Rollup merge of #158233 - kantnero:allow-unstable-attr-on-target, r=JonathanBrouwer

Allow the unstable attribute on foreign type

While working the `FnPtr` trait in #156176 @carbotaniuman was trying to implement `Code` as an extern type but got an error as the current stability infrastructure does not allow `unstable` attribute on foreign type.  #158200

This PR fixes that by allowing the `unstable` attribute on ForeignTy  and adds test to verify.

@rustbot r? @JonathanBrouwer

Author: JonathanBrouwer

compiler/rustc_attr_parsing/src/attributes/stability.rs

@@ -40,6 +40,7 @@ const ALLOWED_TARGETS: AllowedTargets<'_> = AllowedTargets::AllowList(&[
     Allow(Target::Static),
     Allow(Target::ForeignFn),
     Allow(Target::ForeignStatic),
+    Allow(Target::ForeignTy),
     Allow(Target::ExternCrate),
 ]);
 

tests/ui/lint/auxiliary/lint_stability.rs

@@ -1,5 +1,6 @@
 #![crate_name="lint_stability"]
 #![crate_type = "lib"]
+#![feature(extern_types)]
 #![feature(staged_api)]
 #![feature(associated_type_defaults)]
 #![stable(feature = "lint_stability", since = "1.0.0")]
@@ -186,3 +187,8 @@ macro_rules! macro_test_arg {
 macro_rules! macro_test_arg_nested {
     ($func:ident) => (macro_test_arg!($func()));
 }
+
+extern "C" {
+    #[unstable(feature = "unstable_test_feature", issue = "none")]
+    pub type UnstableForeignType;
+}

tests/ui/lint/lint-stability.rs

@@ -6,7 +6,7 @@
 #![allow(deprecated)]
 #![allow(dead_code)]
 #![feature(staged_api)]
-
+#![feature(extern_types)]
 #![stable(feature = "rust1", since = "1.0.0")]
 
 #[macro_use]
@@ -18,6 +18,9 @@ mod cross_crate {
 
     use lint_stability::*;
 
+    fn test_foreign_type(_: &mut UnstableForeignType) { //~ ERROR use of unstable library feature
+    }
+
     fn test() {
         type Foo = MethodTester;
         let foo = MethodTester;