forbid cast as bool

This is currently unsound since `bool` is represented as `i8`. It will
become sound when `bool` is stored as `i8` but always used as `i1`.

However, the current behaviour will always be identical to `x & 1 != 0`,
so there's no need for it. It's also surprising, since `x != 0` is the
expected behaviour.

Closes #7311

Author: thestinger

src/libextra/ebml.rs

@@ -410,7 +410,7 @@ pub mod reader {
         }
 
         fn read_bool(&mut self) -> bool {
-            doc_as_u8(self.next_doc(EsBool)) as bool
+            doc_as_u8(self.next_doc(EsBool)) != 0
         }
 
         fn read_f64(&mut self) -> f64 {

src/librustc/middle/trans/base.rs

@@ -2219,7 +2219,7 @@ pub fn trans_item(ccx: @mut CrateContext, item: &ast::item) {
               }
               let v = ccx.const_values.get_copy(&item.id);
               unsafe {
-                  if !(llvm::LLVMConstIntGetZExtValue(v) as bool) {
+                  if !(llvm::LLVMConstIntGetZExtValue(v) != 0) {
                       ccx.sess.span_fatal(expr.span, "static assertion failed");
                   }
               }

src/librustc/middle/typeck/check/mod.rs

@@ -2696,6 +2696,7 @@ pub fn check_expr_with_unifier(fcx: @mut FnCtxt,
                         }, t_e, None);
                     }
 
+                    let t1 = structurally_resolved_type(fcx, e.span, t_1);
                     let te = structurally_resolved_type(fcx, e.span, t_e);
                     let t_1_is_char = type_is_char(fcx, expr.span, t_1);
 
@@ -2710,6 +2711,9 @@ pub fn check_expr_with_unifier(fcx: @mut FnCtxt,
                                 fmt!("only `u8` can be cast as `char`, not `%s`", actual)
                             }, t_e, None);
                         }
+                    } else if ty::get(t1).sty == ty::ty_bool {
+                        fcx.tcx().sess.span_err(expr.span,
+                                                "cannot cast as `bool`, compare with zero instead");
                     } else if type_is_region_ptr(fcx, expr.span, t_e) &&
                         type_is_unsafe_ptr(fcx, expr.span, t_1) {
 

src/test/compile-fail/cast-as-bool.rs

@@ -0,0 +1,12 @@
+// Copyright 2013 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.
+
+// error-pattern: cannot cast as `bool`, compare with zero instead
+fn main() { let u = (5 as bool); }

src/test/run-pass/cast.rs

@@ -18,6 +18,5 @@ pub fn main() {
     assert_eq!(i as u8, 'Q' as u8);
     assert_eq!(i as u8 as i8, 'Q' as u8 as i8);
     assert_eq!(0x51u8 as char, 'Q');
-    assert_eq!(true, 1 as bool);
     assert_eq!(0 as u32, false as u32);
 }

src/test/run-pass/supported-cast.rs

@@ -26,7 +26,6 @@ pub fn main() {
   info!(1 as int);
   info!(1 as uint);
   info!(1 as float);
-  info!(1 as bool);
   info!(1 as *libc::FILE);
   info!(1 as i8);
   info!(1 as i16);
@@ -42,7 +41,6 @@ pub fn main() {
   info!(1u as int);
   info!(1u as uint);
   info!(1u as float);
-  info!(1u as bool);
   info!(1u as *libc::FILE);
   info!(1u as i8);
   info!(1u as i16);
@@ -58,7 +56,6 @@ pub fn main() {
   info!(1i8 as int);
   info!(1i8 as uint);
   info!(1i8 as float);
-  info!(1i8 as bool);
   info!(1i8 as *libc::FILE);
   info!(1i8 as i8);
   info!(1i8 as i16);
@@ -74,7 +71,6 @@ pub fn main() {
   info!(1u8 as int);
   info!(1u8 as uint);
   info!(1u8 as float);
-  info!(1u8 as bool);
   info!(1u8 as *libc::FILE);
   info!(1u8 as i8);
   info!(1u8 as i16);
@@ -90,7 +86,6 @@ pub fn main() {
   info!(1i16 as int);
   info!(1i16 as uint);
   info!(1i16 as float);
-  info!(1i16 as bool);
   info!(1i16 as *libc::FILE);
   info!(1i16 as i8);
   info!(1i16 as i16);
@@ -106,7 +101,6 @@ pub fn main() {
   info!(1u16 as int);
   info!(1u16 as uint);
   info!(1u16 as float);
-  info!(1u16 as bool);
   info!(1u16 as *libc::FILE);
   info!(1u16 as i8);
   info!(1u16 as i16);
@@ -122,7 +116,6 @@ pub fn main() {
   info!(1i32 as int);
   info!(1i32 as uint);
   info!(1i32 as float);
-  info!(1i32 as bool);
   info!(1i32 as *libc::FILE);
   info!(1i32 as i8);
   info!(1i32 as i16);
@@ -138,7 +131,6 @@ pub fn main() {
   info!(1u32 as int);
   info!(1u32 as uint);
   info!(1u32 as float);
-  info!(1u32 as bool);
   info!(1u32 as *libc::FILE);
   info!(1u32 as i8);
   info!(1u32 as i16);
@@ -154,7 +146,6 @@ pub fn main() {
   info!(1i64 as int);
   info!(1i64 as uint);
   info!(1i64 as float);
-  info!(1i64 as bool);
   info!(1i64 as *libc::FILE);
   info!(1i64 as i8);
   info!(1i64 as i16);
@@ -170,7 +161,6 @@ pub fn main() {
   info!(1u64 as int);
   info!(1u64 as uint);
   info!(1u64 as float);
-  info!(1u64 as bool);
   info!(1u64 as *libc::FILE);
   info!(1u64 as i8);
   info!(1u64 as i16);
@@ -186,7 +176,6 @@ pub fn main() {
   info!(1u64 as int);
   info!(1u64 as uint);
   info!(1u64 as float);
-  info!(1u64 as bool);
   info!(1u64 as *libc::FILE);
   info!(1u64 as i8);
   info!(1u64 as i16);
@@ -202,7 +191,6 @@ pub fn main() {
   info!(true as int);
   info!(true as uint);
   info!(true as float);
-  info!(true as bool);
   info!(true as *libc::FILE);
   info!(true as i8);
   info!(true as i16);
@@ -218,7 +206,6 @@ pub fn main() {
   info!(1. as int);
   info!(1. as uint);
   info!(1. as float);
-  info!(1. as bool);
   info!(1. as i8);
   info!(1. as i16);
   info!(1. as i32);
@@ -233,7 +220,6 @@ pub fn main() {
   info!(1f32 as int);
   info!(1f32 as uint);
   info!(1f32 as float);
-  info!(1f32 as bool);
   info!(1f32 as i8);
   info!(1f32 as i16);
   info!(1f32 as i32);
@@ -248,7 +234,6 @@ pub fn main() {
   info!(1f64 as int);
   info!(1f64 as uint);
   info!(1f64 as float);
-  info!(1f64 as bool);
   info!(1f64 as i8);
   info!(1f64 as i16);
   info!(1f64 as i32);