Cleanup check_cast. Fixes #21554

This also makes the cast error messages somewhat more uniform.

src/test/compile-fail/cast-from-nil.rs

@@ -8,5 +8,5 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: cast from nil: `()` as `u32`
+// error-pattern: non-scalar cast: `()` as `u32`
 fn main() { let u = (assert!(true) as u32); }

src/test/compile-fail/cast-to-nil.rs

@@ -8,5 +8,5 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern: cast to nil: `u32` as `()`
+// error-pattern: non-scalar cast: `u32` as `()`
 fn main() { let u = 0u32 as (); }

src/test/compile-fail/issue-10991.rs

@@ -10,5 +10,5 @@
 
 fn main() {
     let nil = ();
-    let _t = nil as usize; //~ ERROR: cast from nil: `()` as `usize`
+    let _t = nil as usize; //~ ERROR: non-scalar cast: `()` as `usize`
 }

src/test/compile-fail/issue-21554.rs

@@ -0,0 +1,15 @@
+// Copyright 2015 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.
+
+struct Inches(i32);
+
+fn main() {
+    Inches as f32; //~ ERROR illegal cast; cast through an integer first
+}

src/test/compile-fail/typeck-cast-pointer-to-float.rs

@@ -11,5 +11,5 @@
 fn main() {
     let x : i16 = 22;
     ((&x) as *const i16) as f32;
-    //~^ ERROR: cannot cast from pointer to float directly: `*const i16` as `f32`
+    //~^ ERROR illegal cast; cast through an integer first: `*const i16` as `f32`
 }