Rollup merge of #101493 - spastorino:borrow-mut-impl-trait-context, r=oli-obk

Pass ImplTraitContext as &mut to avoid the need of ImplTraitContext::reborrow

`@oli-obk` requested this and other changes as a way of simplifying #101345. This is just going to make the diff of #101345 smaller.

r? `@oli-obk` `@cjgillot`

7 files changed, 125 insertions, 102 deletions

diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
index b0e9fe0469c..90bb01aa216 100644
--- a/compiler/rustc_ast_lowering/src/asm.rs
+++ b/compiler/rustc_ast_lowering/src/asm.rs
@@ -220,7 +220,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 &sym.qself,
                                 &sym.path,
                                 ParamMode::Optional,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             );
                             hir::InlineAsmOperand::SymStatic { path, def_id }
                         } else {
diff --git a/compiler/rustc_ast_lowering/src/block.rs b/compiler/rustc_ast_lowering/src/block.rs
index 7cbfe143b4d..7465706d1a9 100644
--- a/compiler/rustc_ast_lowering/src/block.rs
+++ b/compiler/rustc_ast_lowering/src/block.rs
@@ -84,10 +84,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn lower_local(&mut self, l: &Local) -> &'hir hir::Local<'hir> {
-        let ty = l
-            .ty
-            .as_ref()
-            .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
+        let ty = l.ty.as_ref().map(|t| {
+            self.lower_ty(t, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Variable))
+        });
         let init = l.kind.init().map(|init| self.lower_expr(init));
         let hir_id = self.lower_node_id(l.id);
         let pat = self.lower_pat(&l.pat);
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 77babeb5d39..6fa8d7f0fcd 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -66,7 +66,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         seg,
                         ParamMode::Optional,
                         ParenthesizedGenericArgs::Err,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     ));
                     let receiver = self.lower_expr(receiver);
                     let args =
@@ -89,14 +89,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Cast(ref expr, ref ty) => {
                     let expr = self.lower_expr(expr);
-                    let ty =
-                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty = self
+                        .lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ExprKind::Cast(expr, ty)
                 }
                 ExprKind::Type(ref expr, ref ty) => {
                     let expr = self.lower_expr(expr);
-                    let ty =
-                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty = self
+                        .lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ExprKind::Type(expr, ty)
                 }
                 ExprKind::AddrOf(k, m, ref ohs) => {
@@ -219,7 +219,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     hir::ExprKind::Path(qpath)
                 }
@@ -253,7 +253,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             &se.qself,
                             &se.path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         )),
                         self.arena
                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
@@ -550,12 +550,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         async_gen_kind: hir::AsyncGeneratorKind,
         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
     ) -> hir::ExprKind<'hir> {
-        let output = match ret_ty {
-            Some(ty) => hir::FnRetTy::Return(
-                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
-            ),
-            None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
-        };
+        let output =
+            match ret_ty {
+                Some(ty) => hir::FnRetTy::Return(self.lower_ty(
+                    &ty,
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock),
+                )),
+                None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
+            };
 
         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
         // fully constrained by `future::from_generator`.
@@ -1123,7 +1125,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a tuple struct.
                     let tuple_struct_pat =
@@ -1139,7 +1141,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a unit struct.
                     let unit_struct_pat = hir::PatKind::Path(qpath);
@@ -1163,7 +1165,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &se.qself,
                     &se.path,
                     ParamMode::Optional,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 );
                 let fields_omitted = match &se.rest {
                     StructRest::Base(e) => {
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 76f63d1d78a..f0717b51d4b 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -264,8 +264,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let body_id =
                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 
-                    let itctx = ImplTraitContext::Universal;
-                    let (generics, decl) = this.lower_generics(generics, id, itctx, |this| {
+                    let mut itctx = ImplTraitContext::Universal;
+                    let (generics, decl) = this.lower_generics(generics, id, &mut itctx, |this| {
                         let ret_id = asyncness.opt_return_id();
                         this.lower_fn_decl(&decl, Some(id), FnDeclKind::Fn, ret_id)
                     });
@@ -311,8 +311,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    |this| this.lower_ty(ty, &mut ImplTraitContext::TypeAliasesOpaqueTy),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
             }
@@ -324,7 +324,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
@@ -333,7 +333,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, variants) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.arena.alloc_from_iter(
                             enum_definition.variants.iter().map(|x| this.lower_variant(x)),
@@ -346,7 +346,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, struct_def) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, struct_def),
                 );
                 hir::ItemKind::Struct(struct_def, generics)
@@ -355,7 +355,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, vdata) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, vdata),
                 );
                 hir::ItemKind::Union(vdata, generics)
@@ -383,18 +383,20 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 // 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 mut itctx = ImplTraitContext::Universal;
                 let (generics, (trait_ref, lowered_ty)) =
-                    self.lower_generics(ast_generics, id, itctx, |this| {
+                    self.lower_generics(ast_generics, id, &mut itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
                                 trait_ref,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
+                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )
                         });
 
