Add missing error annotations and .stderr file

Author: CastilloDel

Diff for: tests/ui/feature-gates/unstable-attribute-rejects-already-stable-features.rs

@@ -4,8 +4,8 @@
 #![feature(staged_api)]
 #![stable(feature = "rust_test", since = "1.0.0")]
 
-#[unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
-#[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+#[unstable(feature = "arbitrary_enum_discriminant", issue = "42")] //~ ERROR can't mark as unstable using an already stable feature
+#[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")] //~ ERROR can't mark as unstable using an already stable feature
 const fn my_fun() {}
 
 fn main() {}

Diff for: tests/ui/feature-gates/unstable-attribute-rejects-already-stable-features.stderr

@@ -0,0 +1,31 @@
+error: can't mark as unstable using an already stable feature
+  --> $DIR/unstable-attribute-rejects-already-stable-features.rs:7:1
+   |
+LL | #[unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this feature is already stable
+LL | #[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+LL | const fn my_fun() {}
+   | -------------------- the stability attribute annotates this item
+   |
+help: Consider removing the attribute
+  --> $DIR/unstable-attribute-rejects-already-stable-features.rs:7:1
+   |
+LL | #[unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: can't mark as unstable using an already stable feature
+  --> $DIR/unstable-attribute-rejects-already-stable-features.rs:8:1
+   |
+LL | #[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this feature is already stable
+LL | const fn my_fun() {}
+   | -------------------- the stability attribute annotates this item
+   |
+help: Consider removing the attribute
+  --> $DIR/unstable-attribute-rejects-already-stable-features.rs:8:1
+   |
+LL | #[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+