Stable const things need a stability attribute

Author: oli-obk

src/test/ui/rfc-2632-const-trait-impl/staged-api.rs

@@ -15,6 +15,7 @@ pub struct Foo;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(unstable, rustc_const_unstable(feature = "foo", issue = "none"))]
+#[cfg_attr(stable, rustc_const_stable(feature = "foo", since = "1.0.0"))]
 impl const MyTrait for Foo {
     //[stable]~^ ERROR trait implementations cannot be const stable yet
     fn func() {}

src/test/ui/rfc-2632-const-trait-impl/staged-api.stable.stderr

@@ -1,11 +1,13 @@
-error: implementation has missing const stability attribute
-  --> $DIR/staged-api.rs:18:1
+error: trait implementations cannot be const stable yet
+  --> $DIR/staged-api.rs:19:1
    |
 LL | / impl const MyTrait for Foo {
 LL | |
 LL | |     fn func() {}
 LL | | }
    | |_^
+   |
+   = note: see issue #67792 <https://github.com/rust-lang/rust/issues/67792> for more information
 
 error: aborting due to previous error
 

src/test/ui/rfc-2632-const-trait-impl/staged-api.unstable.stderr

@@ -1,13 +1,13 @@
 error: `<Foo as staged_api::MyTrait>::func` is not yet stable as a const fn
-  --> $DIR/staged-api.rs:34:5
+  --> $DIR/staged-api.rs:35:5
    |
 LL |     Foo::func();
    |     ^^^^^^^^^^^
    |
    = help: add `#![feature(foo)]` to the crate attributes to enable
 
 error: `<Foo as staged_api::MyTrait>::func` is not yet stable as a const fn
-  --> $DIR/staged-api.rs:42:5
+  --> $DIR/staged-api.rs:43:5
    |
 LL |     Foo::func();
    |     ^^^^^^^^^^^