ => None,
            },
            _ => None,
        };

        lit_input.and_then(|l| {
            if const_lit_matches_ty(tcx, &l.lit, l.ty, l.neg) {
                tcx.at(expr.span)
                    .lit_to_const(l)
                    .map(|value| ty::Const::new_value(tcx, value.valtree, value.ty))
            } else {
                None
            }
        })
    }

    fn require_type_const_attribute(
        &self,
        def_id: DefId,
        span: Span,
    ) -> Result<(), ErrorGuaranteed> {
        let tcx = self.tcx();
        if tcx.is_type_const(def_id) {
            Ok(())
        } else {
            let mut err = self.dcx().struct_span_err(
                span,
                "use of `const` in the type system not defined as `type const`",
            );
            if def_id.is_local() {
                let name = tcx.def_path_str(def_id);
                err.span_suggestion(
                    tcx.def_span(def_id).shrink_to_lo(),
                    format!("add `type` before `const` for `{name}`"),
                    format!("type "),
                    Applicability::MaybeIncorrect,
                );
            } else {
                err.note("only consts marked defined as `type const` may be used in types");
            }
            Err(err.emit())
        }
    }

    fn lower_delegation_ty(&self, idx: hir::InferDelegationKind<'tcx>) -> Ty<'tcx> {
        let delegation_sig = self.tcx().inherit_sig_for_delegation_item(self.item_def_id());

        match idx {
            hir::InferDelegationKind::Input(idx) => delegation_sig[idx],
            hir::InferDelegationKind::Output { .. } => *delegation_sig.last().unwrap(),
        }
    }

    /// Lower a type from the HIR to our internal notion of a type.
    #[instrument(level = "debug", skip(self), ret)]
    pub fn lower_ty(&self, hir_ty: &hir::Ty<'tcx>) -> Ty<'tcx> {
        let tcx = self.tcx();

        let result_ty = match &hir_ty.kind {
            hir::TyKind::InferDelegation(_, idx) => self.lower_delegation_ty(*idx),
            hir::TyKind::Slice(ty) => Ty::new_slice(tcx, self.lower_ty(ty)),
            hir::TyKind::Ptr(mt) => Ty::new_ptr(tcx, self.lower_ty(mt.ty), mt.mutbl),
            hir::TyKind::Ref(region, mt) => {
                let r = self.lower_lifetime(region, RegionInferReason::Reference);
                debug!(?r);
                let t = self.lower_ty(mt.ty);
                Ty::new_ref(tcx, r, t, mt.mutbl)
            }
            hir::TyKind::Never => tcx.types.never,
            hir::TyKind::Tup(fields) => {
                Ty::new_tup_from_iter(tcx, fields.iter().map(|t| self.lower_ty(t)))
            }
            hir::TyKind::FnPtr(bf) => {
                check_c_variadic_abi(tcx, bf.decl, bf.abi, hir_ty.span);

