rustc_ast_lowering/

item.rs

use rustc_abi::ExternAbi;
use rustc_ast::visit::AssocCtxt;
use rustc_ast::*;
use rustc_errors::{E0570, ErrorGuaranteed, struct_span_code_err};
use rustc_hir::attrs::{AttributeKind, EiiImplResolution};
use rustc_hir::def::{DefKind, PerNS, Res};
use rustc_hir::def_id::{CRATE_DEF_ID, LocalDefId};
use rustc_hir::{
    self as hir, HirId, ImplItemImplKind, LifetimeSource, PredicateOrigin, Target, find_attr,
};
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::span_bug;
use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
use rustc_span::def_id::DefId;
use rustc_span::edit_distance::find_best_match_for_name;
use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, kw, sym};
use smallvec::{SmallVec, smallvec};
use thin_vec::ThinVec;
use tracing::instrument;

use super::errors::{InvalidAbi, InvalidAbiSuggestion, TupleStructWithDefault, UnionWithDefault};
use super::stability::{enabled_names, gate_unstable_abi};
use super::{
    AstOwner, FnDeclKind, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
    RelaxedBoundForbiddenReason, RelaxedBoundPolicy, ResolverAstLoweringExt,
};

pub(super) struct ItemLowerer<'a, 'hir> {
    pub(super) tcx: TyCtxt<'hir>,
    pub(super) resolver: &'a mut ResolverAstLowering,
    pub(super) ast_index: &'a IndexSlice<LocalDefId, AstOwner<'a>>,
    pub(super) owners: &'a mut IndexVec<LocalDefId, hir::MaybeOwner<'hir>>,
}

/// When we have a ty alias we *may* have two where clauses. To give the best diagnostics, we set the span
/// to the where clause that is preferred, if it exists. Otherwise, it sets the span to the other where
/// clause if it exists.
fn add_ty_alias_where_clause(
    generics: &mut ast::Generics,
    after_where_clause: &ast::WhereClause,
    prefer_first: bool,
) {
    generics.where_clause.predicates.extend_from_slice(&after_where_clause.predicates);

    let mut before = (generics.where_clause.has_where_token, generics.where_clause.span);
    let mut after = (after_where_clause.has_where_token, after_where_clause.span);
    if !prefer_first {
        (before, after) = (after, before);
    }
    (generics.where_clause.has_where_token, generics.where_clause.span) =
        if before.0 || !after.0 { before } else { after };
}

impl<'a, 'hir> ItemLowerer<'a, 'hir> {
    fn with_lctx(
        &mut self,
        owner: NodeId,
        f: impl FnOnce(&mut LoweringContext<'_, 'hir>) -> hir::OwnerNode<'hir>,
    ) {
        let mut lctx = LoweringContext::new(self.tcx, self.resolver);
        lctx.with_hir_id_owner(owner, |lctx| f(lctx));

        for (def_id, info) in lctx.children {
            let owner = self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
            assert!(
                matches!(owner, hir::MaybeOwner::Phantom),
                "duplicate copy of {def_id:?} in lctx.children"
            );
            *owner = info;
        }
    }

    pub(super) fn lower_node(&mut self, def_id: LocalDefId) {
        let owner = self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
        if let hir::MaybeOwner::Phantom = owner {
            let node = self.ast_index[def_id];
            match node {
                AstOwner::NonOwner => {}
                AstOwner::Crate(c) => {
                    assert_eq!(self.resolver.node_id_to_def_id[&CRATE_NODE_ID], CRATE_DEF_ID);
                    self.with_lctx(CRATE_NODE_ID, |lctx| {
                        let module = lctx.lower_mod(&c.items, &c.spans);
                        // FIXME(jdonszelman): is dummy span ever a problem here?
                        lctx.lower_attrs(hir::CRATE_HIR_ID, &c.attrs, DUMMY_SP, Target::Crate);
                        hir::OwnerNode::Crate(module)
                    })
                }
                AstOwner::Item(item) => {
                    self.with_lctx(item.id, |lctx| hir::OwnerNode::Item(lctx.lower_item(item)))
                }
                AstOwner::AssocItem(item, ctxt) => {
                    self.with_lctx(item.id, |lctx| lctx.lower_assoc_item(item, ctxt))
                }
                AstOwner::ForeignItem(item) => self.with_lctx(item.id, |lctx| {
                    hir::OwnerNode::ForeignItem(lctx.lower_foreign_item(item))
                }),
            }
        }
    }
}

impl<'hir> LoweringContext<'_, 'hir> {
    pub(super) fn lower_mod(
        &mut self,
        items: &[Box<Item>],
        spans: &ModSpans,
    ) -> &'hir hir::Mod<'hir> {
        self.arena.alloc(hir::Mod {
            spans: hir::ModSpans {
                inner_span: self.lower_span(spans.inner_span),
                inject_use_span: self.lower_span(spans.inject_use_span),
            },
            item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_ref(x))),
        })
    }

    pub(super) fn lower_item_ref(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
        let mut node_ids = smallvec![hir::ItemId { owner_id: self.owner_id(i.id) }];
        if let ItemKind::Use(use_tree) = &i.kind {
            self.lower_item_id_use_tree(use_tree, &mut node_ids);
        }
        node_ids
    }

    fn lower_item_id_use_tree(&mut self, tree: &UseTree, vec: &mut SmallVec<[hir::ItemId; 1]>) {
        match &tree.kind {
            UseTreeKind::Nested { items, .. } => {
                for &(ref nested, id) in items {
                    vec.push(hir::ItemId { owner_id: self.owner_id(id) });
                    self.lower_item_id_use_tree(nested, vec);
                }
            }
            UseTreeKind::Simple(..) | UseTreeKind::Glob => {}
        }
    }

    fn lower_eii_decl(
        &mut self,
        id: NodeId,
        name: Ident,
        EiiDecl { foreign_item, impl_unsafe }: &EiiDecl,
    ) -> Option<hir::attrs::EiiDecl> {
        self.lower_path_simple_eii(id, foreign_item).map(|did| hir::attrs::EiiDecl {
            foreign_item: did,
            impl_unsafe: *impl_unsafe,
            name,
        })
    }

    fn lower_eii_impl(
        &mut self,
        EiiImpl {
            node_id,
            eii_macro_path,
            impl_safety,
            span,
            inner_span,
            is_default,
            known_eii_macro_resolution,
        }: &EiiImpl,
    ) -> hir::attrs::EiiImpl {
        let resolution = if let Some(target) = known_eii_macro_resolution
            && let Some(decl) = self.lower_eii_decl(
                *node_id,
                // the expect is ok here since we always generate this path in the eii macro.
                eii_macro_path.segments.last().expect("at least one segment").ident,
                target,
            ) {
            EiiImplResolution::Known(decl)
        } else if let Some(macro_did) = self.lower_path_simple_eii(*node_id, eii_macro_path) {
            EiiImplResolution::Macro(macro_did)
        } else {
            EiiImplResolution::Error(
                self.dcx().span_delayed_bug(*span, "eii never resolved without errors given"),
            )
        };

        hir::attrs::EiiImpl {
            span: self.lower_span(*span),
            inner_span: self.lower_span(*inner_span),
            impl_marked_unsafe: self.lower_safety(*impl_safety, hir::Safety::Safe).is_unsafe(),
            is_default: *is_default,
            resolution,
        }
    }

    fn generate_extra_attrs_for_item_kind(
        &mut self,
        id: NodeId,
        i: &ItemKind,
    ) -> Vec<hir::Attribute> {
        match i {
            ItemKind::Fn(box Fn { eii_impls, .. }) if eii_impls.is_empty() => Vec::new(),
            ItemKind::Fn(box Fn { eii_impls, .. }) => {
                vec![hir::Attribute::Parsed(AttributeKind::EiiImpls(
                    eii_impls.iter().map(|i| self.lower_eii_impl(i)).collect(),
                ))]
            }
            ItemKind::MacroDef(name, MacroDef { eii_declaration: Some(target), .. }) => self
                .lower_eii_decl(id, *name, target)
                .map(|decl| vec![hir::Attribute::Parsed(AttributeKind::EiiDeclaration(decl))])
                .unwrap_or_default(),

