allow referencing impl substs from rustc_on_unimplemented

src/test/ui/on-unimplemented/impl-substs.rs

@@ -0,0 +1,15 @@
+#![feature(rustc_attrs)]
+
+trait Foo<A> {
+    fn foo(self);
+}
+
+#[rustc_on_unimplemented = "an impl did not match: {A} {B} {C}"]
+impl<A, B, C> Foo<A> for (A, B, C) {
+    fn foo(self) {}
+}
+
+fn main() {
+    Foo::<usize>::foo((1i32, 1i32, 1i32));
+    //~^ ERROR the trait bound `(i32, i32, i32): Foo<usize>` is not satisfied
+}