Impl trait diagnostic tweaks

compiler/rustc_ast_lowering/messages.ftl

@@ -106,7 +106,8 @@ ast_lowering_misplaced_double_dot =
     .note = only allowed in tuple, tuple struct, and slice patterns
 
 ast_lowering_misplaced_impl_trait =
-    `impl Trait` only allowed in function and inherent method argument and return types, not in {$position}
+    `impl Trait` is not allowed in {$position}
+    .note = `impl Trait` is only allowed in arguments and return types of functions and methods
 
 ast_lowering_misplaced_relax_trait_bound =
     `?Trait` bounds are only permitted at the point where a type parameter is declared

compiler/rustc_ast_lowering/src/errors.rs

@@ -90,6 +90,7 @@ pub enum AssocTyParenthesesSub {
 
 #[derive(Diagnostic)]
 #[diag(ast_lowering_misplaced_impl_trait, code = "E0562")]
+#[note]
 pub struct MisplacedImplTrait<'a> {
     #[primary_span]
     pub span: Span,

compiler/rustc_ast_lowering/src/item.rs

@@ -182,7 +182,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.lower_use_tree(use_tree, &prefix, id, vis_span, ident, attrs)
             }
             ItemKind::Static(box ast::StaticItem { ty: t, mutability: m, expr: e }) => {
-                let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
+                let (ty, body_id) =
+                    self.lower_const_item(t, span, e.as_deref(), ImplTraitPosition::StaticTy);
                 hir::ItemKind::Static(ty, *m, body_id)
             }
             ItemKind::Const(box ast::ConstItem { generics, ty, expr, .. }) => {
@@ -191,7 +192,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     Const::No,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.lower_const_item(ty, span, expr.as_deref()),
+                    |this| {
+                        this.lower_const_item(ty, span, expr.as_deref(), ImplTraitPosition::ConstTy)
+                    },
                 );
                 hir::ItemKind::Const(ty, generics, body_id)
             }
@@ -448,8 +451,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
         ty: &Ty,
         span: Span,
         body: Option<&Expr>,
+        impl_trait_position: ImplTraitPosition,
     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
+        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(impl_trait_position));
         (ty, self.lower_const_body(span, body))
     }
 

compiler/rustc_ast_lowering/src/lib.rs

@@ -304,8 +304,6 @@ enum ImplTraitPosition {
     ClosureParam,
     PointerParam,
     FnTraitParam,
-    TraitParam,
-    ImplParam,
     ExternFnReturn,
     ClosureReturn,
     PointerReturn,
@@ -324,29 +322,27 @@ impl std::fmt::Display for ImplTraitPosition {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let name = match self {
             ImplTraitPosition::Path => "paths",
-            ImplTraitPosition::Variable => "variable bindings",
+            ImplTraitPosition::Variable => "the type of variable bindings",
             ImplTraitPosition::Trait => "traits",
             ImplTraitPosition::AsyncBlock => "async blocks",
             ImplTraitPosition::Bound => "bounds",
             ImplTraitPosition::Generic => "generics",
-            ImplTraitPosition::ExternFnParam => "`extern fn` params",
-            ImplTraitPosition::ClosureParam => "closure params",
-            ImplTraitPosition::PointerParam => "`fn` pointer params",
-            ImplTraitPosition::FnTraitParam => "`Fn` trait params",
-            ImplTraitPosition::TraitParam => "trait method params",
-            ImplTraitPosition::ImplParam => "`impl` method params",
+            ImplTraitPosition::ExternFnParam => "`extern fn` parameters",
+            ImplTraitPosition::ClosureParam => "closure parameters",
+            ImplTraitPosition::PointerParam => "`fn` pointer parameters",
+            ImplTraitPosition::FnTraitParam => "the parameters of `Fn` trait bounds",
             ImplTraitPosition::ExternFnReturn => "`extern fn` return types",
             ImplTraitPosition::ClosureReturn => "closure return types",
             ImplTraitPosition::PointerReturn => "`fn` pointer return types",
-            ImplTraitPosition::FnTraitReturn => "`Fn` trait return types",
+            ImplTraitPosition::FnTraitReturn => "the return type of `Fn` trait bounds",
             ImplTraitPosition::GenericDefault => "generic parameter defaults",
             ImplTraitPosition::ConstTy => "const types",
             ImplTraitPosition::StaticTy => "static types",
             ImplTraitPosition::AssocTy => "associated types",
             ImplTraitPosition::FieldTy => "field types",
-            ImplTraitPosition::Cast => "cast types",
+            ImplTraitPosition::Cast => "cast expression types",
             ImplTraitPosition::ImplSelf => "impl headers",
-            ImplTraitPosition::OffsetOf => "`offset_of!` params",
+            ImplTraitPosition::OffsetOf => "`offset_of!` parameters",
         };
 