            ItemKind::ExternCrate(..)
            | ItemKind::Use(..)
            | ItemKind::Static(..)
            | ItemKind::Const(..)
            | ItemKind::ConstBlock(..)
            | ItemKind::Mod(..)
            | ItemKind::ForeignMod(..)
            | ItemKind::GlobalAsm(..)
            | ItemKind::TyAlias(..)
            | ItemKind::Enum(..)
            | ItemKind::Struct(..)
            | ItemKind::Union(..)
            | ItemKind::Trait(..)
            | ItemKind::TraitAlias(..)
            | ItemKind::Impl(..)
            | ItemKind::MacCall(..)
            | ItemKind::MacroDef(..)
            | ItemKind::Delegation(..)
            | ItemKind::DelegationMac(..) => Vec::new(),
        }
    }

    fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
        let vis_span = self.lower_span(i.vis.span);
        let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);

        let extra_hir_attributes = self.generate_extra_attrs_for_item_kind(i.id, &i.kind);
        let attrs = self.lower_attrs_with_extra(
            hir_id,
            &i.attrs,
            i.span,
            Target::from_ast_item(i),
            &extra_hir_attributes,
        );

        let kind = self.lower_item_kind(i.span, i.id, hir_id, attrs, vis_span, &i.kind);
        let item = hir::Item {
            owner_id: hir_id.expect_owner(),
            kind,
            vis_span,
            span: self.lower_span(i.span),
            has_delayed_lints: !self.delayed_lints.is_empty(),
            eii: find_attr!(attrs, AttributeKind::EiiImpls(..) | AttributeKind::EiiDeclaration(..)),
        };
        self.arena.alloc(item)
    }

    fn lower_item_kind(
        &mut self,
        span: Span,
        id: NodeId,
        hir_id: hir::HirId,
        attrs: &'hir [hir::Attribute],
        vis_span: Span,
        i: &ItemKind,
    ) -> hir::ItemKind<'hir> {
        match i {
            ItemKind::ExternCrate(orig_name, ident) => {
                let ident = self.lower_ident(*ident);
                hir::ItemKind::ExternCrate(*orig_name, ident)
            }
            ItemKind::Use(use_tree) => {
                // Start with an empty prefix.
                let prefix = Path { segments: ThinVec::new(), span: use_tree.span, tokens: None };

                self.lower_use_tree(use_tree, &prefix, id, vis_span, attrs)
            }
            ItemKind::Static(box ast::StaticItem {
                ident,
                ty,
                safety: _,
                mutability: m,
                expr: e,
                define_opaque,
            }) => {
                let ident = self.lower_ident(*ident);
                let ty = self
                    .lower_ty_alloc(ty, ImplTraitContext::Disallowed(ImplTraitPosition::StaticTy));
                let body_id = self.lower_const_body(span, e.as_deref());
                self.lower_define_opaque(hir_id, define_opaque);
                hir::ItemKind::Static(*m, ident, ty, body_id)
            }
            ItemKind::Const(box ConstItem {
                defaultness: _,
                ident,
                generics,
                ty,
                rhs_kind,
                define_opaque,
            }) => {
                let ident = self.lower_ident(*ident);
                let (generics, (ty, rhs)) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        let ty = this.lower_ty_alloc(
                            ty,
                            ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy),
                        );
                        let rhs = this.lower_const_item_rhs(rhs_kind, span);
                        (ty, rhs)
                    },
                );
                self.lower_define_opaque(hir_id, &define_opaque);
                hir::ItemKind::Const(ident, generics, ty, rhs)
            }
            ItemKind::ConstBlock(ConstBlockItem { span, id, block }) => hir::ItemKind::Const(
                self.lower_ident(ConstBlockItem::IDENT),
                hir::Generics::empty(),
                self.arena.alloc(self.ty_tup(DUMMY_SP, &[])),
                hir::ConstItemRhs::Body({
                    let body = hir::Expr {
                        hir_id: self.lower_node_id(*id),
                        kind: hir::ExprKind::Block(self.lower_block(block, false), None),
                        span: self.lower_span(*span),
                    };
                    self.record_body(&[], body)
                }),
            ),
            ItemKind::Fn(box Fn {
                sig: FnSig { decl, header, span: fn_sig_span },
                ident,
                generics,
                body,
                contract,
                define_opaque,
                ..
            }) => {
                self.with_new_scopes(*fn_sig_span, |this| {
                    // Note: we don't need to change the return type from `T` to
                    // `impl Future<Output = T>` here because lower_body
                    // only cares about the input argument patterns in the function
                    // declaration (decl), not the return types.
                    let coroutine_kind = header.coroutine_kind;
                    let body_id = this.lower_maybe_coroutine_body(
                        *fn_sig_span,
                        span,
                        hir_id,
                        decl,
                        coroutine_kind,
                        body.as_deref(),
                        attrs,
                        contract.as_deref(),
                    );

                    let itctx = ImplTraitContext::Universal;
                    let (generics, decl) = this.lower_generics(generics, id, itctx, |this| {
                        this.lower_fn_decl(decl, id, *fn_sig_span, FnDeclKind::Fn, coroutine_kind)
                    });
                    let sig = hir::FnSig {
                        decl,
                        header: this.lower_fn_header(*header, hir::Safety::Safe, attrs),
                        span: this.lower_span(*fn_sig_span),
                    };
                    this.lower_define_opaque(hir_id, define_opaque);
                    let ident = this.lower_ident(*ident);
                    hir::ItemKind::Fn {
                        ident,
                        sig,
                        generics,
                        body: body_id,
                        has_body: body.is_some(),
                    }
                })
            }
            ItemKind::Mod(_, ident, mod_kind) => {
                let ident = self.lower_ident(*ident);
                match mod_kind {
                    ModKind::Loaded(items, _, spans) => {
                        hir::ItemKind::Mod(ident, self.lower_mod(items, spans))
                    }
                    ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
                }
            }
            ItemKind::ForeignMod(fm) => hir::ItemKind::ForeignMod {
                abi: fm.abi.map_or(ExternAbi::FALLBACK, |abi| self.lower_abi(abi)),
                items: self
                    .arena
                    .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
            },
            ItemKind::GlobalAsm(asm) => {
                let asm = self.lower_inline_asm(span, asm);
                let fake_body =
                    self.lower_body(|this| (&[], this.expr(span, hir::ExprKind::InlineAsm(asm))));
                hir::ItemKind::GlobalAsm { asm, fake_body }
            }
            ItemKind::TyAlias(box TyAlias { ident, generics, after_where_clause, ty, .. }) => {
                // We lower
                //
                // type Foo = impl Trait
                //
                // to
                //
                // type Foo = Foo1
                // opaque type Foo1: Trait
                let ident = self.lower_ident(*ident);
                let mut generics = generics.clone();
                add_ty_alias_where_clause(&mut generics, after_where_clause, true);
                let (generics, ty) = self.lower_generics(
                    &generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| match ty {
                        None => {
                            let guar = this.dcx().span_delayed_bug(
                                span,
                                "expected to lower type alias type, but it was missing",
                            );
                            this.arena.alloc(this.ty(span, hir::TyKind::Err(guar)))
                        }
                        Some(ty) => this.lower_ty_alloc(
                            ty,
                            ImplTraitContext::OpaqueTy {
                                origin: hir::OpaqueTyOrigin::TyAlias {
                                    parent: this.local_def_id(id),
                                    in_assoc_ty: false,
                                },
                            },
                        ),
                    },
                );
                hir::ItemKind::TyAlias(ident, generics, ty)
            }
            ItemKind::Enum(ident, generics, enum_definition) => {
                let ident = self.lower_ident(*ident);
                let (generics, variants) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        this.arena.alloc_from_iter(
                            enum_definition.variants.iter().map(|x| this.lower_variant(i, x)),
                        )
                    },
                );
                hir::ItemKind::Enum(ident, generics, hir::EnumDef { variants })
            }
            ItemKind::Struct(ident, generics, struct_def) => {
                let ident = self.lower_ident(*ident);
                let (generics, struct_def) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| this.lower_variant_data(hir_id, i, struct_def),
                );
                hir::ItemKind::Struct(ident, generics, struct_def)
            }
            ItemKind::Union(ident, generics, vdata) => {
                let ident = self.lower_ident(*ident);
                let (generics, vdata) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| this.lower_variant_data(hir_id, i, vdata),
                );
                hir::ItemKind::Union(ident, generics, vdata)
            }
            ItemKind::Impl(Impl {
                generics: ast_generics,
                of_trait,
                self_ty: ty,
                items: impl_items,
                constness,
            }) => {
                // Lower the "impl header" first. This ordering is important
                // for in-band lifetimes! Consider `'a` here:
                //
                //     impl Foo<'a> for u32 {
                //         fn method(&'a self) { .. }
                //     }
                //
                // Because we start by lowering the `Foo<'a> for u32`
                // part, we will add `'a` to the list of generics on
                // the impl. When we then encounter it later in the
                // method, it will not be considered an in-band
                // lifetime to be added, but rather a reference to a
                // parent lifetime.
                let itctx = ImplTraitContext::Universal;
                let (generics, (of_trait, lowered_ty)) =
                    self.lower_generics(ast_generics, id, itctx, |this| {
                        let of_trait = of_trait
                            .as_deref()
                            .map(|of_trait| this.lower_trait_impl_header(of_trait));

