Allow impl Trait inside where clauses

compiler/rustc_ast_lowering/src/item.rs

@@ -1503,14 +1503,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
                 hir_id: self.next_id(),
                 bound_generic_params: self.lower_generic_params(bound_generic_params),
-                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,
-                        &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
-                    )
-                })),
+                bounded_ty: self.lower_ty(bounded_ty, &ImplTraitContext::Universal),
+                bounds: self.arena.alloc_from_iter(
+                    bounds
+                        .iter()
+                        .map(|bound| self.lower_param_bound(bound, &ImplTraitContext::Universal)),
+                ),
                 span: self.lower_span(*span),
                 origin: PredicateOrigin::WhereClause,
             }),

compiler/rustc_error_messages/locales/en-US/ast_lowering.ftl

@@ -17,7 +17,7 @@ ast_lowering_assoc_ty_parentheses =
 ast_lowering_remove_parentheses = remove these parentheses
 
 ast_lowering_misplaced_impl_trait =
-    `impl Trait` only allowed in function and inherent method return types, not in {$position}
+    `impl Trait` isn't allowed within {$position}
 
 ast_lowering_rustc_box_attribute_error =
     #[rustc_box] requires precisely one argument and no other attributes are allowed

src/test/ui/async-await/in-trait/fn-not-async-err2.rs

@@ -11,7 +11,7 @@ trait MyTrait {
 
 impl MyTrait for i32 {
     fn foo(&self) -> impl Future<Output = i32> {
-        //~^ ERROR `impl Trait` only allowed in function and inherent method return types, not in `impl` method return [E0562]
+        //~^ ERROR `impl Trait` isn't allowed within `impl` method return [E0562]
         async {
             *self
         }

src/test/ui/async-await/in-trait/fn-not-async-err2.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `impl` method return
+error[E0562]: `impl Trait` isn't allowed within `impl` method return
   --> $DIR/fn-not-async-err2.rs:13:22
    |
 LL |     fn foo(&self) -> impl Future<Output = i32> {

src/test/ui/feature-gates/feature-gate-associated_type_bounds.rs

@@ -57,20 +57,20 @@ 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 return types
+//~| ERROR `impl Trait` isn't allowed within type [E0562]
 // 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 return types
+//~| ERROR `impl Trait` isn't allowed within type [E0562]
 // 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 return types
+    //~| ERROR `impl Trait` isn't allowed within variable binding [E0562]
     // FIXME: uncomment when `impl_trait_in_bindings` feature is fixed.
     // let _: &dyn Tr1<As1: Copy> = &S1;
 }

src/test/ui/feature-gates/feature-gate-associated_type_bounds.stderr

@@ -115,19 +115,19 @@ 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 return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/feature-gate-associated_type_bounds.rs:58:14
    |
 LL | const _cdef: impl Tr1<As1: Copy> = S1;
    |              ^^^^^^^^^^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/feature-gate-associated_type_bounds.rs:64:15
    |
 LL | static _sdef: impl Tr1<As1: Copy> = S1;
    |               ^^^^^^^^^^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/feature-gate-associated_type_bounds.rs:71:12
    |
 LL |     let _: impl Tr1<As1: Copy> = S1;

src/test/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 return types, not in `Fn` trait return
+//~^ ERROR `impl Trait` isn't allowed within `Fn` trait return
 fn g() -> &'static dyn Fn() -> impl Sized { &|| () }
-//~^ ERROR `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+//~^ ERROR `impl Trait` isn't allowed within `Fn` trait return
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-impl_trait_in_fn_trait_return.stderr

@@ -1,10 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+error[E0562]: `impl Trait` isn't allowed within `Fn` trait return
   --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:1:24
    |
 LL | fn f() -> impl Fn() -> impl Sized { || () }
    |                        ^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+error[E0562]: `impl Trait` isn't allowed within `Fn` trait return
   --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:3:32
    |
 LL | fn g() -> &'static dyn Fn() -> impl Sized { &|| () }

src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.rs

@@ -5,14 +5,14 @@
 #![feature(async_fn_in_trait)]
 
 trait Foo {
-    fn bar() -> impl Sized; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
-    fn baz() -> Box<impl std::fmt::Display>; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
+    fn bar() -> impl Sized; //~ ERROR `impl Trait` isn't allowed within trait method return
+    fn baz() -> Box<impl std::fmt::Display>; //~ ERROR `impl Trait` isn't allowed within trait method return
 }
 
