Lint unstable extern fns that are const stable

Author: jhpratt

compiler/rustc_lint/src/internal.rs

@@ -401,4 +401,29 @@ impl LateLintPass<'_> for IncompatibleStability {
             lint.build("functions cannot be const-stable if they are unstable").emit();
         });
     }
+
+    fn check_foreign_item(&mut self, cx: &LateContext<'_>, item: &rustc_hir::ForeignItem<'_>) {
+        use rustc_attr::{ConstStability, Stability, StabilityLevel};
+
+        if !matches!(item.kind, rustc_hir::ForeignItemKind::Fn(..)) {
+            return;
+        }
+
+        if !matches!(
+            cx.tcx.lookup_stability(item.def_id),
+            Some(Stability { level: StabilityLevel::Unstable { .. }, .. })
+        ) {
+            return;
+        }
+        if !matches!(
+            cx.tcx.lookup_const_stability(item.def_id),
+            Some(ConstStability { level: StabilityLevel::Stable { .. }, .. })
+        ) {
+            return;
+        }
+
+        cx.struct_span_lint(INCOMPATIBLE_STABILITY, item.span, |lint| {
+            lint.build("functions cannot be const-stable if they are unstable").emit();
+        });
+    }
 }

src/test/ui/lint/incompatible_stability.rs

@@ -1,6 +1,6 @@
 // compile-flags: -Zunstable-options
 #![stable(feature = "stable", since = "1.0.0")]
-#![feature(staged_api, rustc_attrs)]
+#![feature(staged_api, rustc_attrs, intrinsics)]
 
 #[unstable(feature = "unstable", issue = "none")]
 #[rustc_const_stable(feature = "stable", since = "1.0.0")]
@@ -13,4 +13,12 @@ mod bar {
     const fn foo() {} //~ ERROR functions cannot be const-stable if they are unstable
 }
 
+mod intrinsics {
+    #![unstable(feature = "unstable", issue = "none")]
+    extern "rust-intrinsic" {
+        #[rustc_const_stable(feature = "stable", since = "1.0.0")]
+        pub fn transmute<T, U>(_: T) -> U; //~ ERROR functions cannot be const-stable if they are unstable
+    }
+}
+
 fn main() {}

src/test/ui/lint/incompatible_stability.stderr

@@ -12,5 +12,11 @@ error: functions cannot be const-stable if they are unstable
 LL |     const fn foo() {}
    |     ^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: functions cannot be const-stable if they are unstable
+  --> $DIR/incompatible_stability.rs:20:9
+   |
+LL |         pub fn transmute<T, U>(_: T) -> U;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors