rustc_hir_typeck/

coercion.rs

//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing. This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference. This is
//! useful for freezing mut things (that is, when the expected type is &T
//! but you have &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T). See
//! the various `tests/ui/coerce/*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When inferring the generic arguments of functions, the argument
//! order is relevant, which can lead to the following edge case:
//!
//! ```ignore (illustrative)
//! fn foo<T>(a: T, b: T) {
//!     // ...
//! }
//!
//! foo(&7i32, &mut 7i32);
//! // This compiles, as we first infer `T` to be `&i32`,
//! // and then coerce `&mut 7i32` to `&7i32`.
//!
//! foo(&mut 7i32, &7i32);
//! // This does not compile, as we first infer `T` to be `&mut i32`
//! // and are then unable to coerce `&7i32` to `&mut i32`.
//! ```

use std::ops::{ControlFlow, Deref};

use rustc_errors::codes::*;
use rustc_errors::{Applicability, Diag, struct_span_code_err};
use rustc_hir::attrs::InlineAttr;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_hir::{self as hir, LangItem};
use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
use rustc_infer::infer::relate::RelateResult;
use rustc_infer::infer::{DefineOpaqueTypes, InferOk, InferResult, RegionVariableOrigin};
use rustc_infer::traits::{
    MatchExpressionArmCause, Obligation, PredicateObligation, PredicateObligations, SelectionError,
};
use rustc_middle::span_bug;
use rustc_middle::ty::adjustment::{
    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, DerefAdjustKind,
    PointerCoercion,
};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
use rustc_span::{BytePos, DUMMY_SP, Span};
use rustc_trait_selection::infer::InferCtxtExt as _;
use rustc_trait_selection::solve::inspect::{self, InferCtxtProofTreeExt, ProofTreeVisitor};
use rustc_trait_selection::solve::{Certainty, Goal, NoSolution};
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::{
    self, ImplSource, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
};
use smallvec::{SmallVec, smallvec};
use tracing::{debug, instrument};

use crate::FnCtxt;
use crate::errors::SuggestBoxingForReturnImplTrait;

struct Coerce<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    cause: ObligationCause<'tcx>,
    use_lub: bool,
    /// Determines whether or not allow_two_phase_borrow is set on any
    /// autoref adjustments we create while coercing. We don't want to
    /// allow deref coercions to create two-phase borrows, at least initially,
    /// but we do need two-phase borrows for function argument reborrows.
    /// See #47489 and #48598
    /// See docs on the "AllowTwoPhase" type for a more detailed discussion
    allow_two_phase: AllowTwoPhase,
    /// Whether we allow `NeverToAny` coercions. This is unsound if we're
    /// coercing a place expression without it counting as a read in the MIR.
    /// This is a side-effect of HIR not really having a great distinction
    /// between places and values.
    coerce_never: bool,
}

impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
    type Target = FnCtxt<'a, 'tcx>;
    fn deref(&self) -> &Self::Target {
        self.fcx
    }
}

type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;

/// Coercing a mutable reference to an immutable works, while
/// coercing `&T` to `&mut T` should be forbidden.
fn coerce_mutbls<'tcx>(
    from_mutbl: hir::Mutability,
    to_mutbl: hir::Mutability,
) -> RelateResult<'tcx, ()> {
    if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
}

/// This always returns `Ok(...)`.
fn success<'tcx>(
    adj: Vec<Adjustment<'tcx>>,
    target: Ty<'tcx>,
    obligations: PredicateObligations<'tcx>,
) -> CoerceResult<'tcx> {
    Ok(InferOk { value: (adj, target), obligations })
}

/// Whether to force a leak check to occur in `Coerce::unify_raw`.
/// Note that leak checks may still occur evn with `ForceLeakCheck::No`.
///
/// FIXME: We may want to change type relations to always leak-check
/// after exiting a binder, at which point we will always do so and
/// no longer need to handle this explicitly
enum ForceLeakCheck {
    Yes,
    No,
}

impl<'f, 'tcx> Coerce<'f, 'tcx> {
    fn new(
        fcx: &'f FnCtxt<'f, 'tcx>,
        cause: ObligationCause<'tcx>,
        allow_two_phase: AllowTwoPhase,
        coerce_never: bool,
    ) -> Self {
        Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
    }

    fn unify_raw(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        leak_check: ForceLeakCheck,
    ) -> InferResult<'tcx, Ty<'tcx>> {
        debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
        self.commit_if_ok(|snapshot| {
            let outer_universe = self.infcx.universe();

            let at = self.at(&self.cause, self.fcx.param_env);

            let res = if self.use_lub {
                at.lub(b, a)
            } else {
                at.sup(DefineOpaqueTypes::Yes, b, a)
                    .map(|InferOk { value: (), obligations }| InferOk { value: b, obligations })
            };

            // In the new solver, lazy norm may allow us to shallowly equate
            // more types, but we emit possibly impossible-to-satisfy obligations.
            // Filter these cases out to make sure our coercion is more accurate.
            let res = match res {
                Ok(InferOk { value, obligations }) if self.next_trait_solver() => {
                    let ocx = ObligationCtxt::new(self);
                    ocx.register_obligations(obligations);
                    if ocx.try_evaluate_obligations().is_empty() {
                        Ok(InferOk { value, obligations: ocx.into_pending_obligations() })
                    } else {
                        Err(TypeError::Mismatch)
                    }
                }
                res => res,
            };

            // We leak check here mostly because lub operations are
            // kind of scuffed around binders. Instead of computing an actual
            // lub'd binder we instead:
            // - Equate the binders
            // - Return the lhs of the lub operation
            //
            // This may lead to incomplete type inference for the resulting type
            // of a `match` or `if .. else`, etc. This is a backwards compat
            // hazard for if/when we start handling `lub` more correctly.
            //
            // In order to actually ensure that equating the binders *does*
            // result in equal binders, and that the lhs is actually a supertype
            // of the rhs, we must perform a leak check here.
            if matches!(leak_check, ForceLeakCheck::Yes) {
                self.leak_check(outer_universe, Some(snapshot))?;
            }

            res
        })
    }

    /// Unify two types (using sub or lub).
    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>, leak_check: ForceLeakCheck) -> CoerceResult<'tcx> {
        self.unify_raw(a, b, leak_check)
            .and_then(|InferOk { value: ty, obligations }| success(vec![], ty, obligations))
    }

    /// Unify two types (using sub or lub) and produce a specific coercion.
    fn unify_and(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        adjustments: impl IntoIterator<Item = Adjustment<'tcx>>,
        final_adjustment: Adjust,
        leak_check: ForceLeakCheck,
    ) -> CoerceResult<'tcx> {
        self.unify_raw(a, b, leak_check).and_then(|InferOk { value: ty, obligations }| {
            success(
                adjustments
                    .into_iter()
                    .chain(std::iter::once(Adjustment { target: ty, kind: final_adjustment }))
                    .collect(),
                ty,
                obligations,
            )
        })
    }

    #[instrument(skip(self), ret)]
    fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        // First, remove any resolved type variables (at the top level, at least):
        let a = self.shallow_resolve(a);
        let b = self.shallow_resolve(b);
        debug!("Coerce.tys({:?} => {:?})", a, b);

        // Coercing from `!` to any type is allowed:
        if a.is_never() {
            if self.coerce_never {
                return success(
                    vec![Adjustment { kind: Adjust::NeverToAny, target: b }],
                    b,
                    PredicateObligations::new(),
                );
            } else {
                // Otherwise the only coercion we can do is unification.
                return self.unify(a, b, ForceLeakCheck::No);
            }
        }

        // Coercing *from* an unresolved inference variable means that
        // we have no information about the source type. This will always
        // ultimately fall back to some form of subtyping.
        if a.is_ty_var() {
            return self.coerce_from_inference_variable(a, b);
        }

        // Consider coercing the subtype to a DST
        //
        // NOTE: this is wrapped in a `commit_if_ok` because it creates
        // a "spurious" type variable, and we don't want to have that
        // type variable in memory if the coercion fails.
        let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
        match unsize {
            Ok(_) => {
                debug!("coerce: unsize successful");
                return unsize;
            }
            Err(error) => {
                debug!(?error, "coerce: unsize failed");
            }
        }

