[dikaiosune]

Many types in the standard library do not consider it sound behavior to have their allocations freed by functions other than their own destructors:

https://doc.rust-lang.org/std/boxed/struct.Box.html#method.i...

    After calling this function, the caller is responsible for the memory 
    previously managed by the Box. In particular, the caller should properly 
    destroy T and release the memory. The proper way to do so is to convert 
    the NonNull<T> pointer into a raw pointer and back into a Box with the 
    Box::from_raw function.

I would guess that this might become a fun latent footgun in crates.io code -- everyone writes their unsafe code in a way that Just Works under the default allocator, but then things break down in confusing ways when enabling alternative allocators, which should be safe to do even when using external crates.

[steveklabnik]

That's already true in cases; for example, I've seen FFI code that has a use-after-free bug in it, but because of the different way that the allocator works on different platforms, on OS X it "worked", but on Linux, it segfaulted.