                Ty::new_fn_ptr(
                    tcx,
                    self.lower_fn_ty(hir_ty.hir_id, bf.safety, bf.abi, bf.decl, None, Some(hir_ty)),
                )
            }
            hir::TyKind::UnsafeBinder(binder) => Ty::new_unsafe_binder(
                tcx,
                ty::Binder::bind_with_vars(
                    self.lower_ty(binder.inner_ty),
                    tcx.late_bound_vars(hir_ty.hir_id),
                ),
            ),
            hir::TyKind::TraitObject(bounds, tagged_ptr) => {
                let lifetime = tagged_ptr.pointer();
                let syntax = tagged_ptr.tag();
                self.lower_trait_object_ty(hir_ty.span, hir_ty.hir_id, bounds, lifetime, syntax)
            }
            // If we encounter a fully qualified path with RTN generics, then it must have
            // *not* gone through `lower_ty_maybe_return_type_notation`, and therefore
            // it's certainly in an illegal position.
            hir::TyKind::Path(hir::QPath::Resolved(_, path))
                if path.segments.last().and_then(|segment| segment.args).is_some_and(|args| {
                    matches!(args.parenthesized, hir::GenericArgsParentheses::ReturnTypeNotation)
                }) =>
            {
                let guar = self.dcx().emit_err(BadReturnTypeNotation { span: hir_ty.span });
                Ty::new_error(tcx, guar)
            }
            hir::TyKind::Path(hir::QPath::Resolved(maybe_qself, path)) => {
                debug!(?maybe_qself, ?path);
                let opt_self_ty = maybe_qself.as_ref().map(|qself| self.lower_ty(qself));
                self.lower_resolved_ty_path(opt_self_ty, path, hir_ty.hir_id, PermitVariants::No)
            }
            &hir::TyKind::OpaqueDef(opaque_ty) => {
                // If this is an RPITIT and we are using the new RPITIT lowering scheme, we
                // generate the def_id of an associated type for the trait and return as
                // type a projection.
                let in_trait = match opaque_ty.origin {
                    hir::OpaqueTyOrigin::FnReturn {
                        parent,
                        in_trait_or_impl: Some(hir::RpitContext::Trait),
                        ..
                    }
                    | hir::OpaqueTyOrigin::AsyncFn {
                        parent,
                        in_trait_or_impl: Some(hir::RpitContext::Trait),
                        ..
                    } => Some(parent),
                    hir::OpaqueTyOrigin::FnReturn {
                        in_trait_or_impl: None | Some(hir::RpitContext::TraitImpl),
                        ..
                    }
                    | hir::OpaqueTyOrigin::AsyncFn {
                        in_trait_or_impl: None | Some(hir::RpitContext::TraitImpl),
                        ..
                    }
                    | hir::OpaqueTyOrigin::TyAlias { .. } => None,
                };

                self.lower_opaque_ty(opaque_ty.def_id, in_trait)
            }
            hir::TyKind::TraitAscription(hir_bounds) => {
                // Impl trait in bindings lower as an infer var with additional
                // set of type bounds.
                let self_ty = self.ty_infer(None, hir_ty.span);
                let mut bounds = Vec::new();
                self.lower_bounds(
                    self_ty,
                    hir_bounds.iter(),
                    &mut bounds,
                    ty::List::empty(),
                    PredicateFilter::All,
                    OverlappingAsssocItemConstraints::Allowed,
                );
                self.add_implicit_sizedness_bounds(
                    &mut bounds,
                    self_ty,
                    hir_bounds,
                    ImpliedBoundsContext::AssociatedTypeOrImplTrait,
                    hir_ty.span,
                );
                self.register_trait_ascription_bounds(bounds, hir_ty.hir_id, hir_ty.span);
                self_ty
            }
            // If we encounter a type relative path with RTN generics, then it must have
            // *not* gone through `lower_ty_maybe_return_type_notation`, and therefore
            // it's certainly in an illegal position.
            hir::TyKind::Path(hir::QPath::TypeRelative(_, segment))
                if segment.args.is_some_and(|args| {
                    matches!(args.parenthesized, hir::GenericArgsParentheses::ReturnTypeNotation)
                }) =>
            {
                let guar = self.dcx().emit_err(BadReturnTypeNotation { span: hir_ty.span });
                Ty::new_error(tcx, guar)
            }
            hir::TyKind::Path(hir::QPath::TypeRelative(hir_self_ty, segment)) => {
                debug!(?hir_self_ty, ?segment);
                let self_ty = self.lower_ty(hir_self_ty);
                self.lower_type_relative_ty_path(
                    self_ty,
                    hir_self_ty,
                    segment,
                    hir_ty.hir_id,
                    hir_ty.span,
                    PermitVariants::No,
                )
                .map(|(ty, _, _)| ty)
                .unwrap_or_else(|guar| Ty::new_error(tcx, guar))
            }
            hir::TyKind::Array(ty, length) => {
                let length = self.lower_const_arg(length, tcx.types.usize);
                Ty::new_array_with_const_len(tcx, self.lower_ty(ty), length)
            }
            hir::TyKind::Infer(()) => {
                // Infer also appears as the type of arguments or return
                // values in an ExprKind::Closure, or as
                // the type of local variables. Both of these cases are
                // handled specially and will not descend into this routine.
                self.ty_infer(None, hir_ty.span)
            }
            hir::TyKind::Pat(ty, pat) => {
                let ty_span = ty.span;
                let ty = self.lower_ty(ty);
                let pat_ty = match self.lower_pat_ty_pat(ty, ty_span, pat) {
                    Ok(kind) => Ty::new_pat(tcx, ty, tcx.mk_pat(kind)),
                    Err(guar) => Ty::new_error(tcx, guar),
                };
                self.record_ty(pat.hir_id, ty, pat.span);
                pat_ty
            }
            hir::TyKind::FieldOf(ty, hir::TyFieldPath { variant, field }) => self.lower_field_of(
                self.lower_ty(ty),
                self.item_def_id(),
                ty.span,
                hir_ty.hir_id,
                *variant,
                *field,
            ),
            hir::TyKind::Err(guar) => Ty::new_error(tcx, *guar),
        };