        // Examine the target type and consider type-specific coercions, such
        // as auto-borrowing, coercing pointer mutability, or pin-ergonomics.
        match *b.kind() {
            ty::RawPtr(_, b_mutbl) => {
                return self.coerce_to_raw_ptr(a, b, b_mutbl);
            }
            ty::Ref(r_b, _, mutbl_b) => {
                return self.coerce_to_ref(a, b, r_b, mutbl_b);
            }
            ty::Adt(pin, _)
                if self.tcx.features().pin_ergonomics()
                    && self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) =>
            {
                let pin_coerce = self.commit_if_ok(|_| self.coerce_to_pin_ref(a, b));
                if pin_coerce.is_ok() {
                    return pin_coerce;
                }
            }
            _ => {}
        }

        match *a.kind() {
            ty::FnDef(..) => {
                // Function items are coercible to any closure
                // type; function pointers are not (that would
                // require double indirection).
                // Additionally, we permit coercion of function
                // items to drop the unsafe qualifier.
                self.coerce_from_fn_item(a, b)
            }
            ty::FnPtr(a_sig_tys, a_hdr) => {
                // We permit coercion of fn pointers to drop the
                // unsafe qualifier.
                self.coerce_from_fn_pointer(a, a_sig_tys.with(a_hdr), b)
            }
            ty::Closure(..) => {
                // Non-capturing closures are coercible to
                // function pointers or unsafe function pointers.
                // It cannot convert closures that require unsafe.
                self.coerce_closure_to_fn(a, b)
            }
            _ => {
                // Otherwise, just use unification rules.
                self.unify(a, b, ForceLeakCheck::No)
            }
        }
    }

    /// Coercing *from* an inference variable. In this case, we have no information
    /// about the source type, so we can't really do a true coercion and we always
    /// fall back to subtyping (`unify_and`).
    fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
        debug_assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        if b.is_ty_var() {
            let mut obligations = PredicateObligations::with_capacity(2);
            let mut push_coerce_obligation = |a, b| {
                obligations.push(Obligation::new(
                    self.tcx(),
                    self.cause.clone(),
                    self.param_env,
                    ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate { a, b })),
                ));
            };

            let target_ty = if self.use_lub {
                // When computing the lub, we create a new target
                // and coerce both `a` and `b` to it.
                let target_ty = self.next_ty_var(self.cause.span);
                push_coerce_obligation(a, target_ty);
                push_coerce_obligation(b, target_ty);
                target_ty
            } else {
                // When subtyping, we don't need to create a new target
                // as we only coerce `a` to `b`.
                push_coerce_obligation(a, b);
                b
            };

            debug!(
                "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
                target_ty, obligations
            );
            success(vec![], target_ty, obligations)
        } else {
            // One unresolved type variable: just apply subtyping, we may be able
            // to do something useful.
            self.unify(a, b, ForceLeakCheck::No)
        }
    }

    /// Handles coercing some arbitrary type `a` to some reference (`b`). This
    /// handles a few cases:
    /// - Introducing reborrows to give more flexible lifetimes
    /// - Deref coercions to allow `&T` to coerce to `&T::Target`
    /// - Coercing mutable references to immutable references
    /// These coercions can be freely intermixed, for example we are able to
    /// coerce `&mut T` to `&mut T::Target`.
    fn coerce_to_ref(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        r_b: ty::Region<'tcx>,
        mutbl_b: hir::Mutability,
    ) -> CoerceResult<'tcx> {
        debug!("coerce_to_ref(a={:?}, b={:?})", a, b);
        debug_assert!(self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        let (r_a, mt_a) = match *a.kind() {
            ty::Ref(r_a, ty, mutbl) => {
                coerce_mutbls(mutbl, mutbl_b)?;
                (r_a, ty::TypeAndMut { ty, mutbl })
            }
            _ => return self.unify(a, b, ForceLeakCheck::No),
        };

        // Look at each step in the `Deref` chain and check if
        // any of the autoref'd `Target` types unify with the
        // coercion target.
        //
        // For example when coercing from `&mut Vec<T>` to `&M [T]` we
        // have three deref steps:
        // 1. `&mut Vec<T>`, skip autoref
        // 2. `Vec<T>`, autoref'd ty: `&M Vec<T>`
        //     - `&M Vec<T>` does not unify with `&M [T]`
        // 3. `[T]`, autoref'd ty: `&M [T]`
        //     - `&M [T]` does unify with `&M [T]`
        let mut first_error = None;
        let mut r_borrow_var = None;
        let mut autoderef = self.autoderef(self.cause.span, a);
        let found = autoderef.by_ref().find_map(|(deref_ty, autoderefs)| {
            if autoderefs == 0 {
                // Don't autoref the first step as otherwise we'd allow
                // coercing `&T` to `&&T`.
                return None;
            }

            // The logic here really shouldn't exist. We don't care about free
            // lifetimes during HIR typeck. Unfortunately later parts of this
            // function rely on structural identity of the autoref'd deref'd ty.
            //
            // This means that what region we use here actually impacts whether
            // we emit a reborrow coercion or not which can affect diagnostics
            // and capture analysis (which in turn affects borrowck).
            let r = if !self.use_lub {
                r_b
            } else if autoderefs == 1 {
                r_a
            } else {
                if r_borrow_var.is_none() {
                    // create var lazily, at most once
                    let coercion = RegionVariableOrigin::Coercion(self.cause.span);
                    let r = self.next_region_var(coercion);
                    r_borrow_var = Some(r);
                }
                r_borrow_var.unwrap()
            };

            let autorefd_deref_ty = Ty::new_ref(self.tcx, r, deref_ty, mutbl_b);

            // Note that we unify the autoref'd `Target` type with `b` rather than
            // the `Target` type with the pointee of `b`. This is necessary
            // to properly account for the differing variances of the pointees
            // of `&` vs `&mut` references.
            match self.unify_raw(autorefd_deref_ty, b, ForceLeakCheck::No) {
                Ok(ok) => Some(ok),
                Err(err) => {
                    if first_error.is_none() {
                        first_error = Some(err);
                    }
                    None
                }
            }
        });

        // Extract type or return an error. We return the first error
        // we got, which should be from relating the "base" type
        // (e.g., in example above, the failure from relating `Vec<T>`
        // to the target type), since that should be the least
        // confusing.
        let Some(InferOk { value: coerced_a, mut obligations }) = found else {
            if let Some(first_error) = first_error {
                debug!("coerce_to_ref: failed with err = {:?}", first_error);
                return Err(first_error);
            } else {
                // This may happen in the new trait solver since autoderef requires
                // the pointee to be structurally normalizable, or else it'll just bail.
                // So when we have a type like `&<not well formed>`, then we get no
                // autoderef steps (even though there should be at least one). That means
                // we get no type mismatches, since the loop above just exits early.
                return Err(TypeError::Mismatch);
            }
        };

        if coerced_a == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
            // As a special case, if we would produce `&'a *x`, that's
            // a total no-op. We end up with the type `&'a T` just as
            // we started with. In that case, just skip it altogether.
            //
            // Unfortunately, this can actually effect capture analysis
            // which in turn means this effects borrow checking. This can
            // also effect diagnostics.
            // FIXME(BoxyUwU): we should always emit reborrow coercions
            //
            // Note that for `&mut`, we DO want to reborrow --
            // otherwise, this would be a move, which might be an
            // error. For example `foo(self.x)` where `self` and
            // `self.x` both have `&mut `type would be a move of
            // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
            // which is a borrow.
            assert!(mutbl_b.is_not()); // can only coerce &T -> &U
            return success(vec![], coerced_a, obligations);
        }

        let InferOk { value: mut adjustments, obligations: o } =
            self.adjust_steps_as_infer_ok(&autoderef);
        obligations.extend(o);
        obligations.extend(autoderef.into_obligations());

        assert!(
            matches!(coerced_a.kind(), ty::Ref(..)),
            "expected a ref type, got {:?}",
            coerced_a
        );

        // Now apply the autoref
        let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
        adjustments
            .push(Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)), target: coerced_a });

        debug!("coerce_to_ref: succeeded coerced_a={:?} adjustments={:?}", coerced_a, adjustments);

        success(adjustments, coerced_a, obligations)
    }

    /// Performs [unsized coercion] by emulating a fulfillment loop on a
    /// `CoerceUnsized` goal until all `CoerceUnsized` and `Unsize` goals
    /// are successfully selected.
    ///
    /// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
    #[instrument(skip(self), level = "debug")]
    fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug!(?source, ?target);
        debug_assert!(self.shallow_resolve(source) == source);
        debug_assert!(self.shallow_resolve(target) == target);

        // We don't apply any coercions incase either the source or target
        // aren't sufficiently well known but tend to instead just equate
        // them both.
        if source.is_ty_var() {
            debug!("coerce_unsized: source is a TyVar, bailing out");
            return Err(TypeError::Mismatch);
        }
        if target.is_ty_var() {
            debug!("coerce_unsized: target is a TyVar, bailing out");
            return Err(TypeError::Mismatch);
        }

