Rollup merge of #74671 - rust-lang:const-generics-coerce-unsized, r=nikomatsakis

add const generics array coercion test

src/test/ui/const-generics/coerce_unsized_array.rs

@@ -0,0 +1,11 @@
+// run-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+fn foo<const N: usize>(v: &[u8; N]) -> &[u8] {
+    v
+}
+
+fn main() {
+    assert_eq!(foo(&[1, 2]), &[1, 2]);
+}