-                        let lowered_ty = this
-                            .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        let lowered_ty = this.lower_ty(
+                            ty,
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
+                        );
 
                         (trait_ref, lowered_ty)
                     });
@@ -433,11 +435,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let bounds = this.lower_param_bounds(
                             bounds,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         );
                         let items = this.arena.alloc_from_iter(
                             items.iter().map(|item| this.lower_trait_item_ref(item)),
@@ -452,11 +454,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, bounds) = self.lower_generics(
                     generics,
                     id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.lower_param_bounds(
                             bounds,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         )
                     },
                 );
@@ -479,7 +481,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         body: Option<&Expr>,
     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+        let ty = self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
         (ty, self.lower_const_body(span, body))
     }
 
@@ -652,9 +654,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
             kind: match i.kind {
                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
                     let fdec = &sig.decl;
-                    let itctx = ImplTraitContext::Universal;
+                    let mut itctx = ImplTraitContext::Universal;
                     let (generics, (fn_dec, fn_args)) =
-                        self.lower_generics(generics, i.id, itctx, |this| {
+                        self.lower_generics(generics, i.id, &mut itctx, |this| {
                             (
                                 // Disallow `impl Trait` in foreign items.
                                 this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
@@ -665,8 +667,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
                 }
                 ForeignItemKind::Static(ref t, m, _) => {
-                    let ty =
-                        self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty = self
+                        .lower_ty(t, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ForeignItemKind::Static(ty, m)
                 }
                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
@@ -734,11 +736,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 qself,
                 path,
                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
-                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
             );
             self.arena.alloc(t)
         } else {
-            self.lower_ty(&f.ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+            self.lower_ty(&f.ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type))
         };
         let hir_id = self.lower_node_id(f.id);
         self.lower_attrs(hir_id, &f.attrs);
@@ -761,7 +763,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind, has_default) = match i.kind {
             AssocItemKind::Const(_, ref ty, ref default) => {
-                let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty =
+                    self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
             }
@@ -796,15 +799,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, kind) = self.lower_generics(
                     &generics,
                     i.id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let ty = ty.as_ref().map(|x| {
-                            this.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                            this.lower_ty(
+                                x,
+                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
+                            )
                         });
                         hir::TraitItemKind::Type(
                             this.lower_param_bounds(
                                 bounds,
-                                ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                             ),
                             ty,
                         )
@@ -857,7 +863,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind) = match &i.kind {
             AssocItemKind::Const(_, ty, expr) => {
-                let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty =
+                    self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 (
                     hir::Generics::empty(),
                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
@@ -884,14 +891,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.lower_generics(
                     &generics,
                     i.id,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| match ty {
                         None => {
                             let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
                             hir::ImplItemKind::TyAlias(ty)
                         }
                         Some(ty) => {
-                            let ty = this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy);
+                            let ty = this.lower_ty(ty, &mut ImplTraitContext::TypeAliasesOpaqueTy);
                             hir::ImplItemKind::TyAlias(ty)
                         }
                     },
@@ -1234,8 +1241,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         is_async: Option<NodeId>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
-        let itctx = ImplTraitContext::Universal;
-        let (generics, decl) = self.lower_generics(generics, id, itctx, |this| {
+        let mut itctx = ImplTraitContext::Universal;
+        let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
             this.lower_fn_decl(&sig.decl, Some(id), kind, is_async)
         });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
@@ -1301,7 +1308,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         generics: &Generics,
         parent_node_id: NodeId,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
         f: impl FnOnce(&mut Self) -> T,
     ) -> (&'hir hir::Generics<'hir>, T) {
         debug_assert!(self.impl_trait_defs.is_empty());
@@ -1406,7 +1413,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         id: NodeId,
         kind: &GenericParamKind,
         bounds: &[GenericBound],
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
         origin: PredicateOrigin,
     ) -> Option<hir::WherePredicate<'hir>> {
         // Do not create a clause if we do not have anything inside it.
@@ -1481,12 +1488,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 span,
             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
                 bound_generic_params: self.lower_generic_params(bound_generic_params),
-                bounded_ty: self
-                    .lower_ty(bounded_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                bounded_ty: self.lower_ty(
+                    bounded_ty,
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
+                ),
                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
                     self.lower_param_bound(
                         bound,
-                        ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                     )
                 })),
                 span: self.lower_span(span),
@@ -1501,16 +1510,20 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 lifetime: self.lower_lifetime(lifetime),
                 bounds: self.lower_param_bounds(
                     bounds,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                 ),
                 in_where_clause: true,
             }),
             WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, span }) => {
                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
-                    lhs_ty: self
-                        .lower_ty(lhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
-                    rhs_ty: self
-                        .lower_ty(rhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                    lhs_ty: self.lower_ty(
+                        lhs_ty,
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
+                    ),
+                    rhs_ty: self.lower_ty(
+                        rhs_ty,
+                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
+                    ),
                     span: self.lower_span(span),
                 })
             }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 29baa3a80c1..11f25d66348 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -953,7 +953,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_assoc_ty_constraint(
         &mut self,
         constraint: &AssocConstraint,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::TypeBinding<'hir> {
         debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", constraint, itctx);
         // lower generic arguments of identifier in constraint
@@ -976,6 +976,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
+        let mut itctx_tait = ImplTraitContext::TypeAliasesOpaqueTy;
 
