Add test

Author: fee1-dead

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

@@ -0,0 +1,9 @@
+#![feature(const_trait_impl)]
+
+pub trait A {}
+//~^ NOTE: this trait must be annotated with `#[const_trait]`
+
+impl const A for () {}
+//~^ ERROR: const `impl`s must be for traits marked with `#[const_trait]`
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/const-impl-requires-const-trait.stderr

@@ -0,0 +1,14 @@
+error: const `impl`s must be for traits marked with `#[const_trait]`
+  --> $DIR/const-impl-requires-const-trait.rs:6:1
+   |
+LL | impl const A for () {}
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+note: this trait must be annotated with `#[const_trait]`
+  --> $DIR/const-impl-requires-const-trait.rs:3:1
+   |
+LL | pub trait A {}
+   | ^^^^^^^^^^^
+
+error: aborting due to previous error
+