Add test for using unstable_feature_bound on method

Author: tiif

tests/ui/internal/unstable_inherent_method.rs

@@ -0,0 +1,23 @@
+#![allow(internal_features)]
+#![feature(staged_api)]
+#![feature(impl_stability)]
+#![stable(feature = "a", since = "1.1.1" )]
+
+
+/// FIXME(tiif): we haven't allowed marking impl method as 
+/// unstable yet, but it should be possible.
+
+#[stable(feature = "a", since = "1.1.1" )]
+pub trait Trait {
+    #[unstable(feature = "feat", issue = "none" )]
+    #[unstable_feature_bound(foo)]
+    fn foo();
+}
+
+#[stable(feature = "a", since = "1.1.1" )]
+impl Trait for u8 {
+    #[unstable_feature_bound(foo)]
+    fn foo() {}
+}
+
+fn main() {}