Tracking Issue for `c_variadic_int128`

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 {}

Known issues

• https://github.com/llvm/llvm-project/issues/213541

Steps / History

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

• ACPish: https://github.com/rust-lang/rust/issues/44930#issuecomment-4289951633
• Implementation: https://github.com/rust-lang/rust/pull/155429
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

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

Start by reviewing the implementation in Rust PR #155429 and the ACPish discussion linked in issue #44930. Resolve the question of which targets support 128-bit integers, then follow the FCP and stabilization checklist; done means the target policy is settled and a stabilization PR is ready.