Forbid #[target_feature] on safe default implementations

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -442,7 +442,7 @@ fn asm_target_features(tcx: TyCtxt<'_>, did: DefId) -> &FxIndexSet<Symbol> {
 pub fn check_target_feature_trait_unsafe(tcx: TyCtxt<'_>, id: LocalDefId, attr_span: Span) {
     if let DefKind::AssocFn = tcx.def_kind(id) {
         let parent_id = tcx.local_parent(id);
-        if let DefKind::Impl { of_trait: true } = tcx.def_kind(parent_id) {
+        if let DefKind::Trait | DefKind::Impl { of_trait: true } = tcx.def_kind(parent_id) {
             tcx.sess
                 .struct_span_err(
                     attr_span,

tests/ui/rfcs/rfc-2396-target_feature-11/trait-impl.rs

@@ -18,4 +18,10 @@ fn foo(&self) {}
     unsafe fn unsf_foo(&self) {}
 }
 
+trait Qux {
+    #[target_feature(enable = "sse2")]
+    //~^ ERROR cannot be applied to safe trait method
+    fn foo(&self) {}
+}
+
 fn main() {}

tests/ui/rfcs/rfc-2396-target_feature-11/trait-impl.stderr

@@ -1,3 +1,12 @@
+error: `#[target_feature(..)]` cannot be applied to safe trait method
+  --> $DIR/trait-impl.rs:22:5
+   |
+LL |     #[target_feature(enable = "sse2")]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot be applied to safe trait method
+LL |
+LL |     fn foo(&self) {}
+   |     ------------- not an `unsafe` function
+
 error: `#[target_feature(..)]` cannot be applied to safe trait method
   --> $DIR/trait-impl.rs:13:5
    |
@@ -7,5 +16,5 @@ LL |
 LL |     fn foo(&self) {}
    |     ------------- not an `unsafe` function
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors