#[track_caller] for allocator APIs?

[anp]

Apologies if this has been raised before, but I've been playing around with trying to track where allocations happen with something like so:

use libc_print::libc_println;
use std::alloc::{GlobalAlloc, Layout};
use std::panic::Location;

pub struct TracedAlloc<T: GlobalAlloc> {
    pub allocator: T,
}

unsafe impl<T> GlobalAlloc for TracedAlloc<T>
where
    T: GlobalAlloc,
{
    #[track_caller]
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        libc_println!("Alloc {:?} at {:?}", layout, Location::caller());
        self.allocator.alloc(layout)
    }

    #[track_caller]
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        libc_println!("Dealloc {:?} at {:?}", layout, Location::caller());
        self.allocator.dealloc(ptr, layout)
    }
}

However the caller location is always the line I've put #[global_allocator] which makes #[track_caller] useless in this context.

Originally posted by @xd009642 in #47809 (comment)

---

note from @anp: I'm not sure how the global allocator hooks in but this might be possible?

[jdm]

I'm not sure if this will work, given the indirection in

rust/src/liballoc/alloc.rs

Lines 77 to 81 in 8cb94fc

	#[stable(feature = "global_alloc", since = "1.28.0")]
	#[inline]
	pub unsafe fn alloc(layout: Layout) -> *mut u8 {
	unsafe { __rust_alloc(layout.size(), layout.align()) }
	}

. That method is called by the methods generated by #[global_alloc], per this comment.

[anp]

Yeah, the only way to carry #[track_caller] through an extern "Rust" block is if both sides have it declared. It's technically possible but it would be a new (breaking) requirement on all allocators so I don't think that's feasible.

[lqd]

cc #74433