Primitive binops are translated wrong when the LHS is mutated

[arielb1]

STR

fn main() {
    let x = Box::new(0);
    println!("{}", *x + { drop(x); let _ = Box::new(main); 0 });
}

Expected Results

If this code compiles, it should print 0.

Actual Results

Garbage is printed (the address of main, in fact).

[eefriedman]

Looks like this is specifically an issue with the implementation of expr::trans_binary: Rust semantics say the value is supposed to be immediately loaded into a temporary, but expr::trans just returns a pointer into the Box.

[alexcrichton]

triage: I-nominated

[eefriedman]

Some more testcases:

fn main() {
    let x = Box::new(0);
    let mut y = 0;
    *{ drop(x); let _ = Box::new(main); &mut y } = *x;
    println!("{}", y);
}

#[derive(Debug, Copy, Clone)]
struct CoolInt(i32);
impl std::ops::Add<CoolInt> for CoolInt {
    type Output = CoolInt;
    fn add(self, other: CoolInt) -> CoolInt { CoolInt(self.0 + other.0) }
}

fn main() {
    let x = Box::new(CoolInt(0));
    let mut y = CoolInt(0);
    println!("{:?}", *x + *{ drop(x); let _ = Box::new(main); &mut y });
}

struct R { x: i32, y: i32, z: i32 }
fn main() {
    let x = Box::new(0);
    let r = R { x: 0, y: 0, z: 0 };
    let r = R { x: *x, y: { drop(x); let _ = Box::new(main); 0 }, ..r};
    println!("{} {} {}", r.x, r.y, r.z);
}

fn main() {
    let x = Box::new(0);
    let mut y = 0;
    *{ drop(x); &mut y } += *x;
    println!("{}", y);
}

[arielb1]

@eefriedman

Your augmented assignment example is actually an order-of-operations mismatch - a different issue.

[eefriedman]

@arielb1 Yes, I realized that later; filed #27868.

[nikomatsakis]

Related to #28160

[nikomatsakis]

triage: P-medium

[eefriedman]

Looks like this is fixed by MIR.

[Mark-Simulacrum]

Verified as fixed by MIR, marking as E-needstest.

Edit: Well, verified as fixed. No idea if it was by MIR, though.

[nikomatsakis]

Yes, MIR would've fixed this.