Add comments and improve error message

Author: tiif

compiler/rustc_feature/src/builtin_attrs.rs

@@ -658,7 +658,7 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     gated!(
         unstable_feature_bound, Normal, template!(Word, List: "feat1, feat2, ..."),
         DuplicatesOk, EncodeCrossCrate::No, impl_stability,
-        "allow unstable impl",
+        "used internally to mark impl as unstable",
     ),
     gated!(
         allow_internal_unsafe, Normal, template!(Word), WarnFollowing,

tests/ui/feature-gates/feature-gate-impl-stability.rs

@@ -4,7 +4,7 @@ trait Foo{}
 struct Bar;
 
 #[unstable_feature_bound(feat_foo)]
-//~^ ERROR: allow unstable impl
+//~^ ERROR: used internally to mark impl as unstable
 impl Foo for Bar{}
 
 fn main() {

tests/ui/feature-gates/feature-gate-impl-stability.stderr

@@ -1,4 +1,4 @@
-error[E0658]: allow unstable impl
+error[E0658]: used internally to mark impl as unstable
   --> $DIR/feature-gate-impl-stability.rs:6:1
    |
 LL | #[unstable_feature_bound(feat_foo)]

tests/ui/internal/unstable-feature-bound-no-effect.rs

@@ -4,16 +4,22 @@
 #![feature(impl_stability)]
 #![allow(dead_code)]
 
+/// If #[unstable(..)] and #[unstable_feature_name(..)] have the same symbol,
+/// The error should not be thrown as it can effectively mark an impl as unstable.
+/// 
+/// If the feature name in #[feature] does not exist in #[unstable_feature_bound(..)]
+/// an error should still be thrown because that feature will not be unstable.
+
 #[stable(feature = "a", since = "1.1.1" )]
 trait Moo {}
 #[stable(feature = "a", since = "1.1.1" )]
 trait Foo {}
 #[stable(feature = "a", since = "1.1.1" )]
+trait Boo {}
+#[stable(feature = "a", since = "1.1.1" )]
 pub struct Bar;
 
 
-// If #[unstable_feature_bound] and #[unstable] has different name,
-// It should throw an error.
 #[unstable(feature = "feat_moo", issue = "none" )]
 #[unstable_feature_bound(feat_foo)] //~^ ERROR: an `#[unstable]` annotation here has no effect
 impl Moo for Bar {}
@@ -22,4 +28,9 @@ impl Moo for Bar {}
 #[unstable_feature_bound(feat_foo)]
 impl Foo for Bar {}
 
+
+#[unstable(feature = "feat_foo", issue = "none" )]
+#[unstable_feature_bound(feat_foo, feat_bar)]
+impl Boo for Bar {}
+
 fn main() {}

tests/ui/internal/unstable-feature-bound-no-effect.stderr

@@ -1,5 +1,5 @@
 error: an `#[unstable]` annotation here has no effect
-  --> $DIR/unstable-feature-bound-no-effect.rs:17:1
+  --> $DIR/unstable-feature-bound-no-effect.rs:23:1
    |
 LL | #[unstable(feature = "feat_moo", issue = "none" )]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^