auto merge of #14786 : pcwalton/rust/enum-to-float-casts, r=alexcrichton

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.

[breaking-change]

Closes #8230.

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")
                 }