        // This is an optimization because coercion is one of the most common
        // operations that we do in typeck, since it happens at every assignment
        // and call arg (among other positions).
        //
        // These targets are known to never be RHS in `LHS: CoerceUnsized<RHS>`.
        // That's because these are built-in types for which a core-provided impl
        // doesn't exist, and for which a user-written impl is invalid.
        //
        // This is technically incomplete when users write impossible bounds like
        // `where T: CoerceUnsized<usize>`, for example, but that trait is unstable
        // and coercion is allowed to be incomplete. The only case where this matters
        // is impossible bounds.
        //
        // Note that some of these types implement `LHS: Unsize<RHS>`, but they
        // do not implement *`CoerceUnsized`* which is the root obligation of the
        // check below.
        match target.kind() {
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
            | ty::Str
            | ty::Array(_, _)
            | ty::Slice(_)
            | ty::FnDef(_, _)
            | ty::FnPtr(_, _)
            | ty::Dynamic(_, _)
            | ty::Closure(_, _)
            | ty::CoroutineClosure(_, _)
            | ty::Coroutine(_, _)
            | ty::CoroutineWitness(_, _)
            | ty::Never
            | ty::Tuple(_) => return Err(TypeError::Mismatch),
            _ => {}
        }
        // `&str: CoerceUnsized<&str>` does not hold but is encountered frequently
        // so we fast path bail out here
        if let ty::Ref(_, source_pointee, ty::Mutability::Not) = *source.kind()
            && source_pointee.is_str()
            && let ty::Ref(_, target_pointee, ty::Mutability::Not) = *target.kind()
            && target_pointee.is_str()
        {
            return Err(TypeError::Mismatch);
        }

        let traits =
            (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
        let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
            debug!("missing Unsize or CoerceUnsized traits");
            return Err(TypeError::Mismatch);
        };

        // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
        // a DST unless we have to. This currently comes out in the wash since
        // we can't unify [T] with U. But to properly support DST, we need to allow
        // that, at which point we will need extra checks on the target here.

        // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
        let reborrow = match (source.kind(), target.kind()) {
            (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
                coerce_mutbls(mutbl_a, mutbl_b)?;

                let coercion = RegionVariableOrigin::Coercion(self.cause.span);
                let r_borrow = self.next_region_var(coercion);

                // We don't allow two-phase borrows here, at least for initial
                // implementation. If it happens that this coercion is a function argument,
                // the reborrow in coerce_borrowed_ptr will pick it up.
                let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);

                Some((
                    Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: ty_a },
                    Adjustment {
                        kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
                        target: Ty::new_ref(self.tcx, r_borrow, ty_a, mutbl_b),
                    },
                ))
            }
            (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(_, mt_b)) => {
                coerce_mutbls(mt_a, mt_b)?;

                Some((
                    Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: ty_a },
                    Adjustment {
                        kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
                        target: Ty::new_ptr(self.tcx, ty_a, mt_b),
                    },
                ))
            }
            _ => None,
        };
        let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);

        // Setup either a subtyping or a LUB relationship between
        // the `CoerceUnsized` target type and the expected type.
        // We only have the latter, so we use an inference variable
        // for the former and let type inference do the rest.
        let coerce_target = self.next_ty_var(self.cause.span);

        let mut coercion = self.unify_and(
            coerce_target,
            target,
            reborrow.into_iter().flat_map(|(deref, autoref)| [deref, autoref]),
            Adjust::Pointer(PointerCoercion::Unsize),
            ForceLeakCheck::No,
        )?;

        // Create an obligation for `Source: CoerceUnsized<Target>`.
        let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
        let pred = ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target]);
        let obligation = Obligation::new(self.tcx, cause, self.fcx.param_env, pred);

        if self.next_trait_solver() {
            coercion.obligations.push(obligation);

            if self
                .infcx
                .visit_proof_tree(
                    Goal::new(self.tcx, self.param_env, pred),
                    &mut CoerceVisitor { fcx: self.fcx, span: self.cause.span },
                )
                .is_break()
            {
                return Err(TypeError::Mismatch);
            }
        } else {
            self.coerce_unsized_old_solver(
                obligation,
                &mut coercion,
                coerce_unsized_did,
                unsize_did,
            )?;
        }

        Ok(coercion)
    }

    fn coerce_unsized_old_solver(
        &self,
        obligation: Obligation<'tcx, ty::Predicate<'tcx>>,
        coercion: &mut InferOk<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>,
        coerce_unsized_did: DefId,
        unsize_did: DefId,
    ) -> Result<(), TypeError<'tcx>> {
        let mut selcx = traits::SelectionContext::new(self);
        // Use a FIFO queue for this custom fulfillment procedure.
        //
        // A Vec (or SmallVec) is not a natural choice for a queue. However,
        // this code path is hot, and this queue usually has a max length of 1
        // and almost never more than 3. By using a SmallVec we avoid an
        // allocation, at the (very small) cost of (occasionally) having to
        // shift subsequent elements down when removing the front element.
        let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![obligation];

        // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
        // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
        // inference might unify those two inner type variables later.
        let traits = [coerce_unsized_did, unsize_did];
        while !queue.is_empty() {
            let obligation = queue.remove(0);
            let trait_pred = match obligation.predicate.kind().no_bound_vars() {
                Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
                    if traits.contains(&trait_pred.def_id()) =>
                {
                    self.resolve_vars_if_possible(trait_pred)
                }
                // Eagerly process alias-relate obligations in new trait solver,
                // since these can be emitted in the process of solving trait goals,
                // but we need to constrain vars before processing goals mentioning
                // them.
                Some(ty::PredicateKind::AliasRelate(..)) => {
                    let ocx = ObligationCtxt::new(self);
                    ocx.register_obligation(obligation);
                    if !ocx.try_evaluate_obligations().is_empty() {
                        return Err(TypeError::Mismatch);
                    }
                    coercion.obligations.extend(ocx.into_pending_obligations());
                    continue;
                }
                _ => {
                    coercion.obligations.push(obligation);
                    continue;
                }
            };
            debug!("coerce_unsized resolve step: {:?}", trait_pred);
            match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
                // Uncertain or unimplemented.
                Ok(None) => {
                    if trait_pred.def_id() == unsize_did {
                        let self_ty = trait_pred.self_ty();
                        let unsize_ty = trait_pred.trait_ref.args[1].expect_ty();
                        debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
                        match (self_ty.kind(), unsize_ty.kind()) {
                            (&ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
                                if self.type_var_is_sized(v) =>
                            {
                                debug!("coerce_unsized: have sized infer {:?}", v);
                                coercion.obligations.push(obligation);
                                // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
                                // for unsizing.
                            }
                            _ => {
                                // Some other case for `$0: Unsize<Something>`. Note that we
                                // hit this case even if `Something` is a sized type, so just
                                // don't do the coercion.
                                debug!("coerce_unsized: ambiguous unsize");
                                return Err(TypeError::Mismatch);
                            }
                        }
                    } else {
                        debug!("coerce_unsized: early return - ambiguous");
                        return Err(TypeError::Mismatch);
                    }
                }
                Err(SelectionError::Unimplemented) => {
                    debug!("coerce_unsized: early return - can't prove obligation");
                    return Err(TypeError::Mismatch);
                }

                Err(SelectionError::TraitDynIncompatible(_)) => {
                    // Dyn compatibility errors in coercion will *always* be due to the
                    // fact that the RHS of the coercion is a non-dyn compatible `dyn Trait`
                    // written in source somewhere (otherwise we will never have lowered
                    // the dyn trait from HIR to middle).
                    //
                    // There's no reason to emit yet another dyn compatibility error,
                    // especially since the span will differ slightly and thus not be
                    // deduplicated at all!
                    self.fcx.set_tainted_by_errors(
                        self.fcx
                            .dcx()
                            .span_delayed_bug(self.cause.span, "dyn compatibility during coercion"),
                    );
                }
                Err(err) => {
                    let guar = self.err_ctxt().report_selection_error(
                        obligation.clone(),
                        &obligation,
                        &err,
                    );
                    self.fcx.set_tainted_by_errors(guar);
                    // Treat this like an obligation and follow through
                    // with the unsizing - the lack of a coercion should
                    // be silent, as it causes a type mismatch later.
                }
                Ok(Some(ImplSource::UserDefined(impl_source))) => {
                    queue.extend(impl_source.nested);
                    // Certain incoherent `CoerceUnsized` implementations may cause ICEs,
                    // so check the impl's validity. Taint the body so that we don't try
                    // to evaluate these invalid coercions in CTFE. We only need to do this
                    // for local impls, since upstream impls should be valid.
                    if impl_source.impl_def_id.is_local()
                        && let Err(guar) =
                            self.tcx.ensure_ok().coerce_unsized_info(impl_source.impl_def_id)
                    {
                        self.fcx.set_tainted_by_errors(guar);
                    }
                }
                Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
            }
        }

        Ok(())
    }

    /// Applies reborrowing for `Pin`
    ///
    /// We currently only support reborrowing `Pin<&mut T>` as `Pin<&mut T>`. This is accomplished
    /// by inserting a call to `Pin::as_mut` during MIR building.
    ///
    /// In the future we might want to support other reborrowing coercions, such as:
    /// - `Pin<&mut T>` as `Pin<&T>`
    /// - `Pin<&T>` as `Pin<&T>`
    /// - `Pin<Box<T>>` as `Pin<&T>`
    /// - `Pin<Box<T>>` as `Pin<&mut T>`
    #[instrument(skip(self), level = "trace")]
    fn coerce_to_pin_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug_assert!(self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        // We need to make sure the two types are compatible for coercion.
        // Then we will build a ReborrowPin adjustment and return that as an InferOk.

