Allow (but don't require) #[unsafe(naked)] so that compiler-builtins can upgrade to it

compiler/rustc_parse/src/validate_attr.rs

@@ -194,6 +194,12 @@ pub fn check_attribute_safety(psess: &ParseSess, safety: AttributeSafety, attr:
             }
         }
     } else if let Safety::Unsafe(unsafe_span) = attr_item.unsafety {
+        // Allow (but don't require) `#[unsafe(naked)]` so that compiler-builtins can upgrade to it.
+        // FIXME(#139797): remove this special case when compiler-builtins has upgraded.
+        if attr.has_name(sym::naked) {
+            return;
+        }
+
         psess.dcx().emit_err(errors::InvalidAttrUnsafe {
             span: unsafe_span,
             name: attr_item.path.clone(),