lint: port unstable feature diagnostics

Signed-off-by: David Wood <[email protected]>

Author: davidtwco

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -352,3 +352,5 @@ lint-builtin-const-no-mangle = const items should never be `#[no_mangle]`
 
 lint-builtin-mutable-transmutes =
     transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
+
+lint-builtin-unstable-features = unstable feature

compiler/rustc_lint/src/builtin.rs

@@ -1323,7 +1323,7 @@ impl<'tcx> LateLintPass<'tcx> for UnstableFeatures {
             if let Some(items) = attr.meta_item_list() {
                 for item in items {
                     cx.struct_span_lint(UNSTABLE_FEATURES, item.span(), |lint| {
-                        lint.build("unstable feature").emit();
+                        lint.build(fluent::lint::builtin_unstable_features).emit();
                     });
                 }
             }