$ cat foo.rs                        
fn main() {
    if true {
        let _a = ~2;
    } else {
        fail!()
    }
}
$ rustc foo.rs 
$ ./foo    
foo(23204,0x107be0000) malloc: *** error for object 0x7fd132c02790: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
zsh: abort      ./foo