        // Right now we can only reborrow if this is a `Pin<&mut T>`.
        let extract_pin_mut = |ty: Ty<'tcx>| {
            // Get the T out of Pin<T>
            let (pin, ty) = match ty.kind() {
                ty::Adt(pin, args) if self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) => {
                    (*pin, args[0].expect_ty())
                }
                _ => {
                    debug!("can't reborrow {:?} as pinned", ty);
                    return Err(TypeError::Mismatch);
                }
            };
            // Make sure the T is something we understand (just `&mut U` for now)
            match ty.kind() {
                ty::Ref(region, ty, mutbl) => Ok((pin, *region, *ty, *mutbl)),
                _ => {
                    debug!("can't reborrow pin of inner type {:?}", ty);
                    Err(TypeError::Mismatch)
                }
            }
        };

        let (pin, a_region, a_ty, mut_a) = extract_pin_mut(a)?;
        let (_, _, _b_ty, mut_b) = extract_pin_mut(b)?;

        coerce_mutbls(mut_a, mut_b)?;

        // update a with b's mutability since we'll be coercing mutability
        let a = Ty::new_adt(
            self.tcx,
            pin,
            self.tcx.mk_args(&[Ty::new_ref(self.tcx, a_region, a_ty, mut_b).into()]),
        );

        // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
        // add the adjustments.
        self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b), ForceLeakCheck::No)
    }

    fn coerce_from_fn_pointer(
        &self,
        a: Ty<'tcx>,
        a_sig: ty::PolyFnSig<'tcx>,
        b: Ty<'tcx>,
    ) -> CoerceResult<'tcx> {
        debug!(?a_sig, ?b, "coerce_from_fn_pointer");
        debug_assert!(self.shallow_resolve(b) == b);

        match b.kind() {
            ty::FnPtr(_, b_hdr) if a_sig.safety().is_safe() && b_hdr.safety.is_unsafe() => {
                let a = self.tcx.safe_to_unsafe_fn_ty(a_sig);
                let adjust = Adjust::Pointer(PointerCoercion::UnsafeFnPointer);
                self.unify_and(a, b, [], adjust, ForceLeakCheck::Yes)
            }
            _ => self.unify(a, b, ForceLeakCheck::Yes),
        }
    }

    fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
        debug_assert!(self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        match b.kind() {
            ty::FnPtr(_, b_hdr) => {
                let a_sig = self.sig_for_fn_def_coercion(a, Some(b_hdr.safety))?;

                let InferOk { value: a_sig, mut obligations } =
                    self.at(&self.cause, self.param_env).normalize(a_sig);
                let a = Ty::new_fn_ptr(self.tcx, a_sig);

                let adjust = Adjust::Pointer(PointerCoercion::ReifyFnPointer(b_hdr.safety));
                let InferOk { value, obligations: o2 } =
                    self.unify_and(a, b, [], adjust, ForceLeakCheck::Yes)?;

                obligations.extend(o2);
                Ok(InferOk { value, obligations })
            }
            _ => self.unify(a, b, ForceLeakCheck::No),
        }
    }

    /// Attempts to coerce from a closure to a function pointer. Fails
    /// if the closure has any upvars.
    fn coerce_closure_to_fn(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
        debug_assert!(self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        match b.kind() {
            ty::FnPtr(_, hdr) => {
                let safety = hdr.safety;
                let terr = TypeError::Sorts(ty::error::ExpectedFound::new(a, b));
                let closure_sig = self.sig_for_closure_coercion(a, Some(hdr.safety), terr)?;
                let pointer_ty = Ty::new_fn_ptr(self.tcx, closure_sig);
                debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);

                let adjust = Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety));
                self.unify_and(pointer_ty, b, [], adjust, ForceLeakCheck::No)
            }
            _ => self.unify(a, b, ForceLeakCheck::No),
        }
    }

    fn coerce_to_raw_ptr(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        mutbl_b: hir::Mutability,
    ) -> CoerceResult<'tcx> {
        debug!("coerce_to_raw_ptr(a={:?}, b={:?})", a, b);
        debug_assert!(self.shallow_resolve(a) == a);
        debug_assert!(self.shallow_resolve(b) == b);

        let (is_ref, mt_a) = match *a.kind() {
            ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
            ty::RawPtr(ty, mutbl) => (false, ty::TypeAndMut { ty, mutbl }),
            _ => return self.unify(a, b, ForceLeakCheck::No),
        };
        coerce_mutbls(mt_a.mutbl, mutbl_b)?;

        // Check that the types which they point at are compatible.
        let a_raw = Ty::new_ptr(self.tcx, mt_a.ty, mutbl_b);
        // Although references and raw ptrs have the same
        // representation, we still register an Adjust::DerefRef so that
        // regionck knows that the region for `a` must be valid here.
        if is_ref {
            self.unify_and(
                a_raw,
                b,
                [Adjustment { kind: Adjust::Deref(DerefAdjustKind::Builtin), target: mt_a.ty }],
                Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
                ForceLeakCheck::No,
            )
        } else if mt_a.mutbl != mutbl_b {
            self.unify_and(
                a_raw,
                b,
                [],
                Adjust::Pointer(PointerCoercion::MutToConstPointer),
                ForceLeakCheck::No,
            )
        } else {
            self.unify(a_raw, b, ForceLeakCheck::No)
        }
    }
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    /// Attempt to coerce an expression to a type, and return the
    /// adjusted type of the expression, if successful.
    /// Adjustments are only recorded if the coercion succeeded.
    /// The expressions *must not* have any preexisting adjustments.
    pub(crate) fn coerce(
        &self,
        expr: &'tcx hir::Expr<'tcx>,
        expr_ty: Ty<'tcx>,
        mut target: Ty<'tcx>,
        allow_two_phase: AllowTwoPhase,
        cause: Option<ObligationCause<'tcx>>,
    ) -> RelateResult<'tcx, Ty<'tcx>> {
        let source = self.try_structurally_resolve_type(expr.span, expr_ty);
        if self.next_trait_solver() {
            target = self.try_structurally_resolve_type(
                cause.as_ref().map_or(expr.span, |cause| cause.span),
                target,
            );
        }
        debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);

        let cause =
            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
        let coerce = Coerce::new(
            self,
            cause,
            allow_two_phase,
            self.tcx.expr_guaranteed_to_constitute_read_for_never(expr),
        );
        let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;

        let (adjustments, _) = self.register_infer_ok_obligations(ok);
        self.apply_adjustments(expr, adjustments);
        Ok(if let Err(guar) = expr_ty.error_reported() {
            Ty::new_error(self.tcx, guar)
        } else {
            target
        })
    }

    /// Probe whether `expr_ty` can be coerced to `target_ty`. This has no side-effects,
    /// and may return false positives if types are not yet fully constrained by inference.
    ///
    /// Returns false if the coercion is not possible, or if the coercion creates any
    /// sub-obligations that result in errors.
    ///
    /// This should only be used for diagnostics.
    pub(crate) fn may_coerce(&self, expr_ty: Ty<'tcx>, target_ty: Ty<'tcx>) -> bool {
        let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
        // We don't ever need two-phase here since we throw out the result of the coercion.
        // We also just always set `coerce_never` to true, since this is a heuristic.
        let coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
        self.probe(|_| {
            // Make sure to structurally resolve the types, since we use
            // the `TyKind`s heavily in coercion.
            let ocx = ObligationCtxt::new(self);
            let structurally_resolve = |ty| {
                let ty = self.shallow_resolve(ty);
                if self.next_trait_solver()
                    && let ty::Alias(..) = ty.kind()
                {
                    ocx.structurally_normalize_ty(&cause, self.param_env, ty)
                } else {
                    Ok(ty)
                }
            };
            let Ok(expr_ty) = structurally_resolve(expr_ty) else {
                return false;
            };
            let Ok(target_ty) = structurally_resolve(target_ty) else {
                return false;
            };

            let Ok(ok) = coerce.coerce(expr_ty, target_ty) else {
                return false;
            };
            ocx.register_obligations(ok.obligations);
            ocx.try_evaluate_obligations().is_empty()
        })
    }