                        let lowered_ty = this.lower_ty_alloc(
                            ty,
                            ImplTraitContext::Disallowed(ImplTraitPosition::ImplSelf),
                        );

                        (of_trait, lowered_ty)
                    });

                let new_impl_items = self
                    .arena
                    .alloc_from_iter(impl_items.iter().map(|item| self.lower_impl_item_ref(item)));

                let constness = self.lower_constness(*constness);

                hir::ItemKind::Impl(hir::Impl {
                    generics,
                    of_trait,
                    self_ty: lowered_ty,
                    items: new_impl_items,
                    constness,
                })
            }
            ItemKind::Trait(box Trait {
                constness,
                is_auto,
                safety,
                ident,
                generics,
                bounds,
                items,
            }) => {
                let constness = self.lower_constness(*constness);
                let ident = self.lower_ident(*ident);
                let (generics, (safety, items, bounds)) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        let bounds = this.lower_param_bounds(
                            bounds,
                            RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::SuperTrait),
                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                        );
                        let items = this.arena.alloc_from_iter(
                            items.iter().map(|item| this.lower_trait_item_ref(item)),
                        );
                        let safety = this.lower_safety(*safety, hir::Safety::Safe);
                        (safety, items, bounds)
                    },
                );
                hir::ItemKind::Trait(constness, *is_auto, safety, ident, generics, bounds, items)
            }
            ItemKind::TraitAlias(box TraitAlias { constness, ident, generics, bounds }) => {
                let constness = self.lower_constness(*constness);
                let ident = self.lower_ident(*ident);
                let (generics, bounds) = self.lower_generics(
                    generics,
                    id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        this.lower_param_bounds(
                            bounds,
                            RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::TraitAlias),
                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                        )
                    },
                );
                hir::ItemKind::TraitAlias(constness, ident, generics, bounds)
            }
            ItemKind::MacroDef(ident, MacroDef { body, macro_rules, eii_declaration: _ }) => {
                let ident = self.lower_ident(*ident);
                let body = Box::new(self.lower_delim_args(body));
                let def_id = self.local_def_id(id);
                let def_kind = self.tcx.def_kind(def_id);
                let DefKind::Macro(macro_kinds) = def_kind else {
                    unreachable!(
                        "expected DefKind::Macro for macro item, found {}",
                        def_kind.descr(def_id.to_def_id())
                    );
                };
                let macro_def = self.arena.alloc(ast::MacroDef {
                    body,
                    macro_rules: *macro_rules,
                    eii_declaration: None,
                });
                hir::ItemKind::Macro(ident, macro_def, macro_kinds)
            }
            ItemKind::Delegation(box delegation) => {
                let delegation_results = self.lower_delegation(delegation, id);
                hir::ItemKind::Fn {
                    sig: delegation_results.sig,
                    ident: delegation_results.ident,
                    generics: delegation_results.generics,
                    body: delegation_results.body_id,
                    has_body: true,
                }
            }
            ItemKind::MacCall(..) | ItemKind::DelegationMac(..) => {
                panic!("macros should have been expanded by now")
            }
        }
    }

    fn lower_path_simple_eii(&mut self, id: NodeId, path: &Path) -> Option<DefId> {
        let res = self.resolver.get_partial_res(id)?;
        let Some(did) = res.expect_full_res().opt_def_id() else {
            self.dcx().span_delayed_bug(path.span, "should have errored in resolve");
            return None;
        };

        Some(did)
    }

    #[instrument(level = "debug", skip(self))]
    fn lower_use_tree(
        &mut self,
        tree: &UseTree,
        prefix: &Path,
        id: NodeId,
        vis_span: Span,
        attrs: &'hir [hir::Attribute],
    ) -> hir::ItemKind<'hir> {
        let path = &tree.prefix;
        let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();

        match tree.kind {
            UseTreeKind::Simple(rename) => {
                let mut ident = tree.ident();

                // First, apply the prefix to the path.
                let mut path = Path { segments, span: path.span, tokens: None };

                // Correctly resolve `self` imports.
                if path.segments.len() > 1
                    && path.segments.last().unwrap().ident.name == kw::SelfLower
                {
                    let _ = path.segments.pop();
                    if rename.is_none() {
                        ident = path.segments.last().unwrap().ident;
                    }
                }

                let res = self.lower_import_res(id, path.span);
                let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                let ident = self.lower_ident(ident);
                hir::ItemKind::Use(path, hir::UseKind::Single(ident))
            }
            UseTreeKind::Glob => {
                let res = self.expect_full_res(id);
                let res = self.lower_res(res);
                // Put the result in the appropriate namespace.
                let res = match res {
                    Res::Def(DefKind::Mod | DefKind::Trait, _) => {
                        PerNS { type_ns: Some(res), value_ns: None, macro_ns: None }
                    }
                    Res::Def(DefKind::Enum, _) => {
                        PerNS { type_ns: None, value_ns: Some(res), macro_ns: None }
                    }
                    Res::Err => {
                        // Propagate the error to all namespaces, just to be sure.
                        let err = Some(Res::Err);
                        PerNS { type_ns: err, value_ns: err, macro_ns: err }
                    }
                    _ => span_bug!(path.span, "bad glob res {:?}", res),
                };
                let path = Path { segments, span: path.span, tokens: None };
                let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                hir::ItemKind::Use(path, hir::UseKind::Glob)
            }
            UseTreeKind::Nested { items: ref trees, .. } => {
                // Nested imports are desugared into simple imports.
                // So, if we start with
                //
                // ```
                // pub(x) use foo::{a, b};
                // ```
                //
                // we will create three items:
                //
                // ```
                // pub(x) use foo::a;
                // pub(x) use foo::b;
                // pub(x) use foo::{}; // <-- this is called the `ListStem`
                // ```
                //
                // The first two are produced by recursively invoking
                // `lower_use_tree` (and indeed there may be things
                // like `use foo::{a::{b, c}}` and so forth). They
                // wind up being directly added to
                // `self.items`. However, the structure of this
                // function also requires us to return one item, and
                // for that we return the `{}` import (called the
                // `ListStem`).

                let span = prefix.span.to(path.span);
                let prefix = Path { segments, span, tokens: None };

                // Add all the nested `PathListItem`s to the HIR.
                for &(ref use_tree, id) in trees {
                    let owner_id = self.owner_id(id);

                    // Each `use` import is an item and thus are owners of the
                    // names in the path. Up to this point the nested import is
                    // the current owner, since we want each desugared import to
                    // own its own names, we have to adjust the owner before
                    // lowering the rest of the import.
                    self.with_hir_id_owner(id, |this| {
                        // `prefix` is lowered multiple times, but in different HIR owners.
                        // So each segment gets renewed `HirId` with the same
                        // `ItemLocalId` and the new owner. (See `lower_node_id`)
                        let kind = this.lower_use_tree(use_tree, &prefix, id, vis_span, attrs);
                        if !attrs.is_empty() {
                            this.attrs.insert(hir::ItemLocalId::ZERO, attrs);
                        }

                        let item = hir::Item {
                            owner_id,
                            kind,
                            vis_span,
                            span: this.lower_span(use_tree.span),
                            has_delayed_lints: !this.delayed_lints.is_empty(),
                            eii: find_attr!(
                                attrs,
                                AttributeKind::EiiImpls(..) | AttributeKind::EiiDeclaration(..)
                            ),
                        };
                        hir::OwnerNode::Item(this.arena.alloc(item))
                    });
                }

