Box sometimes forgets to drop its allocator when the Box is conditionally initialized.

[theemathas]

I tried this code:

#![feature(allocator_api)]

use std::alloc::{Allocator, AllocError, Layout, Global};
use std::ptr::NonNull;

struct LoudDropAllocator;

unsafe impl Allocator for LoudDropAllocator {
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        Global.allocate(layout)
    }
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        Global.deallocate(ptr, layout);
    }
}
impl Drop for LoudDropAllocator {
    fn drop(&mut self) {
        println!("dropping allocator...");
    }
}

struct HasDrop;
impl Drop for HasDrop {
    fn drop(&mut self) {}
}

fn foo() {
    println!("foo()");
    let b = Box::new_in(HasDrop, LoudDropAllocator);
    if true {
        drop(*b);
    } else {
        drop(b);
    }
}

fn bar() {
    println!("bar()");
    let b = Box::new_in(HasDrop, LoudDropAllocator);
    if true {
        drop(*b);
    } else {
        // drop(b);
    }
}

fn main() {
    foo();
    bar();
}

I expected the program to output:

foo()
dropping allocator...
bar()
dropping allocator...

Instead it output:

foo()
bar()
dropping allocator...

Seems like there's something wrong with the drop flags.

Making HasDrop not implement Drop causes the program to behave as expected.

Meta

Reproducible on the playground with version 1.83.0-nightly (2024-09-30 fb4aebddd18d258046dd)

[beepster4096]

@rustbot claim

[beepster4096]

Likely caused by #100036. Tried it out on godbolt, and it doesn't reproduce before 1.72. (with RUSTC_BOOTSTRAP=1)