Fix incremental compilation of cdylib emitting spurious unused_attributes lint

Author: oli-obk

src/libsyntax/feature_gate.rs

@@ -998,7 +998,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     (sym::repr, Normal, template!(List: "C, packed, ..."), Ungated),
     (sym::path, Normal, template!(NameValueStr: "file"), Ungated),
     (sym::automatically_derived, Normal, template!(Word), Ungated),
-    (sym::no_mangle, Normal, template!(Word), Ungated),
+    (sym::no_mangle, Whitelisted, template!(Word), Ungated),
     (sym::no_link, Normal, template!(Word), Ungated),
     (sym::derive, Normal, template!(List: "Trait1, Trait2, ..."), Ungated),
     (

src/test/incremental/no_mangle.rs

@@ -0,0 +1,10 @@
+// revisions:rpass1 rpass2
+// compile-flags: --crate-type cdylib
+// skip-codegen
+
+#![deny(unused_attributes)]
+
+#[no_mangle]
+pub extern "C" fn rust_no_mangle() -> i32 {
+    42
+}