if true { ... } runs destructors twice

[alexcrichton]

$ 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

Doesn't get much worse than that. Nominating because this is terrible.

[alexcrichton]

(editing previous comment about unsure destructors are running twice)

Correction, I'm now 100% sure that destructors are running twice:

struct Foo;                      

impl Drop for Foo {              
    fn drop(&mut self) {         
        println!("dropping");
    }                            
}                                
fn main() {                      
    if true {                    
        let _a = Foo;            
    }                            
}                                

That code prints dropping twice.

[emberian]

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println("dropping");
    }
}

fn main() {
    let x = true;
    if x {
        let a = Foo;
    }
}

does not reproduce. Also cannot reproduce on 0.8; bisecting.

[lilyball]

I can't reproduce with static x: bool = true either, or if { true } { ... }. I can reproduce with if !!!false { ... } though, which is interesting. Also if 0 == 0 { ... }

[emberian]

My bisection says #9751 broke it.

[lilyball]

Disabling the if true { .. } special-case in middle::trans::controlflow::trans_if() fixes the issue.

[emberian]

However, with #[unsafe_no_drop_flag] on the struct, this reproduces on 0.8

[lilyball]

Whoa.

if false {
} else {
    let _a = Foo;
}

drops a whopping three times.