    /// Given a type and a target type, this function will calculate and return
    /// how many dereference steps needed to coerce `expr_ty` to `target`. If
    /// it's not possible, return `None`.
    pub(crate) fn deref_steps_for_suggestion(
        &self,
        expr_ty: Ty<'tcx>,
        target: Ty<'tcx>,
    ) -> Option<usize> {
        let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
        // We don't ever need two-phase here since we throw out the result of the coercion.
        let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
        coerce.autoderef(DUMMY_SP, expr_ty).find_map(|(ty, steps)| {
            self.probe(|_| coerce.unify_raw(ty, target, ForceLeakCheck::No)).ok().map(|_| steps)
        })
    }

    /// Given a type, this function will calculate and return the type given
    /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
    ///
    /// This function is for diagnostics only, since it does not register
    /// trait or region sub-obligations. (presumably we could, but it's not
    /// particularly important for diagnostics...)
    pub(crate) fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
        self.autoderef(DUMMY_SP, expr_ty).silence_errors().nth(1).and_then(|(deref_ty, _)| {
            self.infcx
                .type_implements_trait(
                    self.tcx.lang_items().deref_mut_trait()?,
                    [expr_ty],
                    self.param_env,
                )
                .may_apply()
                .then_some(deref_ty)
        })
    }

    #[instrument(level = "debug", skip(self), ret)]
    fn sig_for_coerce_lub(
        &self,
        ty: Ty<'tcx>,
        closure_upvars_terr: TypeError<'tcx>,
    ) -> Result<ty::PolyFnSig<'tcx>, TypeError<'tcx>> {
        match ty.kind() {
            ty::FnDef(..) => self.sig_for_fn_def_coercion(ty, None),
            ty::Closure(..) => self.sig_for_closure_coercion(ty, None, closure_upvars_terr),
            _ => unreachable!("`sig_for_fn_def_closure_coerce_lub` called with wrong ty: {:?}", ty),
        }
    }

    fn sig_for_fn_def_coercion(
        &self,
        fndef: Ty<'tcx>,
        expected_safety: Option<hir::Safety>,
    ) -> Result<ty::PolyFnSig<'tcx>, TypeError<'tcx>> {
        let tcx = self.tcx;

        let &ty::FnDef(def_id, _) = fndef.kind() else {
            unreachable!("`sig_for_fn_def_coercion` called with non-fndef: {:?}", fndef);
        };

        // Intrinsics are not coercible to function pointers
        if tcx.intrinsic(def_id).is_some() {
            return Err(TypeError::IntrinsicCast);
        }

        let fn_attrs = tcx.codegen_fn_attrs(def_id);
        if matches!(fn_attrs.inline, InlineAttr::Force { .. }) {
            return Err(TypeError::ForceInlineCast);
        }

        let sig = fndef.fn_sig(tcx);
        let sig = if fn_attrs.safe_target_features {
            // Allow the coercion if the current function has all the features that would be
            // needed to call the coercee safely.
            match tcx.adjust_target_feature_sig(def_id, sig, self.body_id.into()) {
                Some(adjusted_sig) => adjusted_sig,
                None if matches!(expected_safety, Some(hir::Safety::Safe)) => {
                    return Err(TypeError::TargetFeatureCast(def_id));
                }
                None => sig,
            }
        } else {
            sig
        };

        if sig.safety().is_safe() && matches!(expected_safety, Some(hir::Safety::Unsafe)) {
            Ok(tcx.safe_to_unsafe_sig(sig))
        } else {
            Ok(sig)
        }
    }

    fn sig_for_closure_coercion(
        &self,
        closure: Ty<'tcx>,
        expected_safety: Option<hir::Safety>,
        closure_upvars_terr: TypeError<'tcx>,
    ) -> Result<ty::PolyFnSig<'tcx>, TypeError<'tcx>> {
        let tcx = self.tcx;

        let ty::Closure(closure_def, closure_args) = closure.kind() else {
            unreachable!("`sig_for_closure_coercion` called with non closure ty: {:?}", closure);
        };

        // At this point we haven't done capture analysis, which means
        // that the ClosureArgs just contains an inference variable instead
        // of tuple of captured types.
        //
        // All we care here is if any variable is being captured and not the exact paths,
        // so we check `upvars_mentioned` for root variables being captured.
        if !tcx.upvars_mentioned(closure_def.expect_local()).is_none_or(|u| u.is_empty()) {
            return Err(closure_upvars_terr);
        }

        // We coerce the closure, which has fn type
        //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
        // to
        //     `fn(arg0,arg1,...) -> _`
        // or
        //     `unsafe fn(arg0,arg1,...) -> _`
        let closure_sig = closure_args.as_closure().sig();
        Ok(tcx.signature_unclosure(closure_sig, expected_safety.unwrap_or(hir::Safety::Safe)))
    }

    /// Given some expressions, their known unified type and another expression,
    /// tries to unify the types, potentially inserting coercions on any of the
    /// provided expressions and returns their LUB (aka "common supertype").
    ///
    /// This is really an internal helper. From outside the coercion
    /// module, you should instantiate a `CoerceMany` instance.
    fn try_find_coercion_lub(
        &self,
        cause: &ObligationCause<'tcx>,
        exprs: &[&'tcx hir::Expr<'tcx>],
        prev_ty: Ty<'tcx>,
        new: &hir::Expr<'_>,
        new_ty: Ty<'tcx>,
    ) -> RelateResult<'tcx, Ty<'tcx>> {
        let prev_ty = self.try_structurally_resolve_type(cause.span, prev_ty);
        let new_ty = self.try_structurally_resolve_type(new.span, new_ty);
        debug!(
            "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
            prev_ty,
            new_ty,
            exprs.len()
        );

        // Fast Path: don't go through the coercion logic if we're coercing
        // a type to itself. This is unfortunately quite perf relevant so
        // we do it even though it may mask bugs in the coercion logic.
        if prev_ty == new_ty {
            return Ok(prev_ty);
        }

        let terr = TypeError::Sorts(ty::error::ExpectedFound::new(prev_ty, new_ty));
        let opt_sigs = match (prev_ty.kind(), new_ty.kind()) {
            // Don't coerce pairs of fndefs or pairs of closures to fn ptrs
            // if they can just be lubbed.
            //
            // See #88097 or `lub_closures_before_fnptr_coercion.rs` for where
            // we would erroneously coerce closures to fnptrs when attempting to
            // coerce a closure to itself.
            (ty::FnDef(..), ty::FnDef(..)) | (ty::Closure(..), ty::Closure(..)) => {
                let lubbed_ty = self.commit_if_ok(|snapshot| {
                    let outer_universe = self.infcx.universe();

                    // We need to eagerly handle nested obligations due to lazy norm.
                    let result = if self.next_trait_solver() {
                        let ocx = ObligationCtxt::new(self);
                        let value = ocx.lub(cause, self.param_env, prev_ty, new_ty)?;
                        if ocx.try_evaluate_obligations().is_empty() {
                            Ok(InferOk { value, obligations: ocx.into_pending_obligations() })
                        } else {
                            Err(TypeError::Mismatch)
                        }
                    } else {
                        self.at(cause, self.param_env).lub(prev_ty, new_ty)
                    };

                    self.leak_check(outer_universe, Some(snapshot))?;
                    result
                });

                match lubbed_ty {
                    Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
                    Err(_) => {
                        let a_sig = self.sig_for_coerce_lub(prev_ty, terr)?;
                        let b_sig = self.sig_for_coerce_lub(new_ty, terr)?;
                        Some((a_sig, b_sig))
                    }
                }
            }

            (ty::Closure(..), ty::FnDef(..)) | (ty::FnDef(..), ty::Closure(..)) => {
                let a_sig = self.sig_for_coerce_lub(prev_ty, terr)?;
                let b_sig = self.sig_for_coerce_lub(new_ty, terr)?;
                Some((a_sig, b_sig))
            }
            // ty::FnPtr x ty::FnPtr is fine to just be handled through a normal `unify`
            // call using `lub` which is what will happen on the normal path.
            (ty::FnPtr(..), ty::FnPtr(..)) => None,
            _ => None,
        };

        if let Some((mut a_sig, mut b_sig)) = opt_sigs {
            // Allow coercing safe sigs to unsafe sigs
            if a_sig.safety().is_safe() && b_sig.safety().is_unsafe() {
                a_sig = self.tcx.safe_to_unsafe_sig(a_sig);
            } else if b_sig.safety().is_safe() && a_sig.safety().is_unsafe() {
                b_sig = self.tcx.safe_to_unsafe_sig(b_sig);
            };

            // The signature must match.
            let (a_sig, b_sig) = self.normalize(new.span, (a_sig, b_sig));
            let sig = self
                .at(cause, self.param_env)
                .lub(a_sig, b_sig)
                .map(|ok| self.register_infer_ok_obligations(ok))?;

