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;`

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;

tests/ui/generic-associated-types/issue-86218-2.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Stream {
     type Item;
@@ -17,7 +17,9 @@ trait Yay<AdditionalValue> {
 
 impl<T> Yay<T> for () {
     type InnerStream<'s> = impl Stream<Item = i32> + 's;
-    fn foo<'s>() -> Self::InnerStream<'s> { () }
+    fn foo<'s>() -> Self::InnerStream<'s> {
+        ()
+    }
 }
 
 fn main() {}

tests/ui/generic-associated-types/issue-86218.rs

@@ -1,6 +1,6 @@
 // check-pass
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Stream {
     type Item;
@@ -18,7 +18,9 @@ trait Yay<AdditionalValue> {
 impl<'a> Yay<&'a ()> for () {
     type InnerStream<'s> = impl Stream<Item = i32> + 's;
     //^ ERROR does not fulfill the required lifetime
-    fn foo<'s>() -> Self::InnerStream<'s> { () }
+    fn foo<'s>() -> Self::InnerStream<'s> {
+        ()
+    }
 }
 
 fn main() {}

tests/ui/generic-associated-types/issue-87258_a.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 // See https://github.com/rust-lang/rust/issues/87258#issuecomment-883293367
 

tests/ui/generic-associated-types/issue-88595.rs

@@ -1,7 +1,8 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 fn main() {}
 
+#[rustfmt::skip]
 trait A<'a> {
     type B<'b>: Clone
     // FIXME(generic_associated_types): Remove one of the below bounds

tests/ui/generic-associated-types/issue-88595.stderr

@@ -1,11 +1,11 @@
 error: non-defining opaque type use in defining scope
-  --> $DIR/issue-88595.rs:20:35
+  --> $DIR/issue-88595.rs:21:35
    |
 LL |     fn a(&'a self) -> Self::B<'a> {}
    |                                   ^^
    |
 note: lifetime used multiple times
-  --> $DIR/issue-88595.rs:17:6
+  --> $DIR/issue-88595.rs:18:6
    |
 LL | impl<'a> A<'a> for C {
    |      ^^

tests/ui/generic-associated-types/issue-89008.rs

@@ -1,7 +1,7 @@
 // check-pass
 // edition:2021
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 use std::future::Future;
 use std::marker::PhantomData;

tests/ui/generic-associated-types/issue-90014.rs

@@ -1,11 +1,13 @@
 // edition:2018
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 use std::future::Future;
 
 trait MakeFut {
-    type Fut<'a> where Self: 'a;
+    type Fut<'a>
+    where
+        Self: 'a;
     fn make_fut<'a>(&'a self) -> Self::Fut<'a>;
 }
 

tests/ui/generic-associated-types/issue-90014.stderr

@@ -1,14 +1,14 @@
 error[E0477]: the type `&mut ()` does not fulfill the required lifetime
-  --> $DIR/issue-90014.rs:13:20
+  --> $DIR/issue-90014.rs:15:20
    |
-LL |     type Fut<'a> where Self: 'a;
+LL |     type Fut<'a>
    |     ------------ definition of `Fut` from trait
 ...
 LL |     type Fut<'a> = impl Future<Output = ()>;
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: type must outlive the lifetime `'a` as defined here
-  --> $DIR/issue-90014.rs:13:14
+  --> $DIR/issue-90014.rs:15:14
    |
 LL |     type Fut<'a> = impl Future<Output = ()>;
    |              ^^

tests/ui/impl-trait/associated-impl-trait-type-generic-trait.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}

tests/ui/impl-trait/associated-impl-trait-type-trivial.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}

tests/ui/impl-trait/associated-impl-trait-type.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 // build-pass (FIXME(62277): could be check-pass?)
 
 trait Bar {}

tests/ui/impl-trait/issue-55872-1.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Bar {
     type E: Copy;

tests/ui/impl-trait/issue-55872-2.rs

@@ -3,7 +3,7 @@
 // [drop_tracking_mir] compile-flags: -Zdrop-tracking-mir
 // edition:2018
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Bar {
     type E: Send;

tests/ui/impl-trait/issue-55872-3.rs

@@ -1,7 +1,7 @@
 // edition:2018
 // ignore-compare-mode-chalk
 
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Bar {
     type E: Copy;

tests/ui/impl-trait/issue-55872.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 pub trait Bar {
     type E: Copy;

tests/ui/impl-trait/issues/issue-82139.rs

@@ -1,4 +1,4 @@
-#![feature(type_alias_impl_trait)]
+#![feature(impl_trait_in_assoc_type)]
 
 trait Trait {
     type Associated;