Heap allocation in Box optimized out with rustc 1.60 but not with 1.61+

[Nekrolm]

I tried this code:

pub struct BBox {
    top: i32,
    left: i32,
    height: i32,
    width: i32
}

fn fiddle(bbox: BBox) -> Box<BBox> {
    Box::new(BBox {
        top: bbox.top * 2,
        ..bbox
    })
}

pub fn main() {
    let bbox = Box::new(BBox {
            top: 551,
            left: 100,
            height: 200,
            width: 180,
    });
    let bbox = fiddle(*bbox);
    let bbox_top = bbox.top;
    println!("{}", bbox_top)
}

With rustc 1.60 all Box allocations are totally optimized out.
But starting from 1.61 it doesn't happen.

godbolt link

Possible it is the same issue as #97751

[bugadani]

Interestingly, a much simpler case isn't optimized at all with either compiler version (though generated code changed between the versions): https://godbolt.org/z/5G9cP5n88

[erikdesjardins]

@bugadani that happens because println takes references to its arguments, which keeps the allocation alive. If you force a move then it is optimized out: https://godbolt.org/z/qz8n7M5jd

[erikdesjardins]

Since this starts getting optimized in 1.59 and stops in 1.61, it is likely #92419, which landed in 1.59 and was reverted in 1.61. (see issues linked in #94823 for more info)

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[Jules-Bertholet]

@rustbot label A-codegen I-heavy I-slow -regression-untriaged +regression-from-stable-to-stable