Support #[align(…)] on fns in extern blocks

compiler/rustc_passes/src/check_attr.rs

@@ -1915,7 +1915,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     /// Checks if the `#[align]` attributes on `item` are valid.
     fn check_align(&self, span: Span, target: Target, align: Align, repr_span: Span) {
         match target {
-            Target::Fn | Target::Method(_) => {}
+            Target::Fn | Target::Method(_) | Target::ForeignFn => {}
             Target::Struct | Target::Union | Target::Enum => {
                 self.dcx().emit_err(errors::AlignShouldBeReprAlign {
                     span: repr_span,

tests/codegen/align-fn.rs

@@ -116,3 +116,17 @@ pub fn align_specified_twice_2() {}
 #[align(32)]
 #[align(256)]
 pub fn align_specified_twice_3() {}
+
+const _: () = {
+    // CHECK-LABEL: align_unmangled
+    // CHECK-SAME: align 256
+    #[unsafe(no_mangle)]
+    #[align(32)]
+    #[align(256)]
+    extern "C" fn align_unmangled() {}
+};
+
+unsafe extern "C" {
+    #[align(256)]
+    fn align_unmangled();
+}