                // Condition should match `build_reduced_graph_for_use_tree`.
                let path = if trees.is_empty()
                    && !(prefix.segments.is_empty()
                        || prefix.segments.len() == 1
                            && prefix.segments[0].ident.name == kw::PathRoot)
                {
                    // For empty lists we need to lower the prefix so it is checked for things
                    // like stability later.
                    let res = self.lower_import_res(id, span);
                    self.lower_use_path(res, &prefix, ParamMode::Explicit)
                } else {
                    // For non-empty lists we can just drop all the data, the prefix is already
                    // present in HIR as a part of nested imports.
                    let span = self.lower_span(span);
                    self.arena.alloc(hir::UsePath { res: PerNS::default(), segments: &[], span })
                };
                hir::ItemKind::Use(path, hir::UseKind::ListStem)
            }
        }
    }

    fn lower_assoc_item(&mut self, item: &AssocItem, ctxt: AssocCtxt) -> hir::OwnerNode<'hir> {
        // Evaluate with the lifetimes in `params` in-scope.
        // This is used to track which lifetimes have already been defined,
        // and which need to be replicated when lowering an async fn.
        match ctxt {
            AssocCtxt::Trait => hir::OwnerNode::TraitItem(self.lower_trait_item(item)),
            AssocCtxt::Impl { of_trait } => {
                hir::OwnerNode::ImplItem(self.lower_impl_item(item, of_trait))
            }
        }
    }

    fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir> {
        let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
        let owner_id = hir_id.expect_owner();
        let attrs =
            self.lower_attrs(hir_id, &i.attrs, i.span, Target::from_foreign_item_kind(&i.kind));
        let (ident, kind) = match &i.kind {
            ForeignItemKind::Fn(box Fn { sig, ident, generics, define_opaque, .. }) => {
                let fdec = &sig.decl;
                let itctx = ImplTraitContext::Universal;
                let (generics, (decl, fn_args)) =
                    self.lower_generics(generics, i.id, itctx, |this| {
                        (
                            // Disallow `impl Trait` in foreign items.
                            this.lower_fn_decl(fdec, i.id, sig.span, FnDeclKind::ExternFn, None),
                            this.lower_fn_params_to_idents(fdec),
                        )
                    });

                // Unmarked safety in unsafe block defaults to unsafe.
                let header = self.lower_fn_header(sig.header, hir::Safety::Unsafe, attrs);

                if define_opaque.is_some() {
                    self.dcx().span_err(i.span, "foreign functions cannot define opaque types");
                }

                (
                    ident,
                    hir::ForeignItemKind::Fn(
                        hir::FnSig { header, decl, span: self.lower_span(sig.span) },
                        fn_args,
                        generics,
                    ),
                )
            }
            ForeignItemKind::Static(box StaticItem {
                ident,
                ty,
                mutability,
                expr: _,
                safety,
                define_opaque,
            }) => {
                let ty = self
                    .lower_ty_alloc(ty, ImplTraitContext::Disallowed(ImplTraitPosition::StaticTy));
                let safety = self.lower_safety(*safety, hir::Safety::Unsafe);
                if define_opaque.is_some() {
                    self.dcx().span_err(i.span, "foreign statics cannot define opaque types");
                }
                (ident, hir::ForeignItemKind::Static(ty, *mutability, safety))
            }
            ForeignItemKind::TyAlias(box TyAlias { ident, .. }) => {
                (ident, hir::ForeignItemKind::Type)
            }
            ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
        };

        let item = hir::ForeignItem {
            owner_id,
            ident: self.lower_ident(*ident),
            kind,
            vis_span: self.lower_span(i.vis.span),
            span: self.lower_span(i.span),
            has_delayed_lints: !self.delayed_lints.is_empty(),
        };
        self.arena.alloc(item)
    }

    fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemId {
        hir::ForeignItemId { owner_id: self.owner_id(i.id) }
    }

    fn lower_variant(&mut self, item_kind: &ItemKind, v: &Variant) -> hir::Variant<'hir> {
        let hir_id = self.lower_node_id(v.id);
        self.lower_attrs(hir_id, &v.attrs, v.span, Target::Variant);
        hir::Variant {
            hir_id,
            def_id: self.local_def_id(v.id),
            data: self.lower_variant_data(hir_id, item_kind, &v.data),
            disr_expr: v
                .disr_expr
                .as_ref()
                .map(|e| self.lower_anon_const_to_anon_const(e, e.value.span)),
            ident: self.lower_ident(v.ident),
            span: self.lower_span(v.span),
        }
    }

    fn lower_variant_data(
        &mut self,
        parent_id: hir::HirId,
        item_kind: &ItemKind,
        vdata: &VariantData,
    ) -> hir::VariantData<'hir> {
        match vdata {
            VariantData::Struct { fields, recovered } => {
                let fields = self
                    .arena
                    .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f)));

                if let ItemKind::Union(..) = item_kind {
                    for field in &fields[..] {
                        if let Some(default) = field.default {
                            // Unions cannot derive `Default`, and it's not clear how to use default
                            // field values of unions if that was supported. Therefore, blanket reject
                            // trying to use field values with unions.
                            if self.tcx.features().default_field_values() {
                                self.dcx().emit_err(UnionWithDefault { span: default.span });
                            } else {
                                let _ = self.dcx().span_delayed_bug(
                                default.span,
                                "expected union default field values feature gate error but none \
                                was produced",
                            );
                            }
                        }
                    }
                }

                hir::VariantData::Struct { fields, recovered: *recovered }
            }
            VariantData::Tuple(fields, id) => {
                let ctor_id = self.lower_node_id(*id);
                self.alias_attrs(ctor_id, parent_id);
                let fields = self
                    .arena
                    .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f)));
                for field in &fields[..] {
                    if let Some(default) = field.default {
                        // Default values in tuple struct and tuple variants are not allowed by the
                        // RFC due to concerns about the syntax, both in the item definition and the
                        // expression. We could in the future allow `struct S(i32 = 0);` and force
                        // users to construct the value with `let _ = S { .. };`.
                        if self.tcx.features().default_field_values() {
                            self.dcx().emit_err(TupleStructWithDefault { span: default.span });
                        } else {
                            let _ = self.dcx().span_delayed_bug(
                                default.span,
                                "expected `default values on `struct` fields aren't supported` \
                                 feature-gate error but none was produced",
                            );
                        }
                    }
                }
                hir::VariantData::Tuple(fields, ctor_id, self.local_def_id(*id))
            }
            VariantData::Unit(id) => {
                let ctor_id = self.lower_node_id(*id);
                self.alias_attrs(ctor_id, parent_id);
                hir::VariantData::Unit(ctor_id, self.local_def_id(*id))
            }
        }
    }

    pub(super) fn lower_field_def(
        &mut self,
        (index, f): (usize, &FieldDef),
    ) -> hir::FieldDef<'hir> {
        let ty =
            self.lower_ty_alloc(&f.ty, ImplTraitContext::Disallowed(ImplTraitPosition::FieldTy));
        let hir_id = self.lower_node_id(f.id);
        self.lower_attrs(hir_id, &f.attrs, f.span, Target::Field);
        hir::FieldDef {
            span: self.lower_span(f.span),
            hir_id,
            def_id: self.local_def_id(f.id),
            ident: match f.ident {
                Some(ident) => self.lower_ident(ident),
                // FIXME(jseyfried): positional field hygiene.
                None => Ident::new(sym::integer(index), self.lower_span(f.span)),
            },
            vis_span: self.lower_span(f.vis.span),
            default: f
                .default
                .as_ref()
                .map(|v| self.lower_anon_const_to_anon_const(v, v.value.span)),
            ty,
            safety: self.lower_safety(f.safety, hir::Safety::Safe),
        }
    }

    fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
        let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
        let attrs = self.lower_attrs(
            hir_id,
            &i.attrs,
            i.span,
            Target::from_assoc_item_kind(&i.kind, AssocCtxt::Trait),
        );
        let trait_item_def_id = hir_id.expect_owner();

        let (ident, generics, kind, has_default) = match &i.kind {
            AssocItemKind::Const(box ConstItem {
                ident,
                generics,
                ty,
                rhs_kind,
                define_opaque,
                ..
            }) => {
                let (generics, kind) = self.lower_generics(
                    generics,
                    i.id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        let ty = this.lower_ty_alloc(
                            ty,
                            ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy),
                        );
                        // Trait associated consts don't need an expression/body.
                        let rhs = if rhs_kind.has_expr() {
                            Some(this.lower_const_item_rhs(rhs_kind, i.span))
                        } else {
                            None
                        };
                        hir::TraitItemKind::Const(ty, rhs, rhs_kind.is_type_const().into())
                    },
                );

                if define_opaque.is_some() {
                    if rhs_kind.has_expr() {
                        self.lower_define_opaque(hir_id, &define_opaque);
                    } else {
                        self.dcx().span_err(
                            i.span,
                            "only trait consts with default bodies can define opaque types",
                        );
                    }
                }

