Pass ImplTraitContext as &, there's no need for that to be &mut

6 files changed, 84 insertions, 107 deletions

diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
index 90bb01aa216..24672efc63c 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,
-                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                                &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 7465706d1a9..e0869bb1063 100644
--- a/compiler/rustc_ast_lowering/src/block.rs
+++ b/compiler/rustc_ast_lowering/src/block.rs
@@ -84,9 +84,10 @@ 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, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Variable))
-        });
+        let ty = l
+            .ty
+            .as_ref()
+            .map(|t| self.lower_ty(t, &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 2993f1939ea..506b69f8789 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,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &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, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty =
+                        self.lower_ty(ty, &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, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty =
+                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ExprKind::Type(expr, ty)
                 }
                 ExprKind::AddrOf(k, m, ref ohs) => {
@@ -225,7 +225,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     hir::ExprKind::Path(qpath)
                 }
@@ -259,7 +259,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             &se.qself,
                             &se.path,
                             ParamMode::Optional,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         )),
                         self.arena
                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
@@ -556,14 +556,12 @@ 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,
-                    &mut 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, &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`.
@@ -1131,7 +1129,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a tuple struct.
                     let tuple_struct_pat = hir::PatKind::TupleStruct(
@@ -1150,7 +1148,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     );
                     // Destructure like a unit struct.
                     let unit_struct_pat = hir::PatKind::Path(qpath);
@@ -1174,7 +1172,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &se.qself,
                     &se.path,
                     ParamMode::Optional,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &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 52273778dcc..550833275e4 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -313,8 +313,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.lower_ty(ty, &mut ImplTraitContext::TypeAliasesOpaqueTy),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    |this| this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
             }
@@ -326,7 +326,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, ty) = self.lower_generics(
                     &generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
                 );
                 hir::ItemKind::TyAlias(ty, generics)
@@ -335,7 +335,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, variants) = self.lower_generics(
                     generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.arena.alloc_from_iter(
                             enum_definition.variants.iter().map(|x| this.lower_variant(x)),
@@ -348,7 +348,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, struct_def) = self.lower_generics(
                     generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, struct_def),
                 );
                 hir::ItemKind::Struct(struct_def, generics)
@@ -357,7 +357,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, vdata) = self.lower_generics(
                     generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| this.lower_variant_data(hir_id, vdata),
                 );
                 hir::ItemKind::Union(vdata, generics)
@@ -391,14 +391,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
                                 trait_ref,
-                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
                             )
                         });
 
-                        let lowered_ty = this.lower_ty(
-                            ty,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
-                        );
+                        let lowered_ty = this
+                            .lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 
                         (trait_ref, lowered_ty)
                     });
@@ -437,11 +435,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
                     generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let bounds = this.lower_param_bounds(
                             bounds,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         );
                         let items = this.arena.alloc_from_iter(
                             items.iter().map(|item| this.lower_trait_item_ref(item)),
@@ -456,11 +454,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, bounds) = self.lower_generics(
                     generics,
                     id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         this.lower_param_bounds(
                             bounds,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                         )
                     },
                 );
@@ -483,7 +481,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         body: Option<&Expr>,
     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
         (ty, self.lower_const_body(span, body))
     }
 
