auto merge of #16718 : Sawyer47/rust/bool-cast, r=pcwalton

Current version of rust fails when casting from bool, e.g.
```rust
fn main() {
    let _a = false as uint;
    let _b = true as uint;
    let _c: [bool, ..false as uint];
    let _d: [bool, ..true as uint];
    // _a and _b work, but _c and _d result in an error
    // error: expected constant expr for vector length: can't cast str to uint
}
```
This commit makes it work as expected.

src/librustc/middle/const_eval.rs

@@ -517,26 +517,29 @@ fn fromb(b: bool) -> Result<const_val, String> { Ok(const_int(b as i64)) }
                 match ty::get(ety).sty {
                     ty::ty_float(_) => {
                         match val {
+                            const_bool(b) => Ok(const_float(b as f64)),
                             const_uint(u) => Ok(const_float(u as f64)),
                             const_int(i) => Ok(const_float(i as f64)),
                             const_float(f) => Ok(const_float(f)),
-                            _ => Err("can't cast float to str".to_string()),
+                            _ => Err("can't cast this type to float".to_string()),
                         }
                     }
                     ty::ty_uint(_) => {
                         match val {
+                            const_bool(b) => Ok(const_uint(b as u64)),
                             const_uint(u) => Ok(const_uint(u)),
                             const_int(i) => Ok(const_uint(i as u64)),
                             const_float(f) => Ok(const_uint(f as u64)),
-                            _ => Err("can't cast str to uint".to_string()),
+                            _ => Err("can't cast this type to uint".to_string()),
                         }
                     }
-                    ty::ty_int(_) | ty::ty_bool => {
+                    ty::ty_int(_) => {
                         match val {
+                            const_bool(b) => Ok(const_int(b as i64)),
                             const_uint(u) => Ok(const_int(u as i64)),
                             const_int(i) => Ok(const_int(i)),
                             const_float(f) => Ok(const_int(f as i64)),
-                            _ => Err("can't cast str to int".to_string()),
+                            _ => Err("can't cast this type to int".to_string()),
                         }
                     }
                     _ => Err("can't cast this type".to_string())

src/test/run-pass/cast-in-array-size.rs

@@ -16,4 +16,5 @@ fn main() {
     let _a: [bool, ..1 as uint];
     let _b: [int, ..SIZE as uint] = [1, ..SIZE as uint];
     let _c: [bool, ..'\n' as uint] = [true, ..'\n' as uint];
+    let _d: [bool, ..true as uint] = [true, ..true as uint];
 }