                (*ident, generics, kind, rhs_kind.has_expr())
            }
            AssocItemKind::Fn(box Fn {
                sig, ident, generics, body: None, define_opaque, ..
            }) => {
                // FIXME(contracts): Deny contract here since it won't apply to
                // any impl method or callees.
                let idents = self.lower_fn_params_to_idents(&sig.decl);
                let (generics, sig) = self.lower_method_sig(
                    generics,
                    sig,
                    i.id,
                    FnDeclKind::Trait,
                    sig.header.coroutine_kind,
                    attrs,
                );
                if define_opaque.is_some() {
                    self.dcx().span_err(
                        i.span,
                        "only trait methods with default bodies can define opaque types",
                    );
                }
                (
                    *ident,
                    generics,
                    hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(idents)),
                    false,
                )
            }
            AssocItemKind::Fn(box Fn {
                sig,
                ident,
                generics,
                body: Some(body),
                contract,
                define_opaque,
                ..
            }) => {
                let body_id = self.lower_maybe_coroutine_body(
                    sig.span,
                    i.span,
                    hir_id,
                    &sig.decl,
                    sig.header.coroutine_kind,
                    Some(body),
                    attrs,
                    contract.as_deref(),
                );
                let (generics, sig) = self.lower_method_sig(
                    generics,
                    sig,
                    i.id,
                    FnDeclKind::Trait,
                    sig.header.coroutine_kind,
                    attrs,
                );
                self.lower_define_opaque(hir_id, &define_opaque);
                (
                    *ident,
                    generics,
                    hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)),
                    true,
                )
            }
            AssocItemKind::Type(box TyAlias {
                ident,
                generics,
                after_where_clause,
                bounds,
                ty,
                ..
            }) => {
                let mut generics = generics.clone();
                add_ty_alias_where_clause(&mut generics, after_where_clause, false);
                let (generics, kind) = self.lower_generics(
                    &generics,
                    i.id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        let ty = ty.as_ref().map(|x| {
                            this.lower_ty_alloc(
                                x,
                                ImplTraitContext::Disallowed(ImplTraitPosition::AssocTy),
                            )
                        });
                        hir::TraitItemKind::Type(
                            this.lower_param_bounds(
                                bounds,
                                RelaxedBoundPolicy::Allowed,
                                ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                            ),
                            ty,
                        )
                    },
                );
                (*ident, generics, kind, ty.is_some())
            }
            AssocItemKind::Delegation(box delegation) => {
                let delegation_results = self.lower_delegation(delegation, i.id);
                let item_kind = hir::TraitItemKind::Fn(
                    delegation_results.sig,
                    hir::TraitFn::Provided(delegation_results.body_id),
                );
                (delegation.ident, delegation_results.generics, item_kind, true)
            }
            AssocItemKind::MacCall(..) | AssocItemKind::DelegationMac(..) => {
                panic!("macros should have been expanded by now")
            }
        };

        let item = hir::TraitItem {
            owner_id: trait_item_def_id,
            ident: self.lower_ident(ident),
            generics,
            kind,
            span: self.lower_span(i.span),
            defaultness: hir::Defaultness::Default { has_value: has_default },
            has_delayed_lints: !self.delayed_lints.is_empty(),
        };
        self.arena.alloc(item)
    }

    fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemId {
        hir::TraitItemId { owner_id: self.owner_id(i.id) }
    }

    /// Construct `ExprKind::Err` for the given `span`.
    pub(crate) fn expr_err(&mut self, span: Span, guar: ErrorGuaranteed) -> hir::Expr<'hir> {
        self.expr(span, hir::ExprKind::Err(guar))
    }

    fn lower_trait_impl_header(
        &mut self,
        trait_impl_header: &TraitImplHeader,
    ) -> &'hir hir::TraitImplHeader<'hir> {
        let TraitImplHeader { safety, polarity, defaultness, ref trait_ref } = *trait_impl_header;
        let safety = self.lower_safety(safety, hir::Safety::Safe);
        let polarity = match polarity {
            ImplPolarity::Positive => ImplPolarity::Positive,
            ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
        };
        // `defaultness.has_value()` is never called for an `impl`, always `true` in order
        // to not cause an assertion failure inside the `lower_defaultness` function.
        let has_val = true;
        let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
        let modifiers = TraitBoundModifiers {
            constness: BoundConstness::Never,
            asyncness: BoundAsyncness::Normal,
            // we don't use this in bound lowering
            polarity: BoundPolarity::Positive,
        };
        let trait_ref = self.lower_trait_ref(
            modifiers,
            trait_ref,
            ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
        );

        self.arena.alloc(hir::TraitImplHeader {
            safety,
            polarity,
            defaultness,
            defaultness_span,
            trait_ref,
        })
    }

    fn lower_impl_item(
        &mut self,
        i: &AssocItem,
        is_in_trait_impl: bool,
    ) -> &'hir hir::ImplItem<'hir> {
        // Since `default impl` is not yet implemented, this is always true in impls.
        let has_value = true;
        let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
        let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
        let attrs = self.lower_attrs(
            hir_id,
            &i.attrs,
            i.span,
            Target::from_assoc_item_kind(&i.kind, AssocCtxt::Impl { of_trait: is_in_trait_impl }),
        );

        let (ident, (generics, kind)) = match &i.kind {
            AssocItemKind::Const(box ConstItem {
                ident,
                generics,
                ty,
                rhs_kind,
                define_opaque,
                ..
            }) => (
                *ident,
                self.lower_generics(
                    generics,
                    i.id,
                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                    |this| {
                        let ty = this.lower_ty_alloc(
                            ty,
                            ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy),
                        );
                        this.lower_define_opaque(hir_id, &define_opaque);
                        let rhs = this.lower_const_item_rhs(rhs_kind, i.span);
                        hir::ImplItemKind::Const(ty, rhs)
                    },
                ),
            ),
            AssocItemKind::Fn(box Fn {
                sig,
                ident,
                generics,
                body,
                contract,
                define_opaque,
                ..
            }) => {
                let body_id = self.lower_maybe_coroutine_body(
                    sig.span,
                    i.span,
                    hir_id,
                    &sig.decl,
                    sig.header.coroutine_kind,
                    body.as_deref(),
                    attrs,
                    contract.as_deref(),
                );
                let (generics, sig) = self.lower_method_sig(
                    generics,
                    sig,
                    i.id,
                    if is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
                    sig.header.coroutine_kind,
                    attrs,
                );
                self.lower_define_opaque(hir_id, &define_opaque);

                (*ident, (generics, hir::ImplItemKind::Fn(sig, body_id)))
            }
            AssocItemKind::Type(box TyAlias {
                ident, generics, after_where_clause, ty, ..
            }) => {
                let mut generics = generics.clone();
                add_ty_alias_where_clause(&mut generics, after_where_clause, false);
                (
                    *ident,
                    self.lower_generics(
                        &generics,
                        i.id,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                        |this| match ty {
                            None => {
                                let guar = this.dcx().span_delayed_bug(
                                    i.span,
                                    "expected to lower associated type, but it was missing",
                                );
                                let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err(guar)));
                                hir::ImplItemKind::Type(ty)
                            }
                            Some(ty) => {
                                let ty = this.lower_ty_alloc(
                                    ty,
                                    ImplTraitContext::OpaqueTy {
                                        origin: hir::OpaqueTyOrigin::TyAlias {
                                            parent: this.local_def_id(i.id),
                                            in_assoc_ty: true,
                                        },
                                    },
                                );
                                hir::ImplItemKind::Type(ty)
                            }
                        },
                    ),
                )
            }
            AssocItemKind::Delegation(box delegation) => {
                let delegation_results = self.lower_delegation(delegation, i.id);
                (
                    delegation.ident,
                    (
                        delegation_results.generics,
                        hir::ImplItemKind::Fn(delegation_results.sig, delegation_results.body_id),
                    ),
                )
            }
            AssocItemKind::MacCall(..) | AssocItemKind::DelegationMac(..) => {
                panic!("macros should have been expanded by now")
            }
        };

