librustc: Use *signed* extension when converting enums to floats.

Previously, constants used unsigned extension, while non-constants used
signed extension. This unifies both paths to use signed extension.

If this breaks your code, take a deep breath, go for a walk, and
consider why you're relying on the sign extension semantics of
enum-to-float casts.

Closes #8230.

[breaking-change]

src/librustc/middle/trans/consts.rs

@@ -503,7 +503,7 @@ fn const_expr_unadjusted(cx: &CrateContext, e: &ast::Expr,
                         let s = ty::type_is_signed(ety) as Bool;
                         llvm::LLVMConstIntCast(iv, llty.to_ref(), s)
                     }
-                    expr::cast_float => llvm::LLVMConstUIToFP(iv, llty.to_ref()),
+                    expr::cast_float => llvm::LLVMConstSIToFP(iv, llty.to_ref()),
                     _ => cx.sess().bug("enum cast destination is not \
                                         integral or float")
                 }

src/test/run-pass/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 do *signed* integer-to-float conversion.
+
+enum E {
+    L0 = -1,
+    H0 = 1
+}
+
+enum F {
+    L1 = 1,
+    H1 = 0xFFFFFFFFFFFFFFFF
+}
+
+static C0: f32 = L0 as f32;
+static C1: f32 = H1 as f32;
+
+pub fn main() {
+    let a = L0 as f32;
+    let b = C0;
+    let c = H1 as f32;
+    let d = C1;
+    assert_eq!(a, -1.0f32);
+    assert_eq!(b, -1.0f32);
+    assert_eq!(c, -1.0f32);
+    assert_eq!(d, -1.0f32);
+}