         write!(f, "{name}")
@@ -364,19 +360,6 @@ enum FnDeclKind {
     Impl,
 }
 
-impl FnDeclKind {
-    fn param_impl_trait_allowed(&self) -> bool {
-        matches!(self, FnDeclKind::Fn | FnDeclKind::Inherent | FnDeclKind::Impl | FnDeclKind::Trait)
-    }
-
-    fn return_impl_trait_allowed(&self) -> bool {
-        match self {
-            FnDeclKind::Fn | FnDeclKind::Inherent | FnDeclKind::Impl | FnDeclKind::Trait => true,
-            _ => false,
-        }
-    }
-}
-
 #[derive(Copy, Clone)]
 enum AstOwner<'a> {
     NonOwner,
@@ -1842,19 +1825,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             inputs = &inputs[..inputs.len() - 1];
         }
         let inputs = self.arena.alloc_from_iter(inputs.iter().map(|param| {
-            let itctx = if kind.param_impl_trait_allowed() {
-                ImplTraitContext::Universal
-            } else {
-                ImplTraitContext::Disallowed(match kind {
-                    FnDeclKind::Fn | FnDeclKind::Inherent => {
-                        unreachable!("fn should allow APIT")
-                    }
-                    FnDeclKind::ExternFn => ImplTraitPosition::ExternFnParam,
-                    FnDeclKind::Closure => ImplTraitPosition::ClosureParam,
-                    FnDeclKind::Pointer => ImplTraitPosition::PointerParam,
-                    FnDeclKind::Trait => ImplTraitPosition::TraitParam,
-                    FnDeclKind::Impl => ImplTraitPosition::ImplParam,
-                })
+            let itctx = match kind {
+                FnDeclKind::Fn | FnDeclKind::Inherent | FnDeclKind::Impl | FnDeclKind::Trait => {
+                    ImplTraitContext::Universal
+                }
+                FnDeclKind::ExternFn => {
+                    ImplTraitContext::Disallowed(ImplTraitPosition::ExternFnParam)
+                }
+                FnDeclKind::Closure => {
+                    ImplTraitContext::Disallowed(ImplTraitPosition::ClosureParam)
+                }
+                FnDeclKind::Pointer => {
+                    ImplTraitContext::Disallowed(ImplTraitPosition::PointerParam)
+                }
             };
             self.lower_ty_direct(&param.ty, &itctx)
         }));
@@ -1866,26 +1849,25 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             }
             None => match &decl.output {
                 FnRetTy::Ty(ty) => {
-                    let context = if kind.return_impl_trait_allowed() {
-                        let fn_def_id = self.local_def_id(fn_node_id);
-                        ImplTraitContext::ReturnPositionOpaqueTy {
-                            origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
+                    let itctx = match kind {
+                        FnDeclKind::Fn
+                        | FnDeclKind::Inherent
+                        | FnDeclKind::Trait
+                        | FnDeclKind::Impl => ImplTraitContext::ReturnPositionOpaqueTy {
+                            origin: hir::OpaqueTyOrigin::FnReturn(self.local_def_id(fn_node_id)),
                             fn_kind: kind,
+                        },
+                        FnDeclKind::ExternFn => {
+                            ImplTraitContext::Disallowed(ImplTraitPosition::ExternFnReturn)
+                        }
+                        FnDeclKind::Closure => {
+                            ImplTraitContext::Disallowed(ImplTraitPosition::ClosureReturn)
+                        }
+                        FnDeclKind::Pointer => {
+                            ImplTraitContext::Disallowed(ImplTraitPosition::PointerReturn)
                         }
-                    } else {
-                        ImplTraitContext::Disallowed(match kind {
-                            FnDeclKind::Fn
-                            | FnDeclKind::Inherent
-                            | FnDeclKind::Trait
-                            | FnDeclKind::Impl => {
-                                unreachable!("fn should allow return-position impl trait in traits")
-                            }
-                            FnDeclKind::ExternFn => ImplTraitPosition::ExternFnReturn,
-                            FnDeclKind::Closure => ImplTraitPosition::ClosureReturn,
-                            FnDeclKind::Pointer => ImplTraitPosition::PointerReturn,
-                        })
                     };
-                    hir::FnRetTy::Return(self.lower_ty(ty, &context))
+                    hir::FnRetTy::Return(self.lower_ty(ty, &itctx))
                 }
                 FnRetTy::Default(span) => hir::FnRetTy::DefaultReturn(self.lower_span(*span)),
             },