         let kind = match constraint.kind {
             AssocConstraintKind::Equality { ref term } => {
@@ -1014,7 +1015,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     //
                     // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
                     ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => {
-                        (true, ImplTraitContext::TypeAliasesOpaqueTy)
+                        (true, &mut itctx_tait)
                     }
 
                     // We are in the parameter position, but not within a dyn type:
@@ -1096,7 +1097,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_generic_arg(
         &mut self,
         arg: &ast::GenericArg,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::GenericArg<'hir> {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(&lt)),
@@ -1158,7 +1159,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn lower_ty(&mut self, t: &Ty, itctx: ImplTraitContext) -> &'hir hir::Ty<'hir> {
+    fn lower_ty(&mut self, t: &Ty, itctx: &mut ImplTraitContext) -> &'hir hir::Ty<'hir> {
         self.arena.alloc(self.lower_ty_direct(t, itctx))
     }
 
@@ -1168,7 +1169,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<QSelf>,
         path: &Path,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::Ty<'hir> {
         // Check whether we should interpret this as a bare trait object.
         // This check mirrors the one in late resolution.  We only introduce this special case in
@@ -1210,7 +1211,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.ty(span, hir::TyKind::Tup(tys))
     }
 
-    fn lower_ty_direct(&mut self, t: &Ty, itctx: ImplTraitContext) -> hir::Ty<'hir> {
+    fn lower_ty_direct(&mut self, t: &Ty, itctx: &mut ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err => hir::TyKind::Err,
@@ -1307,16 +1308,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let span = t.span;
                 match itctx {
                     ImplTraitContext::ReturnPositionOpaqueTy { origin } => {
-                        self.lower_opaque_impl_trait(span, origin, def_node_id, bounds, itctx)
+                        self.lower_opaque_impl_trait(span, *origin, def_node_id, bounds, itctx)
                     }
                     ImplTraitContext::TypeAliasesOpaqueTy => {
-                        let nested_itctx = ImplTraitContext::TypeAliasesOpaqueTy;
+                        let mut nested_itctx = ImplTraitContext::TypeAliasesOpaqueTy;
                         self.lower_opaque_impl_trait(
                             span,
                             hir::OpaqueTyOrigin::TyAlias,
                             def_node_id,
                             bounds,
-                            nested_itctx,
+                            &mut nested_itctx,
                         )
                     }
                     ImplTraitContext::Universal => {
@@ -1388,7 +1389,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         origin: hir::OpaqueTyOrigin,
         opaque_ty_node_id: NodeId,
         bounds: &GenericBounds,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::TyKind<'hir> {
         // Make sure we know that some funky desugaring has been going on here.
         // This is a first: there is code in other places like for loop
@@ -1635,11 +1636,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
         let inputs = self.arena.alloc_from_iter(inputs.iter().map(|param| {
             if fn_node_id.is_some() {
-                self.lower_ty_direct(&param.ty, ImplTraitContext::Universal)
+                self.lower_ty_direct(&param.ty, &mut ImplTraitContext::Universal)
             } else {
                 self.lower_ty_direct(
                     &param.ty,
-                    ImplTraitContext::Disallowed(match kind {
+                    &mut ImplTraitContext::Disallowed(match kind {
                         FnDeclKind::Fn | FnDeclKind::Inherent => {