            // Reify both sides and return the reified fn pointer type.
            let fn_ptr = Ty::new_fn_ptr(self.tcx, sig);
            let prev_adjustment = match prev_ty.kind() {
                ty::Closure(..) => Adjust::Pointer(PointerCoercion::ClosureFnPointer(sig.safety())),
                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer(sig.safety())),
                _ => span_bug!(cause.span, "should not try to coerce a {prev_ty} to a fn pointer"),
            };
            let next_adjustment = match new_ty.kind() {
                ty::Closure(..) => Adjust::Pointer(PointerCoercion::ClosureFnPointer(sig.safety())),
                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer(sig.safety())),
                _ => span_bug!(new.span, "should not try to coerce a {new_ty} to a fn pointer"),
            };
            for expr in exprs.iter() {
                self.apply_adjustments(
                    expr,
                    vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
                );
            }
            self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
            return Ok(fn_ptr);
        }

        // Configure a Coerce instance to compute the LUB.
        // We don't allow two-phase borrows on any autorefs this creates since we
        // probably aren't processing function arguments here and even if we were,
        // they're going to get autorefed again anyway and we can apply 2-phase borrows
        // at that time.
        //
        // NOTE: we set `coerce_never` to `true` here because coercion LUBs only
        // operate on values and not places, so a never coercion is valid.
        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
        coerce.use_lub = true;

        // First try to coerce the new expression to the type of the previous ones,
        // but only if the new expression has no coercion already applied to it.
        let mut first_error = None;
        if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
            let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
            match result {
                Ok(ok) => {
                    let (adjustments, target) = self.register_infer_ok_obligations(ok);
                    self.apply_adjustments(new, adjustments);
                    debug!(
                        "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
                        new_ty, prev_ty, target
                    );
                    return Ok(target);
                }
                Err(e) => first_error = Some(e),
            }
        }

        let ok = self
            .commit_if_ok(|_| coerce.coerce(prev_ty, new_ty))
            // Avoid giving strange errors on failed attempts.
            .map_err(|e| first_error.unwrap_or(e))?;

        let (adjustments, target) = self.register_infer_ok_obligations(ok);
        for expr in exprs {
            self.apply_adjustments(expr, adjustments.clone());
        }
        debug!(
            "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
            prev_ty, new_ty, target
        );
        Ok(target)
    }
}

/// Check whether `ty` can be coerced to `output_ty`.
/// Used from clippy.
pub fn can_coerce<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: LocalDefId,
    ty: Ty<'tcx>,
    output_ty: Ty<'tcx>,
) -> bool {
    let root_ctxt = crate::typeck_root_ctxt::TypeckRootCtxt::new(tcx, body_id);
    let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, body_id);
    fn_ctxt.may_coerce(ty, output_ty)
}

/// CoerceMany encapsulates the pattern you should use when you have
/// many expressions that are all getting coerced to a common
/// type. This arises, for example, when you have a match (the result
/// of each arm is coerced to a common type). It also arises in less
/// obvious places, such as when you have many `break foo` expressions
/// that target the same loop, or the various `return` expressions in
/// a function.
///
/// The basic protocol is as follows:
///
/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
///   This will also serve as the "starting LUB". The expectation is
///   that this type is something which all of the expressions *must*
///   be coercible to. Use a fresh type variable if needed.
/// - For each expression whose result is to be coerced, invoke `coerce()` with.
///   - In some cases we wish to coerce "non-expressions" whose types are implicitly
///     unit. This happens for example if you have a `break` with no expression,
///     or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
///   - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
///     from you so that you don't have to worry your pretty head about it.
///     But if an error is reported, the final type will be `err`.
///   - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
///     previously coerced expressions.
/// - When all done, invoke `complete()`. This will return the LUB of
///   all your expressions.
///   - WARNING: I don't believe this final type is guaranteed to be
///     related to your initial `expected_ty` in any particular way,
///     although it will typically be a subtype, so you should check it.
///     Check the note below for more details.
///   - Invoking `complete()` may cause us to go and adjust the "adjustments" on
///     previously coerced expressions.
///
/// Example:
///
/// ```ignore (illustrative)
/// let mut coerce = CoerceMany::new(expected_ty);
/// for expr in exprs {
///     let expr_ty = fcx.check_expr_with_expectation(expr, expected);
///     coerce.coerce(fcx, &cause, expr, expr_ty);
/// }
/// let final_ty = coerce.complete(fcx);
/// ```
///
/// NOTE: Why does the `expected_ty` participate in the LUB?
/// When coercing, each branch should use the following expectations for type inference:
/// - The branch can be coerced to the expected type of the match/if/whatever.
/// - The branch can be coercion lub'd with the types of the previous branches.
/// Ideally we'd have some sort of `Expectation::ParticipatesInCoerceLub(ongoing_lub_ty, final_ty)`,
/// but adding and using this feels very challenging.
/// What we instead do is to use the expected type of the match/if/whatever as
/// the initial coercion lub. This allows us to use the lub of "expected type of match" with
/// "types from previous branches" as the coercion target, which can contains both expectations.
///
/// Two concerns with this approach:
/// - We may have incompatible `final_ty` if that lub is different from the expected
///   type of the match. However, in this case coercing the final type of the
///   `CoerceMany` to its expected type would have error'd anyways, so we don't care.
/// - We may constrain the `expected_ty` too early. For some branches with
///   type `a` and `b`, we end up with `(a lub expected_ty) lub b` instead of
///   `(a lub b) lub expected_ty`. They should be the same type. However,
///   `a lub expected_ty` may constrain inference variables in `expected_ty`.
///   In this case the difference does matter and we get actually incorrect results.
/// FIXME: Ideally we'd compute the final type without unnecessarily constraining
/// the expected type of the match when computing the types of its branches.
pub(crate) struct CoerceMany<'tcx> {
    expected_ty: Ty<'tcx>,
    final_ty: Option<Ty<'tcx>>,
    expressions: Vec<&'tcx hir::Expr<'tcx>>,
}

impl<'tcx> CoerceMany<'tcx> {
    /// Creates a `CoerceMany` with a default capacity of 1. If the full set of
    /// coercion sites is known before hand, consider `with_capacity()` instead
    /// to avoid allocation.
    pub(crate) fn new(expected_ty: Ty<'tcx>) -> Self {
        Self::with_capacity(expected_ty, 1)
    }

    /// Creates a `CoerceMany` with a given capacity.
    pub(crate) fn with_capacity(expected_ty: Ty<'tcx>, capacity: usize) -> Self {
        CoerceMany { expected_ty, final_ty: None, expressions: Vec::with_capacity(capacity) }
    }

    /// Returns the "expected type" with which this coercion was
    /// constructed. This represents the "downward propagated" type
    /// that was given to us at the start of typing whatever construct
    /// we are typing (e.g., the match expression).
    ///
    /// Typically, this is used as the expected type when
    /// type-checking each of the alternative expressions whose types
    /// we are trying to merge.
    pub(crate) fn expected_ty(&self) -> Ty<'tcx> {
        self.expected_ty
    }

    /// Returns the current "merged type", representing our best-guess
    /// at the LUB of the expressions we've seen so far (if any). This
    /// isn't *final* until you call `self.complete()`, which will return
    /// the merged type.
    pub(crate) fn merged_ty(&self) -> Ty<'tcx> {
        self.final_ty.unwrap_or(self.expected_ty)
    }

    /// Indicates that the value generated by `expression`, which is
    /// of type `expression_ty`, is one of the possibilities that we
    /// could coerce from. This will record `expression`, and later
    /// calls to `coerce` may come back and add adjustments and things
    /// if necessary.
    pub(crate) fn coerce<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        expression: &'tcx hir::Expr<'tcx>,
        expression_ty: Ty<'tcx>,
    ) {
        self.coerce_inner(fcx, cause, Some(expression), expression_ty, |_| {}, false)
    }

    /// Indicates that one of the inputs is a "forced unit". This
    /// occurs in a case like `if foo { ... };`, where the missing else
    /// generates a "forced unit". Another example is a `loop { break;
    /// }`, where the `break` has no argument expression. We treat
    /// these cases slightly differently for error-reporting
    /// purposes. Note that these tend to correspond to cases where
    /// the `()` expression is implicit in the source, and hence we do
    /// not take an expression argument.
    ///
    /// The `augment_error` gives you a chance to extend the error
    /// message, in case any results (e.g., we use this to suggest
    /// removing a `;`).
    pub(crate) fn coerce_forced_unit<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        augment_error: impl FnOnce(&mut Diag<'_>),
        label_unit_as_expected: bool,
    ) {
        self.coerce_inner(
            fcx,
            cause,
            None,
            fcx.tcx.types.unit,
            augment_error,
            label_unit_as_expected,
        )
    }