        self.record_ty(hir_ty.hir_id, result_ty, hir_ty.span);
        result_ty
    }

    fn lower_pat_ty_pat(
        &self,
        ty: Ty<'tcx>,
        ty_span: Span,
        pat: &hir::TyPat<'tcx>,
    ) -> Result<ty::PatternKind<'tcx>, ErrorGuaranteed> {
        let tcx = self.tcx();
        match pat.kind {
            hir::TyPatKind::Range(start, end) => {
                match ty.kind() {
                    // Keep this list of types in sync with the list of types that
                    // the `RangePattern` trait is implemented for.
                    ty::Int(_) | ty::Uint(_) | ty::Char => {
                        let start = self.lower_const_arg(start, ty);
                        let end = self.lower_const_arg(end, ty);
                        Ok(ty::PatternKind::Range { start, end })
                    }
                    _ => Err(self
                        .dcx()
                        .span_delayed_bug(ty_span, "invalid base type for range pattern")),
                }
            }
            hir::TyPatKind::NotNull => Ok(ty::PatternKind::NotNull),
            hir::TyPatKind::Or(patterns) => {
                self.tcx()
                    .mk_patterns_from_iter(patterns.iter().map(|pat| {
                        self.lower_pat_ty_pat(ty, ty_span, pat).map(|pat| tcx.mk_pat(pat))
                    }))
                    .map(ty::PatternKind::Or)
            }
            hir::TyPatKind::Err(e) => Err(e),
        }
    }

    fn lower_field_of(
        &self,
        ty: Ty<'tcx>,
        item_def_id: LocalDefId,
        ty_span: Span,
        hir_id: HirId,
        variant: Option<Ident>,
        field: Ident,
    ) -> Ty<'tcx> {
        let dcx = self.dcx();
        let tcx = self.tcx();
        match ty.kind() {
            ty::Adt(def, _) => {
                let base_did = def.did();
                let kind_name = tcx.def_descr(base_did);
                let (variant_idx, variant) = if def.is_enum() {
                    let Some(variant) = variant else {
                        let err = dcx
                            .create_err(NoVariantNamed { span: field.span, ident: field, ty })
                            .with_span_help(
                                field.span.shrink_to_lo(),
                                "you might be missing a variant here: `Variant.`",
                            )
                            .emit();
                        return Ty::new_error(tcx, err);
                    };

