Rollup merge of rust-lang#89588 - BoxyUwU:add_a_test_uwu, r=lcnr

Add a test for generic_const_exprs

Test that const_eval_resolve evaluates consts with unused inference vars in substs

r? `@lcnr`

src/test/ui/const-generics/generic_const_exprs/const_eval_resolve_canonical.rs

@@ -0,0 +1,29 @@
+// run-pass
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+trait Foo<const N: usize> {
+    type Assoc: Default;
+}
+
+impl Foo<0> for () {
+    type Assoc = u32;
+}
+
+impl Foo<3> for () {
+    type Assoc = i64;
+}
+
+fn foo<T, const N: usize>(_: T) -> <() as Foo<{ N + 1 }>>::Assoc
+where
+    (): Foo<{ N + 1 }>,
+{
+    Default::default()
+}
+
+fn main() {
+    // Test that we can correctly infer `T` which requires evaluating
+    // `{ N + 1 }` which has substs containing an inference var
+    let mut _q = Default::default();
+    _q = foo::<_, 2>(_q);
+}