Tracking Issue for `c_variadic_int128`

[folkertdev]

Feature gate: #![feature(c_variadic_int128)]

This is a tracking issue for the VaArgSafe implementations on 128-bit integers.

These implementations are only available on targets where clang defines __int128.

GCC does not implement __int128 for any 16-bit/32-bit target:

https://gcc.gnu.org/onlinedocs/gcc-15.2.0/gcc/_005f_005fint128.html

There is no support in GCC for expressing an integer constant of type __int128 for targets
with long long integer less than 128 bits wide.

Per https://learn.microsoft.com/en-us/cpp/cpp/data-type-ranges?view=msvc-170, MSVC does not
define __int128.

Clang is slightly more permissive: it defines __int128 on wasm32 (a 32-bit target) and also
does provide __int128 on 64-bit *-pc-windows-msvc, and we follow suit.

Public API

unsafe impl VaArgSafe for i128 {}
unsafe impl VaArgSafe for u128 {}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

• ACPish: Tracking issue for RFC 2137: Support defining C-compatible variadic functions in Rust (c_variadic) #44930 (comment)
• Implementation: Support u128/i128 c-variadic arguments #155429
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Do we support 128-bit integers on the right targets?

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