Stable ABI for floating point numbers

[marioortizmanero]

I've noticed that there's an implementation of StableAbi for most basic types, such as i32, u64, etc with the impl_for_primitive_ints macro. However, f64 isn't in that list, and neither is f32. In fact, f64 isn't mentioned once in the whole repository.

If it helps, their size is always known as per the reference: https://doc.rust-lang.org/stable/reference/type-layout.html#primitive-data-layout. But it's true that they don't have a known representation: https://doc.rust-lang.org/stable/reference/type-layout.html#primitive-representations.

I'm guessing I have to use std::os::raw::c_double and std::os::raw::c_float instead of f64, but is there any way to make this easier to use? It seems to just be an alias to Rust's type (https://doc.rust-lang.org/std/os/raw/type.c_float.html), but I'm not sure if there are any guarantees.

Thanks!

[marioortizmanero]

For what I've investigated, f32 and f64 are indeed FFI-safe, just like i32 or u32. See how this code doesn't warn about FFI issues: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8eefdc9a60206b6bd2f6339c4966df9d.

So even though it doesn't seem to be listed on the reference, it's strictly defined by IEEE 754-2008, meaning that it's equivalent to c_float and that it's stable.

[rodrimati1992]

Added impls of StableAbi for f32 and f64 in the 0.10.3 patch release