auto merge of #14874 : pcwalton/rust/enum-to-float-casts-part-deux, r=alexcrichton

Closes #14794.

If you're casting from an enum to a float, cast through an integer
first.

[breaking-change]

r? @alexcrichton

src/test/compile-fail/enum-to-float-cast.rs

@@ -0,0 +1,35 @@
+// Copyright 2012 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.
+
+// Tests that enum-to-float casts are disallowed.
+
+enum E {
+    L0 = -1,
+    H0 = 1
+}
+
+enum F {
+    L1 = 1,
+    H1 = 0xFFFFFFFFFFFFFFFF
+}
+
+static C0: f32 = L0 as f32; //~ ERROR illegal cast
+static C1: f32 = H1 as f32; //~ ERROR illegal cast
+
+pub fn main() {
+    let a = L0 as f32;  //~ ERROR illegal cast
+    let b = C0;
+    let c = H1 as f32;  //~ ERROR illegal cast
+    let d = C1;
+    assert_eq!(a, -1.0f32);
+    assert_eq!(b, -1.0f32);
+    assert_eq!(c, -1.0f32);
+    assert_eq!(d, -1.0f32);
+}