                    if let Some(res) = def
                        .variants()
                        .iter_enumerated()
                        .find(|(_, f)| f.ident(tcx).normalize_to_macros_2_0() == variant)
                    {
                        res
                    } else {
                        let err = dcx
                            .create_err(NoVariantNamed { span: variant.span, ident: variant, ty })
                            .emit();
                        return Ty::new_error(tcx, err);
                    }
                } else {
                    if let Some(variant) = variant {
                        let adt_path = tcx.def_path_str(base_did);
                        struct_span_code_err!(
                            dcx,
                            variant.span,
                            E0609,
                            "{kind_name} `{adt_path}` does not have any variants",
                        )
                        .with_span_label(variant.span, "variant unknown")
                        .emit();
                    }
                    (FIRST_VARIANT, def.non_enum_variant())
                };
                let block = tcx.local_def_id_to_hir_id(item_def_id);
                let (ident, def_scope) = tcx.adjust_ident_and_get_scope(field, def.did(), block);
                if let Some((field_idx, field)) = variant
                    .fields
                    .iter_enumerated()
                    .find(|(_, f)| f.ident(tcx).normalize_to_macros_2_0() == ident)
                {
                    if field.vis.is_accessible_from(def_scope, tcx) {
                        tcx.check_stability(field.did, Some(hir_id), ident.span, None);
                    } else {
                        let adt_path = tcx.def_path_str(base_did);
                        struct_span_code_err!(
                            dcx,
                            ident.span,
                            E0616,
                            "field `{ident}` of {kind_name} `{adt_path}` is private",
                        )
                        .with_span_label(ident.span, "private field")
                        .emit();
                    }
                    Ty::new_field_representing_type(tcx, ty, variant_idx, field_idx)
                } else {
                    let err =
                        dcx.create_err(NoFieldOnType { span: ident.span, field: ident, ty }).emit();
                    Ty::new_error(tcx, err)
                }
            }
            ty::Tuple(tys) => {
                let index = match field.as_str().parse::<usize>() {
                    Ok(idx) => idx,
                    Err(_) => {
                        let err =
                            dcx.create_err(NoFieldOnType { span: field.span, field, ty }).emit();
                        return Ty::new_error(tcx, err);
                    }
                };
                if field.name != sym::integer(index) {
                    bug!("we parsed above, but now not equal?");
                }
                if tys.get(index).is_some() {
                    Ty::new_field_representing_type(tcx, ty, FIRST_VARIANT, index.into())
                } else {
                    let err = dcx.create_err(NoFieldOnType { span: field.span, field, ty }).emit();
                    Ty::new_error(tcx, err)
                }
            }
            // FIXME(FRTs): support type aliases
            /*
            ty::Alias(AliasTyKind::Free, ty) => {
                return self.lower_field_of(
                    ty,
                    item_def_id,
                    ty_span,
                    hir_id,
                    variant,
                    field,
                );
            }*/
            ty::Alias(..) => Ty::new_error(
                tcx,
                dcx.span_err(ty_span, format!("could not resolve fields of `{ty}`")),
            ),
            ty::Error(err) => Ty::new_error(tcx, *err),
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Foreign(_)
            | ty::Str
            | ty::RawPtr(_, _)
            | ty::Ref(_, _, _)
            | ty::FnDef(_, _)
            | ty::FnPtr(_, _)
            | ty::UnsafeBinder(_)
            | ty::Dynamic(_, _)
            | ty::Closure(_, _)
            | ty::CoroutineClosure(_, _)
            | ty::Coroutine(_, _)
            | ty::CoroutineWitness(_, _)
            | ty::Never
            | ty::Param(_)
            | ty::Bound(_, _)
            | ty::Placeholder(_)
            | ty::Slice(..) => Ty::new_error(
                tcx,
                dcx.span_err(ty_span, format!("type `{ty}` doesn't have fields")),
            ),
            ty::Infer(_) => Ty::new_error(
                tcx,
                dcx.span_err(ty_span, format!("cannot use `{ty}` in this position")),
            ),
            // FIXME(FRTs): support these types?
            ty::Array(..) | ty::Pat(..) => Ty::new_error(
                tcx,
                dcx.span_err(ty_span, format!("type `{ty}` is not yet supported in `field_of!`")),
            ),
        }
    }

    /// Lower an opaque type (i.e., an existential impl-Trait type) from the HIR.
    #[instrument(level = "debug", skip(self), ret)]
    fn lower_opaque_ty(&self, def_id: LocalDefId, in_trait: Option<LocalDefId>) -> Ty<'tcx> {
        let tcx = self.tcx();

        let lifetimes = tcx.opaque_captured_lifetimes(def_id);
        debug!(?lifetimes);

        // If this is an RPITIT and we are using the new RPITIT lowering scheme,
        // do a linear search to map this to the synthetic associated type that
        // it will be lowered to.
        let def_id = if let Some(parent_def_id) = in_trait {
            *tcx.associated_types_for_impl_traits_in_associated_fn(parent_def_id.to_def_id())
                .iter()
                .find(|rpitit| match tcx.opt_rpitit_info(**rpitit) {
                    Some(ty::ImplTraitInTraitData::Trait { opaque_def_id, .. }) => {
                        opaque_def_id.expect_local() == def_id
                    }
                    _ => unreachable!(),
                })
                .unwrap()
        } else {
            def_id.to_def_id()
        };

        let generics = tcx.generics_of(def_id);
        debug!(?generics);

