Generate `unsafe extern` blocks (Edition 2024)

[marmitar]

I know Edition 2024 is very experimental at the moment, so this is more of a heads-up than an actual issue.

Description

Rust has just implemented RFC 3484, requiring all extern blocks to be marked unsafe extern in Edition 2024. Older editions don't yet compile this unsafe extern construct (as of the latest stable, rustc-1.80.0), but they will eventually accept the unsafe keyword for compatibility reasons. This crate needs some way of generating bindings with this future requirements, either via Builder configuration, or by detecting the current edition.

I've implemented a minimal (but complete) example that triggers the error here: bindgen-unsafe-extern.

Input C/C++ Header

void cool_function(int i, char c);

Bindgen Invocation

bindgen::Builder::default()
    .header("src/cool.h")
    .generate()
    .unwrap()
    .write_to_file("src/cool.rs")
    .unwrap()

Actual Results

Generated file:

extern "C" {
    pub fn cool_function(i: ::std::os::raw::c_int, c: ::std::os::raw::c_char);
}

Compiler error:

error: extern blocks must be unsafe
 --> src/cool.rs:3:1
  |
3 | / extern "C" {
4 | |     pub fn cool_function(i: ::std::os::raw::c_int, c: ::std::os::raw::c_char);
5 | | }
  | |_^

Expected Results

For Edition 2024, the generated file should be:

unsafe extern "C" {
    pub fn cool_function(i: ::std::os::raw::c_int, c: ::std::os::raw::c_char);
}

[GKFX]

It's worth noting that Edition 2024 safe extern functions, plus either the proposed [[safe]] attribute for C (N2659) or an equivalent, would solve #2774.

[ojeda]

The proposed [[safe]] attribute did not get enough support from the committee back then, but lately memory safety is talked about more in WG14 and is in the latest charter, so perhaps things change. Rust safe items within unsafe extern blocks could help there.

Or perhaps a C implementation could add something like that to improve interop with Rust for their users... :)

[ehuss]

Just FYI, unsafe extern is now stabilized in all editions in 1.82.0 (2024-10-17). And just a heads up that we are still aiming to have the 2024 edition hit stable in 1.85 (2025-02-20).

[pvdrz]

once #3002 is merged, bindgen will have support for editions. However, it seems to me that this should be enabled regardless of the edition for every rust target greater or equal than 1.82 as not using unsafe extern will generate a warning for all the editions before 2024 anyway. Is that correct?

[ojeda]

Yeah, as far as I can see, the lint is not enabled by default (rust_2024_compatibility or missing_unsafe_on_extern) but it can be used starting with 1.82 even for 2015, so it could help those that want to enable the warning.