`DispatchFromDyn` impl on `Box` with custom allocator is allowed

I'm not sure if this is a bug or not.

The following code currently compiles:

#![feature(allocator_api, dispatch_from_dyn, unsize)]

use std::{
    alloc::{self, Allocator, Global, Layout},
    marker::Unsize,
    ops::DispatchFromDyn,
    ptr::NonNull,
};

struct MyAllocator;

unsafe impl Allocator for MyAllocator {
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, alloc::AllocError> {
        Global.allocate(layout)
    }
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        unsafe { Global.deallocate(ptr, layout) }
    }
}

impl<U: ?Sized> DispatchFromDyn<Box<U, MyAllocator>> for Box<i32, MyAllocator> where i32: Unsize<U> {}

This seems very suspicious, given that there's a comment saying that this impl should not be allowed: https://github.com/rust-lang/rust/blob/6083f7891b5088184968e050d0eaecc7bcfd2b3d/library/alloc/src/boxed.rs#L2330-L2334

I am unable to find a way to do anything interesting with this though. I couldn't even find a way to do dyn-dispatch with this allocator.

Meta

Reproducible on the playground with version 1.98.0-nightly (2026-05-24 423e3d2529999d5e57fa)

Start with the DispatchFromDyn implementation and comment in library/alloc/src/boxed.rs around lines 2330-2334, then compile the provided nightly reproducer. Determine whether the custom-allocator implementation should be rejected despite compiling; done means the behavior is resolved consistently with the comment.