auto merge of #11329 : fhahn/rust/unused-cast-lint2, r=alexcrichton

Updates as mentioned in #11135

src/test/compile-fail/lint-unnecessary-casts.rs

@@ -0,0 +1,24 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[forbid(unnecessary_typecast)];
+
+fn foo_i32(_: i32) {}
+
+fn foo_u64(a: u64) {
+    let b: i32 = a as i32;
+    foo_i32(b as i32); //~ ERROR: unnecessary type cast
+}
+
+fn main() {
+    let x: u64 = 1;
+    let y: u64 = x as u64; //~ ERROR: unnecessary type cast
+    foo_u64(y as u64); //~ ERROR: unnecessary type cast
+}