                             unreachable!("fn should allow in-band lifetimes")
                         }
@@ -1662,7 +1663,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             match decl.output {
                 FnRetTy::Ty(ref ty) => {
-                    let context = match fn_node_id {
+                    let mut context = match fn_node_id {
                         Some(fn_node_id) if kind.impl_trait_return_allowed() => {
                             let fn_def_id = self.local_def_id(fn_node_id);
                             ImplTraitContext::ReturnPositionOpaqueTy {
@@ -1680,7 +1681,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             FnDeclKind::Impl => ImplTraitPosition::ImplReturn,
                         }),
                     };
-                    hir::FnRetTy::Return(self.lower_ty(ty, context))
+                    hir::FnRetTy::Return(self.lower_ty(ty, &mut context))
                 }
                 FnRetTy::Default(span) => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
             }
@@ -1959,10 +1960,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // Not `OpaqueTyOrigin::AsyncFn`: that's only used for the
                 // `impl Future` opaque type that `async fn` implicitly
                 // generates.
-                let context = ImplTraitContext::ReturnPositionOpaqueTy {
+                let mut context = ImplTraitContext::ReturnPositionOpaqueTy {
                     origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
                 };
-                self.lower_ty(ty, context)
+                self.lower_ty(ty, &mut context)
             }
             FnRetTy::Default(ret_ty_span) => self.arena.alloc(self.ty_tup(*ret_ty_span, &[])),
         };
@@ -1988,7 +1989,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bound(
         &mut self,
         tpb: &GenericBound,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         match tpb {
             GenericBound::Trait(p, modifier) => hir::GenericBound::Trait(
@@ -2100,7 +2101,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             GenericParamKind::Type { ref default, .. } => {
                 let kind = hir::GenericParamKind::Type {
                     default: default.as_ref().map(|x| {
-                        self.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                        self.lower_ty(x, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type))
                     }),
                     synthetic: false,
                 };
@@ -2108,7 +2109,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 (hir::ParamName::Plain(self.lower_ident(param.ident)), kind)
             }
             GenericParamKind::Const { ref ty, kw_span: _, ref default } => {
-                let ty = self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty =
+                    self.lower_ty(&ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 let default = default.as_ref().map(|def| self.lower_anon_const(def));
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
@@ -2118,7 +2120,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_trait_ref(&mut self, p: &TraitRef, itctx: ImplTraitContext) -> hir::TraitRef<'hir> {
+    fn lower_trait_ref(
+        &mut self,
+        p: &TraitRef,
+        itctx: &mut ImplTraitContext,
+    ) -> hir::TraitRef<'hir> {
         let path = match self.lower_qpath(p.ref_id, &None, &p.path, ParamMode::Explicit, itctx) {
             hir::QPath::Resolved(None, path) => path,
             qpath => panic!("lower_trait_ref: unexpected QPath `{:?}`", qpath),
@@ -2130,7 +2136,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_poly_trait_ref(
         &mut self,
         p: &PolyTraitRef,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
@@ -2138,23 +2144,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::PolyTraitRef { bound_generic_params, trait_ref, span: self.lower_span(p.span) }
     }
 
