Improve ineffective_unstable_trait_impl error message.

Author: m-ou-se

compiler/rustc_passes/src/stability.rs

@@ -561,10 +561,10 @@ impl Visitor<'tcx> for Checker<'tcx> {
                                 INEFFECTIVE_UNSTABLE_TRAIT_IMPL,
                                 item.hir_id,
                                 span,
-                                |lint| lint.build(
-                                    "An `#[unstable]` annotation here has no effect. \
-                                    See issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information.",
-                                ).emit()
+                                |lint| lint
+                                    .build("an `#[unstable]` annotation here has no effect")
+                                    .note("see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information")
+                                    .emit()
                             );
                         }
                     }

src/test/ui/stability-attribute/stability-attribute-trait-impl.rs

@@ -22,7 +22,7 @@ impl StableTrait for UnstableType {}
 impl UnstableTrait for StableType {}
 
 #[unstable(feature = "x", issue = "none")]
-//~^ ERROR An `#[unstable]` annotation here has no effect.
+//~^ ERROR an `#[unstable]` annotation here has no effect [rustc::ineffective_unstable_trait_impl]
 impl StableTrait for StableType {}
 
 fn main() {}

src/test/ui/stability-attribute/stability-attribute-trait-impl.stderr

@@ -1,10 +1,11 @@
-error: An `#[unstable]` annotation here has no effect. See issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information.
+error: an `#[unstable]` annotation here has no effect
   --> $DIR/stability-attribute-trait-impl.rs:24:1
    |
 LL | #[unstable(feature = "x", issue = "none")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[deny(rustc::ineffective_unstable_trait_impl)]` on by default
+   = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
 
 error: aborting due to previous error