        // We use `generics.count() - lifetimes.len()` here instead of `generics.parent_count`
        // since return-position impl trait in trait squashes all of the generics from its source fn
        // into its own generics, so the opaque's "own" params isn't always just lifetimes.
        let offset = generics.count() - lifetimes.len();

        let args = ty::GenericArgs::for_item(tcx, def_id, |param, _| {
            if let Some(i) = (param.index as usize).checked_sub(offset) {
                let (lifetime, _) = lifetimes[i];
                // FIXME(mgca): should we be calling self.check_params_use_if_mcg here too?
                self.lower_resolved_lifetime(lifetime).into()
            } else {
                tcx.mk_param_from_def(param)
            }
        });
        debug!(?args);

        if in_trait.is_some() {
            Ty::new_projection_from_args(tcx, def_id, args)
        } else {
            Ty::new_opaque(tcx, def_id, args)
        }
    }

    /// Lower a function type from the HIR to our internal notion of a function signature.
    #[instrument(level = "debug", skip(self, hir_id, safety, abi, decl, generics, hir_ty), ret)]
    pub fn lower_fn_ty(
        &self,
        hir_id: HirId,
        safety: hir::Safety,
        abi: rustc_abi::ExternAbi,
        decl: &hir::FnDecl<'tcx>,
        generics: Option<&hir::Generics<'_>>,
        hir_ty: Option<&hir::Ty<'_>>,
    ) -> ty::PolyFnSig<'tcx> {
        let tcx = self.tcx();
        let bound_vars = tcx.late_bound_vars(hir_id);
        debug!(?bound_vars);

        let (input_tys, output_ty) = self.lower_fn_sig(decl, generics, hir_id, hir_ty);

        debug!(?output_ty);

        let fn_ty = tcx.mk_fn_sig(input_tys, output_ty, decl.c_variadic, safety, abi);
        let fn_ptr_ty = ty::Binder::bind_with_vars(fn_ty, bound_vars);

        if let hir::Node::Ty(hir::Ty { kind: hir::TyKind::FnPtr(fn_ptr_ty), span, .. }) =
            tcx.hir_node(hir_id)
        {
            check_abi(tcx, hir_id, *span, fn_ptr_ty.abi);
        }

        // reject function types that violate cmse ABI requirements
        cmse::validate_cmse_abi(self.tcx(), self.dcx(), hir_id, abi, fn_ptr_ty);

        if !fn_ptr_ty.references_error() {
            // Find any late-bound regions declared in return type that do
            // not appear in the arguments. These are not well-formed.
            //
            // Example:
            //     for<'a> fn() -> &'a str <-- 'a is bad
            //     for<'a> fn(&'a String) -> &'a str <-- 'a is ok
            let inputs = fn_ptr_ty.inputs();
            let late_bound_in_args =
                tcx.collect_constrained_late_bound_regions(inputs.map_bound(|i| i.to_owned()));
            let output = fn_ptr_ty.output();
            let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(output);

            self.validate_late_bound_regions(late_bound_in_args, late_bound_in_ret, |br_name| {
                struct_span_code_err!(
                    self.dcx(),
                    decl.output.span(),
                    E0581,
                    "return type references {}, which is not constrained by the fn input types",
                    br_name
                )
            });
        }

        fn_ptr_ty
    }

    /// Given a fn_hir_id for a impl function, suggest the type that is found on the
    /// corresponding function in the trait that the impl implements, if it exists.
    /// If arg_idx is Some, then it corresponds to an input type index, otherwise it
    /// corresponds to the return type.
    pub(super) fn suggest_trait_fn_ty_for_impl_fn_infer(
        &self,
        fn_hir_id: HirId,
        arg_idx: Option<usize>,
    ) -> Option<Ty<'tcx>> {
        let tcx = self.tcx();
        let hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), ident, .. }) =
            tcx.hir_node(fn_hir_id)
        else {
            return None;
        };
        let i = tcx.parent_hir_node(fn_hir_id).expect_item().expect_impl();

        let trait_ref = self.lower_impl_trait_ref(&i.of_trait?.trait_ref, self.lower_ty(i.self_ty));

        let assoc = tcx.associated_items(trait_ref.def_id).find_by_ident_and_kind(
            tcx,
            *ident,
            ty::AssocTag::Fn,
            trait_ref.def_id,
        )?;

        let fn_sig = tcx.fn_sig(assoc.def_id).instantiate(
            tcx,
            trait_ref.args.extend_to(tcx, assoc.def_id, |param, _| tcx.mk_param_from_def(param)),
        );
        let fn_sig = tcx.liberate_late_bound_regions(fn_hir_id.expect_owner().to_def_id(), fn_sig);