        let span = self.lower_span(i.span);
        let item = hir::ImplItem {
            owner_id: hir_id.expect_owner(),
            ident: self.lower_ident(ident),
            generics,
            impl_kind: if is_in_trait_impl {
                ImplItemImplKind::Trait {
                    defaultness,
                    trait_item_def_id: self
                        .resolver
                        .get_partial_res(i.id)
                        .and_then(|r| r.expect_full_res().opt_def_id())
                        .ok_or_else(|| {
                            self.dcx().span_delayed_bug(
                                span,
                                "could not resolve trait item being implemented",
                            )
                        }),
                }
            } else {
                ImplItemImplKind::Inherent { vis_span: self.lower_span(i.vis.span) }
            },
            kind,
            span,
            has_delayed_lints: !self.delayed_lints.is_empty(),
        };
        self.arena.alloc(item)
    }

    fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemId {
        hir::ImplItemId { owner_id: self.owner_id(i.id) }
    }

    fn lower_defaultness(
        &self,
        d: Defaultness,
        has_value: bool,
    ) -> (hir::Defaultness, Option<Span>) {
        match d {
            Defaultness::Default(sp) => {
                (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
            }
            Defaultness::Final => {
                assert!(has_value);
                (hir::Defaultness::Final, None)
            }
        }
    }

    fn record_body(
        &mut self,
        params: &'hir [hir::Param<'hir>],
        value: hir::Expr<'hir>,
    ) -> hir::BodyId {
        let body = hir::Body { params, value: self.arena.alloc(value) };
        let id = body.id();
        assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
        self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
        id
    }

    pub(super) fn lower_body(
        &mut self,
        f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
    ) -> hir::BodyId {
        let prev_coroutine_kind = self.coroutine_kind.take();
        let task_context = self.task_context.take();
        let (parameters, result) = f(self);
        let body_id = self.record_body(parameters, result);
        self.task_context = task_context;
        self.coroutine_kind = prev_coroutine_kind;
        body_id
    }

    fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
        let hir_id = self.lower_node_id(param.id);
        self.lower_attrs(hir_id, &param.attrs, param.span, Target::Param);
        hir::Param {
            hir_id,
            pat: self.lower_pat(&param.pat),
            ty_span: self.lower_span(param.ty.span),
            span: self.lower_span(param.span),
        }
    }

    pub(super) fn lower_fn_body(
        &mut self,
        decl: &FnDecl,
        contract: Option<&FnContract>,
        body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
    ) -> hir::BodyId {
        self.lower_body(|this| {
            let params =
                this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x)));

            // Optionally lower the fn contract
            if let Some(contract) = contract {
                (params, this.lower_contract(body, contract))
            } else {
                (params, body(this))
            }
        })
    }

    fn lower_fn_body_block(
        &mut self,
        decl: &FnDecl,
        body: &Block,
        contract: Option<&FnContract>,
    ) -> hir::BodyId {
        self.lower_fn_body(decl, contract, |this| this.lower_block_expr(body))
    }

    pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
        self.lower_body(|this| {
            (
                &[],
                match expr {
                    Some(expr) => this.lower_expr_mut(expr),
                    None => this.expr_err(span, this.dcx().span_delayed_bug(span, "no block")),
                },
            )
        })
    }

    /// Takes what may be the body of an `async fn` or a `gen fn` and wraps it in an `async {}` or
    /// `gen {}` block as appropriate.
    fn lower_maybe_coroutine_body(
        &mut self,
        fn_decl_span: Span,
        span: Span,
        fn_id: hir::HirId,
        decl: &FnDecl,
        coroutine_kind: Option<CoroutineKind>,
        body: Option<&Block>,
        attrs: &'hir [hir::Attribute],
        contract: Option<&FnContract>,
    ) -> hir::BodyId {
        let Some(body) = body else {
            // Functions without a body are an error, except if this is an intrinsic. For those we
            // create a fake body so that the entire rest of the compiler doesn't have to deal with
            // this as a special case.
            return self.lower_fn_body(decl, contract, |this| {
                if find_attr!(attrs, AttributeKind::RustcIntrinsic)
                    || this.tcx.is_sdylib_interface_build()
                {
                    let span = this.lower_span(span);
                    let empty_block = hir::Block {
                        hir_id: this.next_id(),
                        stmts: &[],
                        expr: None,
                        rules: hir::BlockCheckMode::DefaultBlock,
                        span,
                        targeted_by_break: false,
                    };
                    let loop_ = hir::ExprKind::Loop(
                        this.arena.alloc(empty_block),
                        None,
                        hir::LoopSource::Loop,
                        span,
                    );
                    hir::Expr { hir_id: this.next_id(), kind: loop_, span }
                } else {
                    this.expr_err(span, this.dcx().has_errors().unwrap())
                }
            });
        };
        let Some(coroutine_kind) = coroutine_kind else {
            // Typical case: not a coroutine.
            return self.lower_fn_body_block(decl, body, contract);
        };
        // FIXME(contracts): Support contracts on async fn.
        self.lower_body(|this| {
            let (parameters, expr) = this.lower_coroutine_body_with_moved_arguments(
                decl,
                |this| this.lower_block_expr(body),
                fn_decl_span,
                body.span,
                coroutine_kind,
                hir::CoroutineSource::Fn,
            );

            // FIXME(async_fn_track_caller): Can this be moved above?
            let hir_id = expr.hir_id;
            this.maybe_forward_track_caller(body.span, fn_id, hir_id);

            (parameters, expr)
        })
    }

    /// Lowers a desugared coroutine body after moving all of the arguments
    /// into the body. This is to make sure that the future actually owns the
    /// arguments that are passed to the function, and to ensure things like
    /// drop order are stable.
    pub(crate) fn lower_coroutine_body_with_moved_arguments(
        &mut self,
        decl: &FnDecl,
        lower_body: impl FnOnce(&mut LoweringContext<'_, 'hir>) -> hir::Expr<'hir>,
        fn_decl_span: Span,
        body_span: Span,
        coroutine_kind: CoroutineKind,
        coroutine_source: hir::CoroutineSource,
    ) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>) {
        let mut parameters: Vec<hir::Param<'_>> = Vec::new();
        let mut statements: Vec<hir::Stmt<'_>> = Vec::new();

        // Async function parameters are lowered into the closure body so that they are
        // captured and so that the drop order matches the equivalent non-async functions.
        //
        // from:
        //
        //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
        //         <body>
        //     }
        //
        // into:
        //
        //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
        //       async move {
        //         let __arg2 = __arg2;
        //         let <pattern> = __arg2;
        //         let __arg1 = __arg1;
        //         let <pattern> = __arg1;
        //         let __arg0 = __arg0;
        //         let <pattern> = __arg0;
        //         drop-temps { <body> } // see comments later in fn for details
        //       }
        //     }
        //
        // If `<pattern>` is a simple ident, then it is lowered to a single
        // `let <pattern> = <pattern>;` statement as an optimization.
        //
        // Note that the body is embedded in `drop-temps`; an
        // equivalent desugaring would be `return { <body>
        // };`. The key point is that we wish to drop all the
        // let-bound variables and temporaries created in the body
        // (and its tail expression!) before we drop the
        // parameters (c.f. rust-lang/rust#64512).
        for (index, parameter) in decl.inputs.iter().enumerate() {
            let parameter = self.lower_param(parameter);
            let span = parameter.pat.span;

            // Check if this is a binding pattern, if so, we can optimize and avoid adding a
            // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
            let (ident, is_simple_parameter) = match parameter.pat.kind {
                hir::PatKind::Binding(hir::BindingMode(ByRef::No, _), _, ident, _) => (ident, true),
                // For `ref mut` or wildcard arguments, we can't reuse the binding, but
                // we can keep the same name for the parameter.
                // This lets rustdoc render it correctly in documentation.
                hir::PatKind::Binding(_, _, ident, _) => (ident, false),
                hir::PatKind::Wild => (Ident::with_dummy_span(rustc_span::kw::Underscore), false),
                _ => {
                    // Replace the ident for bindings that aren't simple.
                    let name = format!("__arg{index}");
                    let ident = Ident::from_str(&name);

