Should improper_ctypes_definitions lint warn against boxed DST?

[marmeladema]

Given the following code:

#![deny(improper_ctypes_definitions)]

#[no_mangle]
pub extern "C" fn test_boxed_str() -> Box<str> {
    "test_boxed_str".into()
}

#[no_mangle]
pub extern "C" fn test_boxed_slice() -> Box<[u8]> {
    b"test_boxed_slice"[..].into()
}

Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=dfca164488b7561826bbb61ee137b6bc

The current output is:

• It compiles successfully

Ideally the output should look like:

warning: `extern` fn uses type `Box<str>`, which is not FFI-safe
  --> src/lib.rs:24:32
   |
24 | pub extern "C" fn test_boxed_str() -> Box<str> {
   |                                       ^^^^^^^^ not FFI-safe
   |
   = note: `#[warn(improper_ctypes_definitions)]` on by default
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout

From what I understand, fat pointers layout is not stable and should not be relied upon, pretty much in the same way that relying on the layout of a #[repr(rust)] type is an internal decision to the compiler. If they are now considered stable ABI, then the current lint is fine.

But in any case, I believe the documentation at https://doc.rust-lang.org/std/boxed/index.html#memory-layout could probably be enhanced to speak more about DST. Either to say they should be avoided because the memory layout is not stable, or to explicitly document that it is and makes this a commitment from the compiler.

If those arguments are compelling and the compiler team agrees to those changes, I am willing to implement them, providing a little bit of mentoring from someone 👍

[elichai]

Hi, I'm not part of the compiler team but IMHO this does deserve a warning, because DST do have unspecified layout,
If you want to give it a go the code for this is in rustc_lint::types::check_type_for_ffi, otherwise I don't mind fixing this myself.

If you do work on this and have questions free to ping me on Zulip/Discord(elichai2)/IRC(libera #rust channel, elichai2).

[marmeladema]

I'll try to come up with something but I'll probably ping you for help 🙂 Thanks!