Add tests for unsized-locals functions stability.

src/test/run-pass/unsized-locals/unsized-exprs.rs

@@ -34,4 +34,5 @@ fn main() {
     udrop::<[u8]>((*foo()));
     udrop::<[u8]>((*tfoo()).1);
     *afoo() + 42;
+    udrop as fn([u8]);
 }

src/test/ui/unsized-locals/auxiliary/ufuncs.rs

@@ -0,0 +1,3 @@
+#![feature(unsized_locals)]
+
+pub fn udrop<T: ?Sized>(_x: T) {}

src/test/ui/unsized-locals/unsized-exprs3.rs

@@ -0,0 +1,10 @@
+// aux-build:ufuncs.rs
+
+extern crate ufuncs;
+
+use ufuncs::udrop;
+
+fn main() {
+    udrop as fn([u8]);
+    //~^ERROR E0277
+}

src/test/ui/unsized-locals/unsized-exprs3.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-exprs3.rs:8:5
+   |
+LL |     udrop as fn([u8]);
+   |     ^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `[u8]`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: all function arguments 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`.