Add test for #[unstable] impl for fn() -> !.

Author: m-ou-se

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

@@ -33,4 +33,8 @@ impl StableTrait for extern "C-unwind" fn() {}
 //~^ ERROR an `#[unstable]` annotation here has no effect [ineffective_unstable_trait_impl]
 impl StableTrait for StableType {}
 
+#[unstable(feature = "k", issue = "none")]
+//~^ ERROR an `#[unstable]` annotation here has no effect [ineffective_unstable_trait_impl]
+impl StableTrait for fn() -> ! {}
+
 fn main() {}

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

@@ -7,6 +7,14 @@ LL | #[unstable(feature = "j", issue = "none")]
    = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
    = note: `#[deny(ineffective_unstable_trait_impl)]` on by default
 
+error: an `#[unstable]` annotation here has no effect
+  --> $DIR/stability-attribute-trait-impl.rs:36:1
+   |
+LL | #[unstable(feature = "k", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
+
 error: module has missing stability attribute
   --> $DIR/stability-attribute-trait-impl.rs:1:1
    |
@@ -19,5 +27,5 @@ LL | |
 LL | | fn main() {}
    | |____________^
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors