add tests and fix comments

compiler/rustc_feature/src/active.rs

@@ -533,8 +533,8 @@ declare_features! (
     ///
     /// NOTE: A limited form of `union U { ... }` was accepted in 1.19.0.
     (active, untagged_unions, "1.13.0", Some(55149), None),
-    /// Allows using the `#[used(retain)]` attribute.
-    (active, used_with_arg, "1.60.0", None, None),
+    /// Allows using the `#[used(linker)]` (or `#[used(compiler)]`) attribute.
+    (active, used_with_arg, "1.60.0", Some(00000), None),
     /// Allows `extern "wasm" fn`
     (active, wasm_abi, "1.53.0", Some(83788), None),
     // !!!!    !!!!    !!!!    !!!!   !!!!    !!!!    !!!!    !!!!    !!!!    !!!!    !!!!

compiler/rustc_feature/src/builtin_attrs.rs

@@ -324,7 +324,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(export_name, Normal, template!(NameValueStr: "name"), FutureWarnPreceding),
     ungated!(link_section, Normal, template!(NameValueStr: "name"), FutureWarnPreceding),
     ungated!(no_mangle, Normal, template!(Word), WarnFollowing),
-    ungated!(used, Normal, template!(Word, List: "used"), WarnFollowing),
+    ungated!(used, Normal, template!(Word, List: "compiler|linker"), WarnFollowing),
 
     // Limits:
     ungated!(recursion_limit, CrateLevel, template!(NameValueStr: "N"), FutureWarnFollowing),

src/test/codegen/used_with_arg.rs

@@ -0,0 +1,12 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+#![feature(used_with_arg)]
+
+// CHECK: @llvm.used = appending global [1 x i8*]
+#[used(linker)]
+static mut USED_LINKER: [usize; 1] = [0];
+
+// CHECK-NEXT: @llvm.compiler.used = appending global [1 x i8*]
+#[used(compiler)]
+static mut USED_COMPILER: [usize; 1] = [0];

src/test/ui/feature-gates/feature-gate-used_with_arg.rs

@@ -0,0 +1,7 @@
+#[used(linker)] //~ ERROR `#[used(linker)]` is currently unstable
+static mut USED_LINKER: [usize; 1] = [0];
+
+#[used(compiler)] //~ ERROR `#[used(compiler)]` is currently unstable
+static mut USED_COMPILER: [usize; 1] = [0];
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-used_with_arg.stderr

@@ -0,0 +1,19 @@
+error[E0658]: `#[used(linker)]` is currently unstable
+  --> $DIR/feature-gate-used_with_arg.rs:1:1
+   |
+LL | #[used(linker)]
+   | ^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(used_with_arg)]` to the crate attributes to enable
+
+error[E0658]: `#[used(compiler)]` is currently unstable
+  --> $DIR/feature-gate-used_with_arg.rs:4:1
+   |
+LL | #[used(compiler)]
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(used_with_arg)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.