[Feature]: Make FlagSet struct FFI-safe

[Kiuhbit]

Is there an existing issue for this?

• I have searched the existing issues

Description

Currently, the FlagSet struct doesn't have a repr attribute on it, which means it's technically not FFI-safe.

When writing an extern fn which takes a FlagSet or a struct with a FlagSet field, I get the following compiler warning:

warning: `extern` fn uses type `FlagSet<ExampleFlags>`, which is not FFI-safe
  --> src/lib.rs:16:43
   |
16 | pub extern fn do_thing_with_flags(_flags: FlagSet<ExampleFlags>) {
   |                                           ^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout
   = note: `#[warn(improper_ctypes_definitions)]` on by default

Here's the complete example code I used to generate that warning:

lib.rs:

use flagset::{FlagSet, flags};

flags! {
    #[repr(C)]
    pub enum ExampleFlags: u32 {
        Flag1,
        Flag2,
    }
}

#[repr(C)]
pub struct ExampleStruct {
    pub flags: FlagSet<ExampleFlags>
}

pub extern "C" fn do_thing_with_flags(_flags: FlagSet<ExampleFlags>) {
    // code here
}

pub extern "C" fn do_things_with_struct(_example_struct: ExampleStruct) {
    // code here
}

Cargo.toml:

[package]
name = "ffitest"
version = "0.1.0"
edition = "2021"

[dependencies]
flagset = "0.4.3"

Acceptance Criteria

FlagSet structs can be used in extern functions without improper_ctypes_definitions compiler warnings.

Suggestions for a technical implementation

Add #[repr(transparent)] or #[repr(C)] to the FlagSet struct. I think this would technically be considered a breaking change, although I doubt it makes any difference to the actual memory layout of a tuple struct with a single value.