    /// The inner coercion "engine". If `expression` is `None`, this
    /// is a forced-unit case, and hence `expression_ty` must be
    /// `Nil`.
    #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
    pub(crate) fn coerce_inner<'a>(
        &mut self,
        fcx: &FnCtxt<'a, 'tcx>,
        cause: &ObligationCause<'tcx>,
        expression: Option<&'tcx hir::Expr<'tcx>>,
        mut expression_ty: Ty<'tcx>,
        augment_error: impl FnOnce(&mut Diag<'_>),
        label_expression_as_expected: bool,
    ) {
        // Incorporate whatever type inference information we have
        // until now; in principle we might also want to process
        // pending obligations, but doing so should only improve
        // compatibility (hopefully that is true) by helping us
        // uncover never types better.
        if expression_ty.is_ty_var() {
            expression_ty = fcx.infcx.shallow_resolve(expression_ty);
        }

        // If we see any error types, just propagate that error
        // upwards.
        if let Err(guar) = (expression_ty, self.merged_ty()).error_reported() {
            self.final_ty = Some(Ty::new_error(fcx.tcx, guar));
            return;
        }

        let (expected, found) = if label_expression_as_expected {
            // In the case where this is a "forced unit", like
            // `break`, we want to call the `()` "expected"
            // since it is implied by the syntax.
            // (Note: not all force-units work this way.)"
            (expression_ty, self.merged_ty())
        } else {
            // Otherwise, the "expected" type for error
            // reporting is the current unification type,
            // which is basically the LUB of the expressions
            // we've seen so far (combined with the expected
            // type)
            (self.merged_ty(), expression_ty)
        };

        // Handle the actual type unification etc.
        let result = if let Some(expression) = expression {
            if self.expressions.is_empty() {
                // Special-case the first expression we are coercing.
                // To be honest, I'm not entirely sure why we do this.
                // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
                fcx.coerce(
                    expression,
                    expression_ty,
                    self.expected_ty,
                    AllowTwoPhase::No,
                    Some(cause.clone()),
                )
            } else {
                fcx.try_find_coercion_lub(
                    cause,
                    &self.expressions,
                    self.merged_ty(),
                    expression,
                    expression_ty,
                )
            }
        } else {
            // this is a hack for cases where we default to `()` because
            // the expression etc has been omitted from the source. An
            // example is an `if let` without an else:
            //
            //     if let Some(x) = ... { }
            //
            // we wind up with a second match arm that is like `_ =>
            // ()`. That is the case we are considering here. We take
            // a different path to get the right "expected, found"
            // message and so forth (and because we know that
            // `expression_ty` will be unit).
            //
            // Another example is `break` with no argument expression.
            assert!(expression_ty.is_unit(), "if let hack without unit type");
            fcx.at(cause, fcx.param_env)
                .eq(
                    // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
                    DefineOpaqueTypes::Yes,
                    expected,
                    found,
                )
                .map(|infer_ok| {
                    fcx.register_infer_ok_obligations(infer_ok);
                    expression_ty
                })
        };

        debug!(?result);
        match result {
            Ok(v) => {
                self.final_ty = Some(v);
                if let Some(e) = expression {
                    self.expressions.push(e);
                }
            }
            Err(coercion_error) => {
                // Mark that we've failed to coerce the types here to suppress
                // any superfluous errors we might encounter while trying to
                // emit or provide suggestions on how to fix the initial error.
                fcx.set_tainted_by_errors(
                    fcx.dcx().span_delayed_bug(cause.span, "coercion error but no error emitted"),
                );
                let (expected, found) = fcx.resolve_vars_if_possible((expected, found));

                let mut err;
                let mut unsized_return = false;
                match *cause.code() {
                    ObligationCauseCode::ReturnNoExpression => {
                        err = struct_span_code_err!(
                            fcx.dcx(),
                            cause.span,
                            E0069,
                            "`return;` in a function whose return type is not `()`"
                        );
                        if let Some(value) = fcx.err_ctxt().ty_kind_suggestion(fcx.param_env, found)
                        {
                            err.span_suggestion_verbose(
                                cause.span.shrink_to_hi(),
                                "give the `return` a value of the expected type",
                                format!(" {value}"),
                                Applicability::HasPlaceholders,
                            );
                        }
                        err.span_label(cause.span, "return type is not `()`");
                    }
                    ObligationCauseCode::BlockTailExpression(blk_id, ..) => {
                        err = self.report_return_mismatched_types(
                            cause,
                            expected,
                            found,
                            coercion_error,
                            fcx,
                            blk_id,
                            expression,
                        );
                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
                    }
                    ObligationCauseCode::ReturnValue(return_expr_id) => {
                        err = self.report_return_mismatched_types(
                            cause,
                            expected,
                            found,
                            coercion_error,
                            fcx,
                            return_expr_id,
                            expression,
                        );
                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
                    }
                    ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                        arm_span,
                        arm_ty,
                        prior_arm_ty,
                        ref prior_non_diverging_arms,
                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
                        ..
                    }) => {
                        err = fcx.err_ctxt().report_mismatched_types(
                            cause,
                            fcx.param_env,
                            expected,
                            found,
                            coercion_error,
                        );
                        // Check that we're actually in the second or later arm
                        if prior_non_diverging_arms.len() > 0 {
                            self.suggest_boxing_tail_for_return_position_impl_trait(
                                fcx,
                                &mut err,
                                rpit_def_id,
                                arm_ty,
                                prior_arm_ty,
                                prior_non_diverging_arms
                                    .iter()
                                    .chain(std::iter::once(&arm_span))
                                    .copied(),
                            );
                        }
                    }
                    ObligationCauseCode::IfExpression {
                        expr_id,
                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
                    } => {
                        let hir::Node::Expr(hir::Expr {
                            kind: hir::ExprKind::If(_, then_expr, Some(else_expr)),
                            ..
                        }) = fcx.tcx.hir_node(expr_id)
                        else {
                            unreachable!();
                        };
                        err = fcx.err_ctxt().report_mismatched_types(
                            cause,
                            fcx.param_env,
                            expected,
                            found,
                            coercion_error,
                        );
                        let then_span = fcx.find_block_span_from_hir_id(then_expr.hir_id);
                        let else_span = fcx.find_block_span_from_hir_id(else_expr.hir_id);
                        // Don't suggest wrapping whole block in `Box::new`.
                        if then_span != then_expr.span && else_span != else_expr.span {
                            let then_ty = fcx.typeck_results.borrow().expr_ty(then_expr);
                            let else_ty = fcx.typeck_results.borrow().expr_ty(else_expr);
                            self.suggest_boxing_tail_for_return_position_impl_trait(
                                fcx,
                                &mut err,
                                rpit_def_id,
                                then_ty,
                                else_ty,
                                [then_span, else_span].into_iter(),
                            );
                        }
                    }
                    _ => {
                        err = fcx.err_ctxt().report_mismatched_types(
                            cause,
                            fcx.param_env,
                            expected,
                            found,
                            coercion_error,
                        );
                    }
                }

                augment_error(&mut err);

                if let Some(expr) = expression {
                    if let hir::ExprKind::Loop(
                        _,
                        _,
                        loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
                        _,
                    ) = expr.kind
                    {
                        let loop_type = if loop_src == hir::LoopSource::While {
                            "`while` loops"
                        } else {
                            "`for` loops"
                        };

                        err.note(format!("{loop_type} evaluate to unit type `()`"));
                    }

                    fcx.emit_coerce_suggestions(
                        &mut err,
                        expr,
                        found,
                        expected,
                        None,
                        Some(coercion_error),
                    );
                }

                let reported = err.emit_unless_delay(unsized_return);

                self.final_ty = Some(Ty::new_error(fcx.tcx, reported));
            }
        }
    }

    fn suggest_boxing_tail_for_return_position_impl_trait(
        &self,
        fcx: &FnCtxt<'_, 'tcx>,
        err: &mut Diag<'_>,
        rpit_def_id: LocalDefId,
        a_ty: Ty<'tcx>,
        b_ty: Ty<'tcx>,
        arm_spans: impl Iterator<Item = Span>,
    ) {
        let compatible = |ty: Ty<'tcx>| {
            fcx.probe(|_| {
                let ocx = ObligationCtxt::new(fcx);
                ocx.register_obligations(
                    fcx.tcx.item_self_bounds(rpit_def_id).iter_identity().filter_map(|clause| {
                        let predicate = clause
                            .kind()
                            .map_bound(|clause| match clause {
                                ty::ClauseKind::Trait(trait_pred) => Some(ty::ClauseKind::Trait(
                                    trait_pred.with_replaced_self_ty(fcx.tcx, ty),
                                )),
                                ty::ClauseKind::Projection(proj_pred) => {
                                    Some(ty::ClauseKind::Projection(
                                        proj_pred.with_replaced_self_ty(fcx.tcx, ty),
                                    ))
                                }
                                _ => None,
                            })
                            .transpose()?;
                        Some(Obligation::new(
                            fcx.tcx,
                            ObligationCause::dummy(),
                            fcx.param_env,
                            predicate,
                        ))
                    }),
                );
                ocx.try_evaluate_obligations().is_empty()
            })
        };