 // Both return_position_impl_trait_in_trait and async_fn_in_trait are required for this (see also
 // feature-gate-async_fn_in_trait.rs)
 trait AsyncFoo {
-    async fn bar() -> impl Sized; //~ ERROR `impl Trait` only allowed in function and inherent method return types, not in trait method return
+    async fn bar() -> impl Sized; //~ ERROR `impl Trait` isn't allowed within trait method return [E0562]
 }
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
+error[E0562]: `impl Trait` isn't allowed within trait method return
   --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:8:17
    |
 LL |     fn bar() -> impl Sized;
@@ -7,7 +7,7 @@ LL |     fn bar() -> impl Sized;
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
    = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
+error[E0562]: `impl Trait` isn't allowed within trait method return
   --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:9:21
    |
 LL |     fn baz() -> Box<impl std::fmt::Display>;
@@ -16,7 +16,7 @@ LL |     fn baz() -> Box<impl std::fmt::Display>;
    = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
    = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
+error[E0562]: `impl Trait` isn't allowed within trait method return
   --> $DIR/feature-gate-return_position_impl_trait_in_trait.rs:15:23
    |
 LL |     async fn bar() -> impl Sized;

src/test/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 return types
+    //~^ `impl Trait` isn't allowed within variable binding [E0562]
     println!("{:?}", x);
 }

src/test/ui/impl-trait/issues/issue-54600.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-54600.rs:4:19
    |
 LL |     let x: Option<impl Debug> = Some(44_u32);

src/test/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 return types
+    //~^ `impl Trait` isn't allowed within variable binding [E0562]
 }

src/test/ui/impl-trait/issues/issue-54840.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-54840.rs:5:13
    |
 LL |     let j: &impl Add = &i;

src/test/ui/impl-trait/issues/issue-58504.rs

@@ -8,5 +8,5 @@ fn mk_gen() -> impl Generator<Return=!, Yield=()> {
 
 fn main() {
     let gens: [impl Generator<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
-    //~^ `impl Trait` only allowed in function and inherent method return types
+    //~^ `impl Trait` isn't allowed within variable binding
 }

src/test/ui/impl-trait/issues/issue-58504.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-58504.rs:10:16
    |
 LL |     let gens: [impl Generator<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];

src/test/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 return types
+    //~^ `impl Trait` isn't allowed within type
     let x: Wrap<impl Lam> = Wrap(B);
-    //~^ `impl Trait` only allowed in function and inherent method return types
+    //~^ `impl Trait` isn't allowed within variable binding
     x.0
 };
 

src/test/ui/impl-trait/issues/issue-58956.stderr

@@ -1,10 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/issue-58956.rs:7:11
    |
 LL | const _A: impl Lam = {
    |           ^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-58956.rs:9:17
    |
 LL |     let x: Wrap<impl Lam> = Wrap(B);

src/test/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 return types
+    //~^ `impl Trait` isn't allowed within variable binding
 }

src/test/ui/impl-trait/issues/issue-70971.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-70971.rs:2:14
    |
 LL |     let x : (impl Copy,) = (true,);

src/test/ui/impl-trait/issues/issue-79099.rs

@@ -1,7 +1,7 @@
 struct Bug {
     V1: [(); {
         let f: impl core::future::Future<Output = u8> = async { 1 };
-        //~^ `impl Trait` only allowed in function and inherent method return types
+        //~^ `impl Trait` isn't allowed within variable binding [E0562]
         //~| expected identifier
         1
     }],

src/test/ui/impl-trait/issues/issue-79099.stderr

@@ -9,7 +9,7 @@ LL |         let f: impl core::future::Future<Output = u8> = async { 1 };
    = help: pass `--edition 2021` to `rustc`
    = note: for more on editions, read https://doc.rust-lang.org/edition-guide
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-79099.rs:3:16
    |
 LL |         let f: impl core::future::Future<Output = u8> = async { 1 };

