Add ui test

src/test/ui/rfc-2632-const-trait-impl/std-impl-gate.rs

@@ -0,0 +1,21 @@
+// This tests feature gates for const impls in the standard library.
+
+// revisions: stock gated
+//[gated] run-pass
+
+#![cfg_attr(gated, feature(const_trait_impl, const_default_impls))]
+
+fn non_const_context() -> Vec<usize> {
+    Default::default()
+}
+
+const fn const_context() -> Vec<usize> {
+    Default::default()
+    //[stock]~^ ERROR calls in constant functions are limited
+}
+
+fn main() {
+    const VAL: Vec<usize> = const_context();
+
+    assert_eq!(VAL, non_const_context());
+}

src/test/ui/rfc-2632-const-trait-impl/std-impl-gate.stock.stderr

@@ -0,0 +1,9 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/std-impl-gate.rs:13:5
+   |
+LL |     Default::default()
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.