tests/ui/associated-consts/issue-105330.stderr

@@ -33,11 +33,13 @@ LL | fn main<A: TraitWAssocConst<A=32>>() {
    = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
    = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in impl headers
+error[E0562]: `impl Trait` is not allowed in impl headers
   --> $DIR/issue-105330.rs:6:27
    |
 LL | impl TraitWAssocConst for impl Demo {
    |                           ^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error[E0131]: `main` function is not allowed to have generic parameters
   --> $DIR/issue-105330.rs:15:8

tests/ui/feature-gates/feature-gate-associated_type_bounds.rs

@@ -54,20 +54,20 @@ fn _rpit_dyn() -> Box<dyn Tr1<As1: Copy>> { Box::new(S1) }
 
 const _cdef: impl Tr1<As1: Copy> = S1;
 //~^ ERROR associated type bounds are unstable
-//~| ERROR `impl Trait` only allowed in function and inherent method argument and return types
+//~| ERROR `impl Trait` is not allowed in const types
 // FIXME: uncomment when `impl_trait_in_bindings` feature is fixed.
 // const _cdef_dyn: &dyn Tr1<As1: Copy> = &S1;
 
 static _sdef: impl Tr1<As1: Copy> = S1;
 //~^ ERROR associated type bounds are unstable
-//~| ERROR `impl Trait` only allowed in function and inherent method argument and return types
+//~| ERROR `impl Trait` is not allowed in static types
 // FIXME: uncomment when `impl_trait_in_bindings` feature is fixed.
 // static _sdef_dyn: &dyn Tr1<As1: Copy> = &S1;
 
 fn main() {
     let _: impl Tr1<As1: Copy> = S1;
     //~^ ERROR associated type bounds are unstable
-    //~| ERROR `impl Trait` only allowed in function and inherent method argument and return types
+    //~| ERROR `impl Trait` is not allowed in the type of variable bindings
     // FIXME: uncomment when `impl_trait_in_bindings` feature is fixed.
     // let _: &dyn Tr1<As1: Copy> = &S1;
 }

tests/ui/feature-gates/feature-gate-associated_type_bounds.stderr

@@ -115,23 +115,29 @@ LL |     let _: impl Tr1<As1: Copy> = S1;
    = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
    = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in const types
+error[E0562]: `impl Trait` is not allowed in const types
   --> $DIR/feature-gate-associated_type_bounds.rs:55:14
    |
 LL | const _cdef: impl Tr1<As1: Copy> = S1;
    |              ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in const types
+error[E0562]: `impl Trait` is not allowed in static types
   --> $DIR/feature-gate-associated_type_bounds.rs:61:15
    |
 LL | static _sdef: impl Tr1<As1: Copy> = S1;
    |               ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in variable bindings
+error[E0562]: `impl Trait` is not allowed in the type of variable bindings
   --> $DIR/feature-gate-associated_type_bounds.rs:68:12
    |
 LL |     let _: impl Tr1<As1: Copy> = S1;
    |            ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error[E0277]: the trait bound `<<Self as _Tr3>::A as Iterator>::Item: Copy` is not satisfied
   --> $DIR/feature-gate-associated_type_bounds.rs:12:28

tests/ui/feature-gates/feature-gate-impl_trait_in_fn_trait_return.rs

@@ -1,6 +1,6 @@
 fn f() -> impl Fn() -> impl Sized { || () }
-//~^ ERROR `impl Trait` only allowed in function and inherent method argument and return types, not in `Fn` trait return
+//~^ ERROR `impl Trait` is not allowed in the return type of `Fn` trait bounds
 fn g() -> &'static dyn Fn() -> impl Sized { &|| () }
-//~^ ERROR `impl Trait` only allowed in function and inherent method argument and return types, not in `Fn` trait return
+//~^ ERROR `impl Trait` is not allowed in the return type of `Fn` trait bounds
 
