Add test.

tests/ui/mir/unsize-trait.rs

@@ -0,0 +1,15 @@
+// Check that the interpreter does not ICE when trying to unsize `B` to `[u8]`.
+// This is a `build` test to ensure that const-prop-lint runs.
+// build-pass
+
+#![feature(unsize)]
+
+fn foo<B>(buffer: &mut [B; 2])
+    where B: std::marker::Unsize<[u8]>,
+{
+    let buffer: &[u8] = &buffer[0];
+}
+
+fn main() {
+    foo(&mut [[0], [5]]);
+}