                    (ident, false)
                }
            };

            let desugared_span = self.mark_span_with_reason(DesugaringKind::Async, span, None);

            // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
            // async function.
            //
            // If this is the simple case, this parameter will end up being the same as the
            // original parameter, but with a different pattern id.
            let stmt_attrs = self.attrs.get(&parameter.hir_id.local_id).copied();
            let (new_parameter_pat, new_parameter_id) = self.pat_ident(desugared_span, ident);
            let new_parameter = hir::Param {
                hir_id: parameter.hir_id,
                pat: new_parameter_pat,
                ty_span: self.lower_span(parameter.ty_span),
                span: self.lower_span(parameter.span),
            };

            if is_simple_parameter {
                // If this is the simple case, then we only insert one statement that is
                // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
                // `HirId`s are densely assigned.
                let expr = self.expr_ident(desugared_span, ident, new_parameter_id);
                let stmt = self.stmt_let_pat(
                    stmt_attrs,
                    desugared_span,
                    Some(expr),
                    parameter.pat,
                    hir::LocalSource::AsyncFn,
                );
                statements.push(stmt);
            } else {
                // If this is not the simple case, then we construct two statements:
                //
                // ```
                // let __argN = __argN;
                // let <pat> = __argN;
                // ```
                //
                // The first statement moves the parameter into the closure and thus ensures
                // that the drop order is correct.
                //
                // The second statement creates the bindings that the user wrote.

                // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
                // because the user may have specified a `ref mut` binding in the next
                // statement.
                let (move_pat, move_id) =
                    self.pat_ident_binding_mode(desugared_span, ident, hir::BindingMode::MUT);
                let move_expr = self.expr_ident(desugared_span, ident, new_parameter_id);
                let move_stmt = self.stmt_let_pat(
                    None,
                    desugared_span,
                    Some(move_expr),
                    move_pat,
                    hir::LocalSource::AsyncFn,
                );

                // Construct the `let <pat> = __argN;` statement. We re-use the original
                // parameter's pattern so that `HirId`s are densely assigned.
                let pattern_expr = self.expr_ident(desugared_span, ident, move_id);
                let pattern_stmt = self.stmt_let_pat(
                    stmt_attrs,
                    desugared_span,
                    Some(pattern_expr),
                    parameter.pat,
                    hir::LocalSource::AsyncFn,
                );

                statements.push(move_stmt);
                statements.push(pattern_stmt);
            };

            parameters.push(new_parameter);
        }

        let mkbody = |this: &mut LoweringContext<'_, 'hir>| {
            // Create a block from the user's function body:
            let user_body = lower_body(this);

            // Transform into `drop-temps { <user-body> }`, an expression:
            let desugared_span =
                this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
            let user_body = this.expr_drop_temps(desugared_span, this.arena.alloc(user_body));

            // As noted above, create the final block like
            //
            // ```
            // {
            //   let $param_pattern = $raw_param;
            //   ...
            //   drop-temps { <user-body> }
            // }
            // ```
            let body = this.block_all(
                desugared_span,
                this.arena.alloc_from_iter(statements),
                Some(user_body),
            );

            this.expr_block(body)
        };
        let desugaring_kind = match coroutine_kind {
            CoroutineKind::Async { .. } => hir::CoroutineDesugaring::Async,
            CoroutineKind::Gen { .. } => hir::CoroutineDesugaring::Gen,
            CoroutineKind::AsyncGen { .. } => hir::CoroutineDesugaring::AsyncGen,
        };
        let closure_id = coroutine_kind.closure_id();

        let coroutine_expr = self.make_desugared_coroutine_expr(
            // The default capture mode here is by-ref. Later on during upvar analysis,
            // we will force the captured arguments to by-move, but for async closures,
            // we want to make sure that we avoid unnecessarily moving captures, or else
            // all async closures would default to `FnOnce` as their calling mode.
            CaptureBy::Ref,
            closure_id,
            None,
            fn_decl_span,
            body_span,
            desugaring_kind,
            coroutine_source,
            mkbody,
        );

        let expr = hir::Expr {
            hir_id: self.lower_node_id(closure_id),
            kind: coroutine_expr,
            span: self.lower_span(body_span),
        };

        (self.arena.alloc_from_iter(parameters), expr)
    }

    fn lower_method_sig(
        &mut self,
        generics: &Generics,
        sig: &FnSig,
        id: NodeId,
        kind: FnDeclKind,
        coroutine_kind: Option<CoroutineKind>,
        attrs: &[hir::Attribute],
    ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
        let header = self.lower_fn_header(sig.header, hir::Safety::Safe, attrs);
        let itctx = ImplTraitContext::Universal;
        let (generics, decl) = self.lower_generics(generics, id, itctx, |this| {
            this.lower_fn_decl(&sig.decl, id, sig.span, kind, coroutine_kind)
        });
        (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
    }

    pub(super) fn lower_fn_header(
        &mut self,
        h: FnHeader,
        default_safety: hir::Safety,
        attrs: &[hir::Attribute],
    ) -> hir::FnHeader {
        let asyncness = if let Some(CoroutineKind::Async { span, .. }) = h.coroutine_kind {
            hir::IsAsync::Async(self.lower_span(span))
        } else {
            hir::IsAsync::NotAsync
        };

        let safety = self.lower_safety(h.safety, default_safety);

        // Treat safe `#[target_feature]` functions as unsafe, but also remember that we did so.
        let safety = if find_attr!(attrs, AttributeKind::TargetFeature { was_forced: false, .. })
            && safety.is_safe()
            && !self.tcx.sess.target.is_like_wasm
        {
            hir::HeaderSafety::SafeTargetFeatures
        } else {
            safety.into()
        };

        hir::FnHeader {
            safety,
            asyncness,
            constness: self.lower_constness(h.constness),
            abi: self.lower_extern(h.ext),
        }
    }

    pub(super) fn lower_abi(&mut self, abi_str: StrLit) -> ExternAbi {
        let ast::StrLit { symbol_unescaped, span, .. } = abi_str;
        let extern_abi = symbol_unescaped.as_str().parse().unwrap_or_else(|_| {
            self.error_on_invalid_abi(abi_str);
            ExternAbi::Rust
        });
        let tcx = self.tcx;

        // we can't do codegen for unsupported ABIs, so error now so we won't get farther
        if !tcx.sess.target.is_abi_supported(extern_abi) {
            let mut err = struct_span_code_err!(
                tcx.dcx(),
                span,
                E0570,
                "{extern_abi} is not a supported ABI for the current target",
            );

            if let ExternAbi::Stdcall { unwind } = extern_abi {
                let c_abi = ExternAbi::C { unwind };
                let system_abi = ExternAbi::System { unwind };
                err.help(format!("if you need `extern {extern_abi}` on win32 and `extern {c_abi}` everywhere else, \
                    use `extern {system_abi}`"
                ));
            }
            err.emit();
        }
        // Show required feature gate even if we already errored, as the user is likely to build the code
        // for the actually intended target next and then they will need the feature gate.
        gate_unstable_abi(tcx.sess, tcx.features(), span, extern_abi);
        extern_abi
    }

    pub(super) fn lower_extern(&mut self, ext: Extern) -> ExternAbi {
        match ext {
            Extern::None => ExternAbi::Rust,
            Extern::Implicit(_) => ExternAbi::FALLBACK,
            Extern::Explicit(abi, _) => self.lower_abi(abi),
        }
    }

    fn error_on_invalid_abi(&self, abi: StrLit) {
        let abi_names = enabled_names(self.tcx.features(), abi.span)
            .iter()
            .map(|s| Symbol::intern(s))
            .collect::<Vec<_>>();
        let suggested_name = find_best_match_for_name(&abi_names, abi.symbol_unescaped, None);
        self.dcx().emit_err(InvalidAbi {
            abi: abi.symbol_unescaped,
            span: abi.span,
            suggestion: suggested_name.map(|suggested_name| InvalidAbiSuggestion {
                span: abi.span,
                suggestion: suggested_name.to_string(),
            }),
            command: "rustc --print=calling-conventions".to_string(),
        });
    }

    pub(super) fn lower_constness(&mut self, c: Const) -> hir::Constness {
        match c {
            Const::Yes(_) => hir::Constness::Const,
            Const::No => hir::Constness::NotConst,
        }
    }

    pub(super) fn lower_safety(&self, s: Safety, default: hir::Safety) -> hir::Safety {
        match s {
            Safety::Unsafe(_) => hir::Safety::Unsafe,
            Safety::Default => default,
            Safety::Safe(_) => hir::Safety::Safe,
        }
    }

