Add test for unstable trait impl lint.

Author: m-ou-se

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

@@ -0,0 +1,28 @@
+#![feature(staged_api)]
+
+#[stable(feature = "x", since = "1")]
+struct StableType;
+
+#[unstable(feature = "x", issue = "none")]
+struct UnstableType;
+
+#[stable(feature = "x", since = "1")]
+trait StableTrait {}
+
+#[unstable(feature = "x", issue = "none")]
+trait UnstableTrait {}
+
+#[unstable(feature = "x", issue = "none")]
+impl UnstableTrait for UnstableType {}
+
+#[unstable(feature = "x", issue = "none")]
+impl StableTrait for UnstableType {}
+
+#[unstable(feature = "x", issue = "none")]
+impl UnstableTrait for StableType {}
+
+#[unstable(feature = "x", issue = "none")]
+//~^ ERROR An `#[unstable]` annotation here has no effect.
+impl StableTrait for StableType {}
+
+fn main() {}

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

@@ -0,0 +1,10 @@
+error: An `#[unstable]` annotation here has no effect. See issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information.
+  --> $DIR/stability-attribute-trait-impl.rs:24:1
+   |
+LL | #[unstable(feature = "x", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(rustc::ineffective_unstable_trait_impl)]` on by default
+
+error: aborting due to previous error
+