Fix transmutes with boolean source values

Fixes #25746

Author: dotdash

src/librustc_trans/trans/intrinsic.rs

@@ -223,7 +223,7 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
                     let val = if datum.kind.is_by_ref() {
                         load_ty(bcx, datum.val, datum.ty)
                     } else {
-                        datum.val
+                        from_arg_ty(bcx, datum.val, datum.ty)
                     };
 
                     let cast_val = BitCast(bcx, val, llret_ty);

src/test/run-pass/issue-25746-bool-transmute.rs

@@ -0,0 +1,20 @@
+// 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.
+
+use std::mem::transmute;
+
+fn main() {
+    unsafe {
+        let _: i8 = transmute(false);
+        let _: i8 = transmute(true);
+        let _: bool = transmute(0u8);
+        let _: bool = transmute(1u8);
+    }
+}