    /// Return the pair of the lowered `generics` as `hir::Generics` and the evaluation of `f` with
    /// the carried impl trait definitions and bounds.
    #[instrument(level = "debug", skip(self, f))]
    fn lower_generics<T>(
        &mut self,
        generics: &Generics,
        parent_node_id: NodeId,
        itctx: ImplTraitContext,
        f: impl FnOnce(&mut Self) -> T,
    ) -> (&'hir hir::Generics<'hir>, T) {
        assert!(self.impl_trait_defs.is_empty());
        assert!(self.impl_trait_bounds.is_empty());

        let mut predicates: SmallVec<[hir::WherePredicate<'hir>; 4]> = SmallVec::new();
        predicates.extend(generics.params.iter().filter_map(|param| {
            self.lower_generic_bound_predicate(
                param.ident,
                param.id,
                &param.kind,
                &param.bounds,
                param.colon_span,
                generics.span,
                RelaxedBoundPolicy::Allowed,
                itctx,
                PredicateOrigin::GenericParam,
            )
        }));
        predicates.extend(
            generics
                .where_clause
                .predicates
                .iter()
                .map(|predicate| self.lower_where_predicate(predicate, &generics.params)),
        );

        let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> = self
            .lower_generic_params_mut(&generics.params, hir::GenericParamSource::Generics)
            .collect();

        // Introduce extra lifetimes if late resolution tells us to.
        let extra_lifetimes = self.resolver.extra_lifetime_params(parent_node_id);
        params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
            self.lifetime_res_to_generic_param(
                ident,
                node_id,
                res,
                hir::GenericParamSource::Generics,
            )
        }));

        let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
        let where_clause_span = self.lower_span(generics.where_clause.span);
        let span = self.lower_span(generics.span);
        let res = f(self);

        let impl_trait_defs = std::mem::take(&mut self.impl_trait_defs);
        params.extend(impl_trait_defs.into_iter());

        let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
        predicates.extend(impl_trait_bounds.into_iter());

        let lowered_generics = self.arena.alloc(hir::Generics {
            params: self.arena.alloc_from_iter(params),
            predicates: self.arena.alloc_from_iter(predicates),
            has_where_clause_predicates,
            where_clause_span,
            span,
        });

        (lowered_generics, res)
    }

    pub(super) fn lower_define_opaque(
        &mut self,
        hir_id: HirId,
        define_opaque: &Option<ThinVec<(NodeId, Path)>>,
    ) {
        assert_eq!(self.define_opaque, None);
        assert!(hir_id.is_owner());
        let Some(define_opaque) = define_opaque.as_ref() else {
            return;
        };
        let define_opaque = define_opaque.iter().filter_map(|(id, path)| {
            let res = self.resolver.get_partial_res(*id);
            let Some(did) = res.and_then(|res| res.expect_full_res().opt_def_id()) else {
                self.dcx().span_delayed_bug(path.span, "should have errored in resolve");
                return None;
            };
            let Some(did) = did.as_local() else {
                self.dcx().span_err(
                    path.span,
                    "only opaque types defined in the local crate can be defined",
                );
                return None;
            };
            Some((self.lower_span(path.span), did))
        });
        let define_opaque = self.arena.alloc_from_iter(define_opaque);
        self.define_opaque = Some(define_opaque);
    }

    pub(super) fn lower_generic_bound_predicate(
        &mut self,
        ident: Ident,
        id: NodeId,
        kind: &GenericParamKind,
        bounds: &[GenericBound],
        colon_span: Option<Span>,
        parent_span: Span,
        rbp: RelaxedBoundPolicy<'_>,
        itctx: ImplTraitContext,
        origin: PredicateOrigin,
    ) -> Option<hir::WherePredicate<'hir>> {
        // Do not create a clause if we do not have anything inside it.
        if bounds.is_empty() {
            return None;
        }

        let bounds = self.lower_param_bounds(bounds, rbp, itctx);

        let param_span = ident.span;

        // Reconstruct the span of the entire predicate from the individual generic bounds.
        let span_start = colon_span.unwrap_or_else(|| param_span.shrink_to_hi());
        let span = bounds.iter().fold(span_start, |span_accum, bound| {
            match bound.span().find_ancestor_inside(parent_span) {
                Some(bound_span) => span_accum.to(bound_span),
                None => span_accum,
            }
        });
        let span = self.lower_span(span);
        let hir_id = self.next_id();
        let kind = self.arena.alloc(match kind {
            GenericParamKind::Const { .. } => return None,
            GenericParamKind::Type { .. } => {
                let def_id = self.local_def_id(id).to_def_id();
                let hir_id = self.next_id();
                let res = Res::Def(DefKind::TyParam, def_id);
                let ident = self.lower_ident(ident);
                let ty_path = self.arena.alloc(hir::Path {
                    span: self.lower_span(param_span),
                    res,
                    segments: self
                        .arena
                        .alloc_from_iter([hir::PathSegment::new(ident, hir_id, res)]),
                });
                let ty_id = self.next_id();
                let bounded_ty =
                    self.ty_path(ty_id, param_span, hir::QPath::Resolved(None, ty_path));
                hir::WherePredicateKind::BoundPredicate(hir::WhereBoundPredicate {
                    bounded_ty: self.arena.alloc(bounded_ty),
                    bounds,
                    bound_generic_params: &[],
                    origin,
                })
            }
            GenericParamKind::Lifetime => {
                let lt_id = self.next_node_id();
                let lifetime =
                    self.new_named_lifetime(id, lt_id, ident, LifetimeSource::Other, ident.into());
                hir::WherePredicateKind::RegionPredicate(hir::WhereRegionPredicate {
                    lifetime,
                    bounds,
                    in_where_clause: false,
                })
            }
        });
        Some(hir::WherePredicate { hir_id, span, kind })
    }

    fn lower_where_predicate(
        &mut self,
        pred: &WherePredicate,
        params: &[ast::GenericParam],
    ) -> hir::WherePredicate<'hir> {
        let hir_id = self.lower_node_id(pred.id);
        let span = self.lower_span(pred.span);
        self.lower_attrs(hir_id, &pred.attrs, span, Target::WherePredicate);
        let kind = self.arena.alloc(match &pred.kind {
            WherePredicateKind::BoundPredicate(WhereBoundPredicate {
                bound_generic_params,
                bounded_ty,
                bounds,
            }) => {
                let rbp = if bound_generic_params.is_empty() {
                    RelaxedBoundPolicy::AllowedIfOnTyParam(bounded_ty.id, params)
                } else {
                    RelaxedBoundPolicy::Forbidden(RelaxedBoundForbiddenReason::LateBoundVarsInScope)
                };
                hir::WherePredicateKind::BoundPredicate(hir::WhereBoundPredicate {
                    bound_generic_params: self.lower_generic_params(
                        bound_generic_params,
                        hir::GenericParamSource::Binder,
                    ),
                    bounded_ty: self.lower_ty_alloc(
                        bounded_ty,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                    ),
                    bounds: self.lower_param_bounds(
                        bounds,
                        rbp,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                    ),
                    origin: PredicateOrigin::WhereClause,
                })
            }
            WherePredicateKind::RegionPredicate(WhereRegionPredicate { lifetime, bounds }) => {
                hir::WherePredicateKind::RegionPredicate(hir::WhereRegionPredicate {
                    lifetime: self.lower_lifetime(
                        lifetime,
                        LifetimeSource::Other,
                        lifetime.ident.into(),
                    ),
                    bounds: self.lower_param_bounds(
                        bounds,
                        RelaxedBoundPolicy::Allowed,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                    ),
                    in_where_clause: true,
                })
            }
            WherePredicateKind::EqPredicate(WhereEqPredicate { lhs_ty, rhs_ty }) => {
                hir::WherePredicateKind::EqPredicate(hir::WhereEqPredicate {
                    lhs_ty: self.lower_ty_alloc(
                        lhs_ty,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                    ),
                    rhs_ty: self.lower_ty_alloc(
                        rhs_ty,
                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                    ),
                })
            }
        });
        hir::WherePredicate { hir_id, span, kind }
    }
}