@@ -675,8 +673,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
                 }
                 ForeignItemKind::Static(ref t, m, _) => {
-                    let ty = self
-                        .lower_ty(t, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty =
+                        self.lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                     hir::ForeignItemKind::Static(ty, m)
                 }
                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
@@ -744,11 +742,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 qself,
                 path,
                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
-                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
             );
             self.arena.alloc(t)
         } else {
-            self.lower_ty(&f.ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+            self.lower_ty(&f.ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
         };
         let hir_id = self.lower_node_id(f.id);
         self.lower_attrs(hir_id, &f.attrs);
@@ -771,8 +769,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind, has_default) = match i.kind {
             AssocItemKind::Const(_, ref ty, ref default) => {
-                let ty =
-                    self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(ty, &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())
             }
@@ -813,18 +810,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let (generics, kind) = self.lower_generics(
                     &generics,
                     i.id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let ty = ty.as_ref().map(|x| {
-                            this.lower_ty(
-                                x,
-                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
-                            )
+                            this.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
                         });
                         hir::TraitItemKind::Type(
                             this.lower_param_bounds(
                                 bounds,
-                                &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                             ),
                             ty,
                         )
@@ -877,8 +871,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind) = match &i.kind {
             AssocItemKind::Const(_, ty, expr) => {
-                let ty =
-                    self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 (
                     hir::Generics::empty(),
                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
@@ -905,14 +898,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.lower_generics(
                     &generics,
                     i.id,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
+                    &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, &mut ImplTraitContext::TypeAliasesOpaqueTy);
+                            let ty = this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy);
                             hir::ImplItemKind::TyAlias(ty)
                         }
                     },
@@ -1322,7 +1315,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         generics: &Generics,
         parent_node_id: NodeId,
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
         f: impl FnOnce(&mut Self) -> T,
     ) -> (&'hir hir::Generics<'hir>, T) {
         debug_assert!(self.impl_trait_defs.is_empty());
@@ -1427,7 +1420,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         id: NodeId,
         kind: &GenericParamKind,
         bounds: &[GenericBound],
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
         origin: PredicateOrigin,
     ) -> Option<hir::WherePredicate<'hir>> {
         // Do not create a clause if we do not have anything inside it.
@@ -1502,14 +1495,12 @@ 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,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
-                ),
+                bounded_ty: self
+                    .lower_ty(bounded_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
                     self.lower_param_bound(
                         bound,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                        &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
                     )
                 })),
                 span: self.lower_span(span),
@@ -1524,20 +1515,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 lifetime: self.lower_lifetime(lifetime),
                 bounds: self.lower_param_bounds(
                     bounds,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
+                    &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,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
-                    ),
-                    rhs_ty: self.lower_ty(
-                        rhs_ty,
-                        &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
-                    ),
+                    lhs_ty: self
+                        .lower_ty(lhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                    rhs_ty: self
+                        .lower_ty(rhs_ty, &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 c96e419566d..bda80c3eb19 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -984,7 +984,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_assoc_ty_constraint(
         &mut self,
         constraint: &AssocConstraint,
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::TypeBinding<'hir> {
         debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", constraint, itctx);
         // lower generic arguments of identifier in constraint
@@ -1003,7 +1003,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
-        let mut itctx_tait = ImplTraitContext::TypeAliasesOpaqueTy;
+        let itctx_tait = &ImplTraitContext::TypeAliasesOpaqueTy;
 
         let kind = match constraint.kind {
             AssocConstraintKind::Equality { ref term } => {
@@ -1041,9 +1041,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // then to an opaque type).
                     //
                     // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
-                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => {
-                        (true, &mut itctx_tait)
-                    }
+                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => (true, itctx_tait),
 
                     // We are in the parameter position, but not within a dyn type:
                     //
@@ -1122,7 +1120,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_generic_arg(
         &mut self,
         arg: &ast::GenericArg,
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericArg<'hir> {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(&lt)),
@@ -1184,7 +1182,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn lower_ty(&mut self, t: &Ty, itctx: &mut ImplTraitContext) -> &'hir hir::Ty<'hir> {
+    fn lower_ty(&mut self, t: &Ty, itctx: &ImplTraitContext) -> &'hir hir::Ty<'hir> {
         self.arena.alloc(self.lower_ty_direct(t, itctx))
     }
 
@@ -1194,7 +1192,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<QSelf>,
         path: &Path,
         param_mode: ParamMode,
-        itctx: &mut ImplTraitContext,
+        itctx: &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
@@ -1237,7 +1235,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.ty(span, hir::TyKind::Tup(tys))
     }
 