 fn main() {}

tests/ui/feature-gates/feature-gate-impl_trait_in_fn_trait_return.stderr

@@ -1,18 +1,20 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in `Fn` trait return types
+error[E0562]: `impl Trait` is not allowed in the return type of `Fn` trait bounds
   --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:1:24
    |
 LL | fn f() -> impl Fn() -> impl Sized { || () }
    |                        ^^^^^^^^^^
    |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
    = note: see issue #99697 <https://github.com/rust-lang/rust/issues/99697> for more information
    = help: add `#![feature(impl_trait_in_fn_trait_return)]` to the crate attributes to enable
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in `Fn` trait return types
+error[E0562]: `impl Trait` is not allowed in the return type of `Fn` trait bounds
   --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:3:32
    |
 LL | fn g() -> &'static dyn Fn() -> impl Sized { &|| () }
    |                                ^^^^^^^^^^
    |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
    = note: see issue #99697 <https://github.com/rust-lang/rust/issues/99697> for more information
    = help: add `#![feature(impl_trait_in_fn_trait_return)]` to the crate attributes to enable
 

tests/ui/impl-trait/issues/issue-54600.rs

@@ -2,6 +2,6 @@ use std::fmt::Debug;
 
 fn main() {
     let x: Option<impl Debug> = Some(44_u32);
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in the type of variable bindings
     println!("{:?}", x);
 }

tests/ui/impl-trait/issues/issue-54600.stderr

@@ -1,8 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in variable bindings
+error[E0562]: `impl Trait` is not allowed in the type of variable bindings
   --> $DIR/issue-54600.rs:4:19
    |
 LL |     let x: Option<impl Debug> = Some(44_u32);
    |                   ^^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/issues/issue-54840.rs

@@ -3,5 +3,5 @@ use std::ops::Add;
 fn main() {
     let i: i32 = 0;
     let j: &impl Add = &i;
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in the type of variable bindings
 }

tests/ui/impl-trait/issues/issue-54840.stderr

@@ -1,8 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in variable bindings
+error[E0562]: `impl Trait` is not allowed in the type of variable bindings
   --> $DIR/issue-54840.rs:5:13
    |
 LL |     let j: &impl Add = &i;
    |             ^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/issues/issue-58504.rs

@@ -8,5 +8,5 @@ fn mk_gen() -> impl Coroutine<Return=!, Yield=()> {
 
 fn main() {
     let gens: [impl Coroutine<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in the type of variable bindings
 }

tests/ui/impl-trait/issues/issue-58504.stderr

@@ -1,8 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in variable bindings
+error[E0562]: `impl Trait` is not allowed in the type of variable bindings
   --> $DIR/issue-58504.rs:10:16
    |
 LL |     let gens: [impl Coroutine<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error: aborting due to 1 previous error
 

tests/ui/impl-trait/issues/issue-58956.rs

@@ -5,9 +5,9 @@ impl Lam for B {}
 pub struct Wrap<T>(T);
 
 const _A: impl Lam = {
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in const types
     let x: Wrap<impl Lam> = Wrap(B);
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in the type of variable bindings
     x.0
 };
 

tests/ui/impl-trait/issues/issue-58956.stderr

@@ -1,14 +1,18 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in const types
+error[E0562]: `impl Trait` is not allowed in const types
   --> $DIR/issue-58956.rs:7:11
    |
 LL | const _A: impl Lam = {
    |           ^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
-error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in variable bindings
+error[E0562]: `impl Trait` is not allowed in the type of variable bindings
   --> $DIR/issue-58956.rs:9:17
    |
 LL |     let x: Wrap<impl Lam> = Wrap(B);
    |                 ^^^^^^^^
+   |
+   = note: `impl Trait` is only allowed in arguments and return types of functions and methods
 
 error: aborting due to 2 previous errors
 

tests/ui/impl-trait/issues/issue-70971.rs

@@ -1,4 +1,4 @@
 fn main() {
     let x : (impl Copy,) = (true,);
-    //~^ `impl Trait` only allowed in function and inherent method argument and return types
+    //~^ `impl Trait` is not allowed in the type of variable bindings
 }