Rollup merge of rust-lang#110237 - oli-obk:impl_trait_in_assoc_tys, r=jackh726

Split out a separate feature gate for impl trait in associated types

in rust-lang#107645 it was decided that we'll take a new route for type alias impl trait. The exact route isn't clear yet, so while I'm working on implementing some of these proposed changes (e.g. in rust-lang#110010) to be able to experiment with them, I will also work on stabilizing another sugar version first: impl trait in associated types. Similarly I'll look into creating feature gates for impl trait in const/static types.

This PR does nothing but split the feature gate, so that you need to enable a different feature gate for

```rust
impl Trait for Type {
    type Assoc = impl SomeTrait;
}
```

than what you need for `type Foo = impl SomeTrait;`

Author: matthiaskrgr

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -121,24 +121,34 @@ impl<'a> PostExpansionVisitor<'a> {
     }
 
     /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
-    fn check_impl_trait(&self, ty: &ast::Ty) {
+    fn check_impl_trait(&self, ty: &ast::Ty, in_associated_ty: bool) {
         struct ImplTraitVisitor<'a> {
             vis: &'a PostExpansionVisitor<'a>,
+            in_associated_ty: bool,
         }
         impl Visitor<'_> for ImplTraitVisitor<'_> {
             fn visit_ty(&mut self, ty: &ast::Ty) {
                 if let ast::TyKind::ImplTrait(..) = ty.kind {
-                    gate_feature_post!(
-                        &self.vis,
-                        type_alias_impl_trait,
-                        ty.span,
-                        "`impl Trait` in type aliases is unstable"
-                    );
+                    if self.in_associated_ty {
+                        gate_feature_post!(
+                            &self.vis,
+                            impl_trait_in_assoc_type,
+                            ty.span,
+                            "`impl Trait` in associated types is unstable"
+                        );
+                    } else {
+                        gate_feature_post!(
+                            &self.vis,
+                            type_alias_impl_trait,
+                            ty.span,
+                            "`impl Trait` in type aliases is unstable"
+                        );
+                    }
                 }
                 visit::walk_ty(self, ty);
             }
         }
-        ImplTraitVisitor { vis: self }.visit_ty(ty);
+        ImplTraitVisitor { vis: self, in_associated_ty }.visit_ty(ty);
     }
 
     fn check_late_bound_lifetime_defs(&self, params: &[ast::GenericParam]) {
@@ -294,7 +304,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             }
 
             ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ty), .. }) => {
-                self.check_impl_trait(&ty)
+                self.check_impl_trait(&ty, false)
             }
 
             _ => {}
@@ -520,7 +530,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     );
                 }
                 if let Some(ty) = ty {
-                    self.check_impl_trait(ty);
+                    self.check_impl_trait(ty, true);
                 }
                 false
             }

compiler/rustc_feature/src/active.rs

@@ -416,6 +416,8 @@ declare_features! (
     (active, half_open_range_patterns_in_slices, "1.66.0", Some(67264), None),
     /// Allows `if let` guard in match arms.
     (active, if_let_guard, "1.47.0", Some(51114), None),
+    /// Allows `impl Trait` to be used inside associated types (RFC 2515).
+    (active, impl_trait_in_assoc_type, "CURRENT_RUSTC_VERSION", Some(63063), None),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.
     (active, impl_trait_in_fn_trait_return, "1.64.0", Some(99697), None),
     /// Allows referencing `Self` and projections in impl-trait.

compiler/rustc_span/src/symbol.rs

@@ -801,6 +801,7 @@ symbols! {
         ignore,
         impl_header_lifetime_elision,
         impl_lint_pass,
+        impl_trait_in_assoc_type,
         impl_trait_in_bindings,
         impl_trait_in_fn_trait_return,
         impl_trait_projections,

src/librustdoc/lib.rs

@@ -15,6 +15,7 @@
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
 #![feature(type_alias_impl_trait)]
+#![cfg_attr(not(bootstrap), feature(impl_trait_in_assoc_type))]
 #![recursion_limit = "256"]
 #![warn(rustc::internal)]
 #![allow(clippy::collapsible_if, clippy::collapsible_else_if)]

tests/rustdoc/auxiliary/issue-73061.rs

@@ -1,6 +1,6 @@
 //edition:2018
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Foo {
     type X: std::future::Future<Output = ()>;

tests/ui/associated-types/issue-63591.rs

@@ -1,19 +1,23 @@
 // check-pass
 
 #![feature(associated_type_bounds)]
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 fn main() {}
 
-trait Bar { type Assoc; }
+trait Bar {
+    type Assoc;
+}
 
 trait Thing {
     type Out;
     fn func() -> Self::Out;
 }
 
 struct AssocIsCopy;
-impl Bar for AssocIsCopy { type Assoc = u8; }
+impl Bar for AssocIsCopy {
+    type Assoc = u8;
+}
 
 impl Thing for AssocIsCopy {
     type Out = impl Bar<Assoc: Copy>;

tests/ui/async-await/in-trait/async-associated-types2.rs

@@ -4,7 +4,7 @@
 // revisions: current next
 
 #![feature(async_fn_in_trait)]
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 #![allow(incomplete_features)]
 
 use std::future::Future;
@@ -23,9 +23,7 @@ impl MyTrait for i32 {
         Self: 'a;
 
     fn foo<'a>(&'a self) -> Self::Fut<'a> {
-        async {
-            *self
-        }
+        async { *self }
     }
 }
 

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

@@ -0,0 +1,18 @@
+trait Foo {
+    type Bar;
+}
+
+impl Foo for () {
+    type Bar = impl std::fmt::Debug;
+    //~^ ERROR: `impl Trait` in associated types is unstable
+}
+
+struct Mop;
+
+impl Mop {
+    type Bop = impl std::fmt::Debug;
+    //~^ ERROR: `impl Trait` in associated types is unstable
+    //~| ERROR: inherent associated types are unstable
+}
+
+fn main() {}

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

@@ -0,0 +1,30 @@
+error[E0658]: `impl Trait` in associated types is unstable
+  --> $DIR/feature-gate-impl_trait_in_assoc_type.rs:6:16
+   |
+LL |     type Bar = impl std::fmt::Debug;
+   |                ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(impl_trait_in_assoc_type)]` to the crate attributes to enable
+
+error[E0658]: `impl Trait` in associated types is unstable
+  --> $DIR/feature-gate-impl_trait_in_assoc_type.rs:13:16
+   |
+LL |     type Bop = impl std::fmt::Debug;
+   |                ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(impl_trait_in_assoc_type)]` to the crate attributes to enable
+
+error[E0658]: inherent associated types are unstable
+  --> $DIR/feature-gate-impl_trait_in_assoc_type.rs:13:5
+   |
+LL |     type Bop = impl std::fmt::Debug;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #8995 <https://github.com/rust-lang/rust/issues/8995> for more information
+   = help: add `#![feature(inherent_associated_types)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/generator/issue-87142.rs

@@ -4,7 +4,7 @@
 // Regression test for #87142
 // This test needs the above flags and the "lib" crate type.
 
-#![feature(type_alias_impl_trait, generator_trait, generators)]
+#![feature(impl_trait_in_assoc_type, generator_trait, generators)]
 #![crate_type = "lib"]
 
 use std::ops::Generator;