        Some(if let Some(arg_idx) = arg_idx {
            *fn_sig.inputs().get(arg_idx)?
        } else {
            fn_sig.output()
        })
    }

    #[instrument(level = "trace", skip(self, generate_err))]
    fn validate_late_bound_regions<'cx>(
        &'cx self,
        constrained_regions: FxIndexSet<ty::BoundRegionKind<'tcx>>,
        referenced_regions: FxIndexSet<ty::BoundRegionKind<'tcx>>,
        generate_err: impl Fn(&str) -> Diag<'cx>,
    ) {
        for br in referenced_regions.difference(&constrained_regions) {
            let br_name = if let Some(name) = br.get_name(self.tcx()) {
                format!("lifetime `{name}`")
            } else {
                "an anonymous lifetime".to_string()
            };

            let mut err = generate_err(&br_name);

            if !br.is_named(self.tcx()) {
                // The only way for an anonymous lifetime to wind up
                // in the return type but **also** be unconstrained is
                // if it only appears in "associated types" in the
                // input. See #47511 and #62200 for examples. In this case,
                // though we can easily give a hint that ought to be
                // relevant.
                err.note(
                    "lifetimes appearing in an associated or opaque type are not considered constrained",
                );
                err.note("consider introducing a named lifetime parameter");
            }

            err.emit();
        }
    }

    /// Given the bounds on an object, determines what single region bound (if any) we can
    /// use to summarize this type.
    ///
    /// The basic idea is that we will use the bound the user
    /// provided, if they provided one, and otherwise search the supertypes of trait bounds
    /// for region bounds. It may be that we can derive no bound at all, in which case
    /// we return `None`.
    #[instrument(level = "debug", skip(self, span), ret)]
    fn compute_object_lifetime_bound(
        &self,
        span: Span,
        existential_predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
    ) -> Option<ty::Region<'tcx>> // if None, use the default
    {
        let tcx = self.tcx();

        // No explicit region bound specified. Therefore, examine trait
        // bounds and see if we can derive region bounds from those.
        let derived_region_bounds = object_region_bounds(tcx, existential_predicates);

        // If there are no derived region bounds, then report back that we
        // can find no region bound. The caller will use the default.
        if derived_region_bounds.is_empty() {
            return None;
        }

        // If any of the derived region bounds are 'static, that is always
        // the best choice.
        if derived_region_bounds.iter().any(|r| r.is_static()) {
            return Some(tcx.lifetimes.re_static);
        }

        // Determine whether there is exactly one unique region in the set
        // of derived region bounds. If so, use that. Otherwise, report an
        // error.
        let r = derived_region_bounds[0];
        if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
            self.dcx().emit_err(AmbiguousLifetimeBound { span });
        }
        Some(r)
    }

    fn construct_const_ctor_value(
        &self,
        ctor_def_id: DefId,
        ctor_of: CtorOf,
        args: GenericArgsRef<'tcx>,
    ) -> Const<'tcx> {
        let tcx = self.tcx();
        let parent_did = tcx.parent(ctor_def_id);

        let adt_def = tcx.adt_def(match ctor_of {
            CtorOf::Variant => tcx.parent(parent_did),
            CtorOf::Struct => parent_did,
        });

        let variant_idx = adt_def.variant_index_with_id(parent_did);

        let valtree = if adt_def.is_enum() {
            let discr = ty::ValTree::from_scalar_int(tcx, variant_idx.as_u32().into());
            ty::ValTree::from_branches(tcx, [ty::Const::new_value(tcx, discr, tcx.types.u32)])
        } else {
            ty::ValTree::zst(tcx)
        };

        let adt_ty = Ty::new_adt(tcx, adt_def, args);
        ty::Const::new_value(tcx, valtree, adt_ty)
    }
}