src/test/ui/impl-trait/issues/issue-83929-impl-trait-in-generic-default.rs

@@ -1,8 +1,8 @@
 struct Foo<T = impl Copy>(T);
-//~^ ERROR `impl Trait` only allowed in function and inherent method return types
+//~^ ERROR `impl Trait` isn't allowed within type [E0562]
 
 type Result<T, E = impl std::error::Error> = std::result::Result<T, E>;
-//~^ ERROR `impl Trait` only allowed in function and inherent method return types
+//~^ ERROR `impl Trait` isn't allowed within type [E0562]
 
 // should not cause ICE
 fn x() -> Foo {

src/test/ui/impl-trait/issues/issue-83929-impl-trait-in-generic-default.stderr

@@ -1,10 +1,10 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/issue-83929-impl-trait-in-generic-default.rs:1:16
    |
 LL | struct Foo<T = impl Copy>(T);
    |                ^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/issue-83929-impl-trait-in-generic-default.rs:4:20
    |
 LL | type Result<T, E = impl std::error::Error> = std::result::Result<T, E>;

src/test/ui/impl-trait/issues/issue-84919.rs

@@ -3,7 +3,7 @@ impl Trait for () {}
 
 fn foo<'a: 'a>() {
     let _x: impl Trait = ();
-    //~^ `impl Trait` only allowed in function and inherent method return types
+    //~^ `impl Trait` isn't allowed within variable binding [E0562]
 }
 
 fn main() {}

src/test/ui/impl-trait/issues/issue-84919.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-84919.rs:5:13
    |
 LL |     let _x: impl Trait = ();

src/test/ui/impl-trait/issues/issue-86642.rs

@@ -1,5 +1,5 @@
 static x: impl Fn(&str) -> Result<&str, ()> = move |source| {
-    //~^ `impl Trait` only allowed in function and inherent method return types
+    //~^ `impl Trait` isn't allowed within type [E0562]
     let res = (move |source| Ok(source))(source);
     let res = res.or((move |source| Ok(source))(source));
     res

src/test/ui/impl-trait/issues/issue-86642.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
+error[E0562]: `impl Trait` isn't allowed within type
   --> $DIR/issue-86642.rs:1:11
    |
 LL | static x: impl Fn(&str) -> Result<&str, ()> = move |source| {

src/test/ui/impl-trait/issues/issue-87295.rs

@@ -14,5 +14,5 @@ impl<F> Struct<F> {
 
 fn main() {
     let _do_not_waste: Struct<impl Trait<Output = i32>> = Struct::new(());
-    //~^ `impl Trait` only allowed in function and inherent method return types
+    //~^ `impl Trait` isn't allowed within variable binding [E0562]
 }

src/test/ui/impl-trait/issues/issue-87295.stderr

@@ -1,4 +1,4 @@
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
+error[E0562]: `impl Trait` isn't allowed within variable binding
   --> $DIR/issue-87295.rs:16:31
    |
 LL |     let _do_not_waste: Struct<impl Trait<Output = i32>> = Struct::new(());

src/test/ui/impl-trait/nested_impl_trait.rs

@@ -9,7 +9,7 @@ fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
 
 fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
 //~^ ERROR nested `impl Trait` is not allowed
-//~| `impl Trait` only allowed in function and inherent method return types
+//~| `impl Trait` isn't allowed within `fn` pointer return [E0562]
 
 fn bad_in_arg_position(_: impl Into<impl Debug>) { }
 //~^ ERROR nested `impl Trait` is not allowed

src/test/ui/impl-trait/nested_impl_trait.stderr

@@ -34,7 +34,7 @@ LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  |         nested `impl Trait` here
    |                                  outer `impl Trait`
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `fn` pointer return
+error[E0562]: `impl Trait` isn't allowed within `fn` pointer return
   --> $DIR/nested_impl_trait.rs:10:32
    |
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}