Rollup merge of #72008 - lcnr:patch-3, r=varkor

Add const-generics test

Taken from #71973 as this apparently already compiles.

r? @varkor

src/test/ui/const-generics/trait-const-args.rs

@@ -0,0 +1,29 @@
+// check-pass
+#![allow(incomplete_features)]
+#![feature(const_generics)]
+
+struct Const<const N: usize>;
+trait Foo<const N: usize> {}
+
+impl<const N: usize> Foo<N> for Const<N> {}
+
+fn foo_impl(_: impl Foo<3>) {}
+
+fn foo_explicit<T: Foo<3>>(_: T) {}
+
+fn foo_where<T>(_: T)
+where
+    T: Foo<3>,
+{
+}
+
+fn main() {
+    foo_impl(Const);
+    foo_impl(Const::<3>);
+
+    foo_explicit(Const);
+    foo_explicit(Const::<3>);
+
+    foo_where(Const);
+    foo_where(Const::<3>);
+}