-    fn lower_ty_direct(&mut self, t: &Ty, itctx: &mut ImplTraitContext) -> hir::Ty<'hir> {
+    fn lower_ty_direct(&mut self, t: &Ty, itctx: &ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match t.kind {
             TyKind::Infer => hir::TyKind::Infer,
             TyKind::Err => hir::TyKind::Err,
@@ -1348,7 +1346,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         def_node_id,
                         bounds,
                         false,
-                        &mut ImplTraitContext::TypeAliasesOpaqueTy,
+                        &ImplTraitContext::TypeAliasesOpaqueTy,
                     ),
                     ImplTraitContext::Universal => {
                         let span = t.span;
@@ -1435,7 +1433,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         opaque_ty_node_id: NodeId,
         bounds: &GenericBounds,
         in_trait: bool,
-        itctx: &mut ImplTraitContext,
+        itctx: &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
@@ -1681,11 +1679,11 @@ 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, &mut ImplTraitContext::Universal)
+                self.lower_ty_direct(&param.ty, &ImplTraitContext::Universal)
             } else {
                 self.lower_ty_direct(
                     &param.ty,
-                    &mut ImplTraitContext::Disallowed(match kind {
+                    &ImplTraitContext::Disallowed(match kind {
                         FnDeclKind::Fn | FnDeclKind::Inherent => {
                             unreachable!("fn should allow in-band lifetimes")
                         }
@@ -2084,7 +2082,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bound(
         &mut self,
         tpb: &GenericBound,
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         match tpb {
             GenericBound::Trait(p, modifier) => hir::GenericBound::Trait(
@@ -2200,7 +2198,7 @@ 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, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                        self.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
                     }),
                     synthetic: false,
                 };
@@ -2208,8 +2206,7 @@ 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, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
                 let default = default.as_ref().map(|def| self.lower_anon_const(def));
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
@@ -2219,11 +2216,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
-    fn lower_trait_ref(
-        &mut self,
-        p: &TraitRef,
-        itctx: &mut ImplTraitContext,
-    ) -> hir::TraitRef<'hir> {
+    fn lower_trait_ref(&mut self, p: &TraitRef, itctx: &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),
@@ -2235,7 +2228,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_poly_trait_ref(
         &mut self,
         p: &PolyTraitRef,
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::PolyTraitRef<'hir> {
         let bound_generic_params =
             self.lower_lifetime_binder(p.trait_ref.ref_id, &p.bound_generic_params);
@@ -2243,7 +2236,7 @@ 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: &mut ImplTraitContext) -> hir::MutTy<'hir> {
+    fn lower_mt(&mut self, mt: &MutTy, itctx: &ImplTraitContext) -> hir::MutTy<'hir> {
         hir::MutTy { ty: self.lower_ty(&mt.ty, itctx), mutbl: mt.mutbl }
     }
 
@@ -2251,7 +2244,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bounds(
         &mut self,
         bounds: &[GenericBound],
-        itctx: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> hir::GenericBounds<'hir> {
         self.arena.alloc_from_iter(self.lower_param_bounds_mut(bounds, itctx))
     }
@@ -2259,7 +2252,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_param_bounds_mut<'s, 'b>(
         &'s mut self,
         bounds: &'s [GenericBound],
-        itctx: &'b mut ImplTraitContext,
+        itctx: &'b ImplTraitContext,
     ) -> impl Iterator<Item = hir::GenericBound<'hir>> + Captures<'s> + Captures<'a> + Captures<'b>
     {
         bounds.iter().map(move |bound| self.lower_param_bound(bound, itctx))
@@ -2291,7 +2284,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             node_id,
             &GenericParamKind::Type { default: None },
             bounds,
-            &mut ImplTraitContext::Universal,
+            &ImplTraitContext::Universal,
             hir::PredicateOrigin::ImplTrait,
         );
 
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index a3d864023a9..6bb1bb9eace 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: &mut ImplTraitContext,
+        itctx: &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,
-                    &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                    &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: &mut ImplTraitContext,
+        itctx: &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: &mut ImplTraitContext,
+        itctx: &ImplTraitContext,
     ) -> (GenericArgsCtor<'hir>, bool) {
         let has_non_lt_args = data.args.iter().any(|arg| match arg {
             AngleBracketedArg::Arg(ast::GenericArg::Lifetime(_))
@@ -350,14 +350,12 @@ 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,
-                &mut ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam),
-            )
+            self.lower_ty_direct(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
         }));
         let output_ty = match output {
-            FnRetTy::Ty(ty) => self
-                .lower_ty(&ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn)),
+            FnRetTy::Ty(ty) => {
+                self.lower_ty(&ty, &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)))];