-    fn lower_mt(&mut self, mt: &MutTy, itctx: ImplTraitContext) -> hir::MutTy<'hir> {
+    fn lower_mt(&mut self, mt: &MutTy, itctx: &mut ImplTraitContext) -> hir::MutTy<'hir> {
         hir::MutTy { ty: self.lower_ty(&mt.ty, itctx), mutbl: mt.mutbl }
     }
 
     fn lower_param_bounds(
         &mut self,
         bounds: &[GenericBound],
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::GenericBounds<'hir> {
         self.arena.alloc_from_iter(self.lower_param_bounds_mut(bounds, itctx))
     }
 
-    fn lower_param_bounds_mut<'s>(
+    fn lower_param_bounds_mut<'s, 'b>(
         &'s mut self,
         bounds: &'s [GenericBound],
-        itctx: ImplTraitContext,
-    ) -> impl Iterator<Item = hir::GenericBound<'hir>> + Captures<'s> + Captures<'a> {
+        itctx: &'b mut ImplTraitContext,
+    ) -> impl Iterator<Item = hir::GenericBound<'hir>> + Captures<'s> + Captures<'a> + Captures<'b>
+    {
         bounds.iter().map(move |bound| self.lower_param_bound(bound, itctx))
     }
 
@@ -2183,7 +2190,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             node_id,
             &GenericParamKind::Type { default: None },
             bounds,
-            ImplTraitContext::Universal,
+            &mut ImplTraitContext::Universal,
             hir::PredicateOrigin::ImplTrait,
         );
 
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index a1eee1be798..69e6c2c8ad0 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -37,7 +37,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         let (pats, ddpos) = self.lower_pat_tuple(pats, "tuple struct");
                         break hir::PatKind::TupleStruct(qpath, pats, ddpos);
@@ -53,7 +53,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         break hir::PatKind::Path(qpath);
                     }
@@ -63,7 +63,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
 
                         let fs = self.arena.alloc_from_iter(fields.iter().map(|f| {
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index de1467b1b07..a3d864023a9 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -22,7 +22,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<QSelf>,
         p: &Path,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::QPath<'hir> {
         let qself_position = qself.as_ref().map(|q| q.position);
         let qself = qself.as_ref().map(|q| self.lower_ty(&q.ty, itctx));
@@ -156,7 +156,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     segment,
                     param_mode,
                     ParenthesizedGenericArgs::Err,
-                    ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 )
             })),
             span: self.lower_span(p.span),
@@ -180,7 +180,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         segment: &PathSegment,
         param_mode: ParamMode,
         parenthesized_generic_args: ParenthesizedGenericArgs,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> hir::PathSegment<'hir> {
         debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment,);
         let (mut generic_args, infer_args) = if let Some(ref generic_args) = segment.args {
@@ -316,7 +316,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         data: &AngleBracketedArgs,
         param_mode: ParamMode,
-        itctx: ImplTraitContext,
+        itctx: &mut ImplTraitContext,
     ) -> (GenericArgsCtor<'hir>, bool) {
         let has_non_lt_args = data.args.iter().any(|arg| match arg {
             AngleBracketedArg::Arg(ast::GenericArg::Lifetime(_))
@@ -350,12 +350,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // we generally don't permit such things (see #51008).
         let ParenthesizedArgs { span, inputs, inputs_span, output } = data;
         let inputs = self.arena.alloc_from_iter(inputs.iter().map(|ty| {
-            self.lower_ty_direct(ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
+            self.lower_ty_direct(
+                ty,
+                &mut ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam),
+            )
         }));
         let output_ty = match output {
-            FnRetTy::Ty(ty) => {
-                self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
-            }
+            FnRetTy::Ty(ty) => self
+                .lower_ty(&ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn)),
             FnRetTy::Default(_) => self.arena.alloc(self.ty_tup(*span, &[])),
         };
         let args = smallvec![GenericArg::Type(self.arena.alloc(self.ty_tup(*inputs_span, inputs)))];