Feature gate: #![feature(c_size_t)].

This is a tracking issue for std::os::raw::{c_size_t, c_ssize_t}, which are guaranteed to be the same size as the underlying C size_t and ssize_t types from stddef.h.

Currently, on all targets, this is equivalent to usize and isize, however Rust has historically gone somewhat out of its way to avoid promising this. There are some targets with vaguely-planned support where this is not true (W65, used for SNES homebrew, for example)

Further reading here is available:

• https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/.60usize.60.20vs.20.60size_t.60
• https://internals.rust-lang.org/t/abi-discussion-for-w65/15092
• size_t vs usize rust-bindgen#1671

And probably more.

Public API

// std::os::raw

pub type c_size_t = usize;
pub type c_ssize_t = isize;

Steps / History

• Implementation: Add c_size_t and c_ssize_t to std::os::raw. #88340
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• Do we want to instead guarantee usize and size_t are the same? See https://internals.rust-lang.org/t/pre-rfc-usize-is-not-size-t/15369
• Should this instead live in libcore somehow, given that probably libstd may never support platforms where this is not true. (This may not be true, since I imagine CHERI will support libstd)
• Do we need both the signed and unsigned version, given that size_t is more common in function signatures.
• ...