Test that unsized locals fail when turning unsized_fn_params feature flag on

src/test/ui/unsized-locals/unsized-locals-using-unsized-fn-params.rs

@@ -0,0 +1,7 @@
+#![feature(unsized_fn_params)]
+
+fn main() {
+    let foo: Box<[u8]> = Box::new(*b"foo");
+    let _foo: [u8] = *foo;
+    //~^ ERROR: the size for values of type `[u8]` cannot be known at compilation time [E0277]
+}

src/test/ui/unsized-locals/unsized-locals-using-unsized-fn-params.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-locals-using-unsized-fn-params.rs:5:9
+   |
+LL |     let _foo: [u8] = *foo;
+   |         ^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+   = note: all local variables must have a statically known size
+   = help: unsized locals are gated as an unstable feature
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.