Add UI test for the variance of types appearing in consts

src/test/ui/variance/variance-associated-consts.rs

@@ -0,0 +1,17 @@
+// Test that the variance computation considers types that
+// appear in const expressions to be invariant.
+
+#![feature(rustc_attrs)]
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+trait Trait {
+    const Const: usize;
+}
+
+#[rustc_variance]
+struct Foo<T: Trait> { //~ ERROR [o]
+    field: [u8; <T as Trait>::Const]
+}
+
+fn main() { }

src/test/ui/variance/variance-associated-consts.stderr

@@ -0,0 +1,10 @@
+error[E0208]: [o]
+  --> $DIR/variance-associated-consts.rs:13:1
+   |
+LL | / struct Foo<T: Trait> {
+LL | |     field: [u8; <T as Trait>::Const]
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+