util/attributes: Error on #[repr] applied to functions

The #[repr] attribute is only valid for structs, enums, and unions.
Applying it to a function is invalid and should result in an error.
This patch adds a check in the attribute visitor to reject #[repr]
on functions, matching rustc behavior.

Fixes #4232

gcc/rust/ChangeLog:

	* util/rust-attributes.cc (AttributeChecker::check_attributes): Emit error for #[repr].

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4232.rs: New test.

Signed-off-by: Jayant Chauhan <[email protected]>

Author: Pasta-coder

gcc/rust/util/rust-attributes.cc

@@ -912,6 +912,12 @@ AttributeChecker::visit (AST::Function &fun)
 	{
 	  check_link_section_attribute (attribute);
 	}
+      else if (result.name == Attrs::REPR)
+	{
+	  rust_error_at (
+	    attribute.get_locus (),
+	    "attribute should be applied to a struct, enum, or union");
+	}
     }
 
   if (fun.has_body ())

gcc/testsuite/rust/compile/issue-4232.rs

@@ -0,0 +1,3 @@
+// { dg-options "-w" }
+#[repr(C)] // { dg-error "attribute should be applied to a struct, enum, or union" }
+fn a() {}