        if !compatible(a_ty) || !compatible(b_ty) {
            return;
        }

        let rpid_def_span = fcx.tcx.def_span(rpit_def_id);
        err.subdiagnostic(SuggestBoxingForReturnImplTrait::ChangeReturnType {
            start_sp: rpid_def_span.with_hi(rpid_def_span.lo() + BytePos(4)),
            end_sp: rpid_def_span.shrink_to_hi(),
        });

        let (starts, ends) =
            arm_spans.map(|span| (span.shrink_to_lo(), span.shrink_to_hi())).unzip();
        err.subdiagnostic(SuggestBoxingForReturnImplTrait::BoxReturnExpr { starts, ends });
    }

    fn report_return_mismatched_types<'infcx>(
        &self,
        cause: &ObligationCause<'tcx>,
        expected: Ty<'tcx>,
        found: Ty<'tcx>,
        ty_err: TypeError<'tcx>,
        fcx: &'infcx FnCtxt<'_, 'tcx>,
        block_or_return_id: hir::HirId,
        expression: Option<&'tcx hir::Expr<'tcx>>,
    ) -> Diag<'infcx> {
        let mut err =
            fcx.err_ctxt().report_mismatched_types(cause, fcx.param_env, expected, found, ty_err);

        let due_to_block = matches!(fcx.tcx.hir_node(block_or_return_id), hir::Node::Block(..));
        let parent = fcx.tcx.parent_hir_node(block_or_return_id);
        if let Some(expr) = expression
            && let hir::Node::Expr(&hir::Expr {
                kind: hir::ExprKind::Closure(&hir::Closure { body, .. }),
                ..
            }) = parent
        {
            let needs_block =
                !matches!(fcx.tcx.hir_body(body).value.kind, hir::ExprKind::Block(..));
            fcx.suggest_missing_semicolon(&mut err, expr, expected, needs_block, true);
        }
        // Verify that this is a tail expression of a function, otherwise the
        // label pointing out the cause for the type coercion will be wrong
        // as prior return coercions would not be relevant (#57664).
        if let Some(expr) = expression
            && due_to_block
        {
            fcx.suggest_missing_semicolon(&mut err, expr, expected, false, false);
            let pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
                &mut err,
                expr,
                expected,
                found,
                block_or_return_id,
            );
            if let Some(cond_expr) = fcx.tcx.hir_get_if_cause(expr.hir_id)
                && expected.is_unit()
                && !pointing_at_return_type
                // If the block is from an external macro or try (`?`) desugaring, then
                // do not suggest adding a semicolon, because there's nowhere to put it.
                // See issues #81943 and #87051.
                // Similarly, if the block is from a loop desugaring, then also do not
                // suggest adding a semicolon. See issue #150850.
                && cond_expr.span.desugaring_kind().is_none()
                && !cond_expr.span.in_external_macro(fcx.tcx.sess.source_map())
                && !matches!(
                    cond_expr.kind,
                    hir::ExprKind::Match(.., hir::MatchSource::TryDesugar(_))
                )
            {
                if let ObligationCauseCode::BlockTailExpression(hir_id, hir::MatchSource::Normal) =
                    cause.code()
                    && let hir::Node::Block(block) = fcx.tcx.hir_node(*hir_id)
                    && let hir::Node::Expr(expr) = fcx.tcx.parent_hir_node(block.hir_id)
                    && let hir::Node::Expr(if_expr) = fcx.tcx.parent_hir_node(expr.hir_id)
                    && let hir::ExprKind::If(_cond, _then, None) = if_expr.kind
                {
                    err.span_label(
                        cond_expr.span,
                        "`if` expressions without `else` arms expect their inner expression to be `()`",
                    );
                } else {
                    err.span_label(cond_expr.span, "expected this to be `()`");
                }
                if expr.can_have_side_effects() {
                    fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
                }
            }
        }

        // If this is due to an explicit `return`, suggest adding a return type.
        if let Some((fn_id, fn_decl)) = fcx.get_fn_decl(block_or_return_id)
            && !due_to_block
        {
            fcx.suggest_missing_return_type(&mut err, fn_decl, expected, found, fn_id);
        }

        // If this is due to a block, then maybe we forgot a `return`/`break`.
        if due_to_block
            && let Some(expr) = expression
            && let Some(parent_fn_decl) =
                fcx.tcx.hir_fn_decl_by_hir_id(fcx.tcx.local_def_id_to_hir_id(fcx.body_id))
        {
            fcx.suggest_missing_break_or_return_expr(
                &mut err,
                expr,
                parent_fn_decl,
                expected,
                found,
                block_or_return_id,
                fcx.body_id,
            );
        }

        let ret_coercion_span = fcx.ret_coercion_span.get();

        if let Some(sp) = ret_coercion_span
            // If the closure has an explicit return type annotation, or if
            // the closure's return type has been inferred from outside
            // requirements (such as an Fn* trait bound), then a type error
            // may occur at the first return expression we see in the closure
            // (if it conflicts with the declared return type). Skip adding a
            // note in this case, since it would be incorrect.
            && let Some(fn_sig) = fcx.body_fn_sig()
            && fn_sig.output().is_ty_var()
        {
            err.span_note(sp, format!("return type inferred to be `{expected}` here"));
        }

        err
    }

    /// Checks whether the return type is unsized via an obligation, which makes
    /// sure we consider `dyn Trait: Sized` where clauses, which are trivially
    /// false but technically valid for typeck.
    fn is_return_ty_definitely_unsized(&self, fcx: &FnCtxt<'_, 'tcx>) -> bool {
        if let Some(sig) = fcx.body_fn_sig() {
            !fcx.predicate_may_hold(&Obligation::new(
                fcx.tcx,
                ObligationCause::dummy(),
                fcx.param_env,
                ty::TraitRef::new(
                    fcx.tcx,
                    fcx.tcx.require_lang_item(hir::LangItem::Sized, DUMMY_SP),
                    [sig.output()],
                ),
            ))
        } else {
            false
        }
    }

    pub(crate) fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
        if let Some(final_ty) = self.final_ty {
            final_ty
        } else {
            // If we only had inputs that were of type `!` (or no
            // inputs at all), then the final type is `!`.
            assert!(self.expressions.is_empty());
            fcx.tcx.types.never
        }
    }
}

/// Recursively visit goals to decide whether an unsizing is possible.
/// `Break`s when it isn't, and an error should be raised.
/// `Continue`s when an unsizing ok based on an implementation of the `Unsize` trait / lang item.
struct CoerceVisitor<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    span: Span,
}

impl<'tcx> ProofTreeVisitor<'tcx> for CoerceVisitor<'_, 'tcx> {
    type Result = ControlFlow<()>;

    fn span(&self) -> Span {
        self.span
    }

    fn visit_goal(&mut self, goal: &inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
        let Some(pred) = goal.goal().predicate.as_trait_clause() else {
            return ControlFlow::Continue(());
        };

        // Make sure this predicate is referring to either an `Unsize` or `CoerceUnsized` trait,
        // Otherwise there's nothing to do.
        if !self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::Unsize)
            && !self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::CoerceUnsized)
        {
            return ControlFlow::Continue(());
        }

        match goal.result() {
            // If we prove the `Unsize` or `CoerceUnsized` goal, continue recursing.
            Ok(Certainty::Yes) => ControlFlow::Continue(()),
            Err(NoSolution) => {
                // Even if we find no solution, continue recursing if we find a single candidate
                // for which we're shallowly certain it holds to get the right error source.
                if let [only_candidate] = &goal.candidates()[..]
                    && only_candidate.shallow_certainty() == Certainty::Yes
                {
                    only_candidate.visit_nested_no_probe(self)
                } else {
                    ControlFlow::Break(())
                }
            }
            Ok(Certainty::Maybe { .. }) => {
                // FIXME: structurally normalize?
                if self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::Unsize)
                    && let ty::Dynamic(..) = pred.skip_binder().trait_ref.args.type_at(1).kind()
                    && let ty::Infer(ty::TyVar(vid)) = *pred.self_ty().skip_binder().kind()
                    && self.fcx.type_var_is_sized(vid)
                {
                    // We get here when trying to unsize a type variable to a `dyn Trait`,
                    // knowing that that variable is sized. Unsizing definitely has to happen in that case.
                    // If the variable weren't sized, we may not need an unsizing coercion.
                    // In general, we don't want to add coercions too eagerly since it makes error messages much worse.
                    ControlFlow::Continue(())
                } else if let Some(cand) = goal.unique_applicable_candidate()
                    && cand.shallow_certainty() == Certainty::Yes
                {
                    cand.visit_nested_no_probe(self)
                } else {
                    ControlFlow::Break(())
                }
            }
        }
    }
}