Years ago, I've helped giving Rust support for 128 bit integers as one of my first big contributions to rustc. As part of that work, I ported some of the routines concerning 128 bit numbers from C to Rust. That language conversion alone made the implementation more portable, as clang doesn't support 128 bit numbers on all targets. On the flip side, this led to the occasional LLVM bug/limitation being hit because backends are usually written with C in mind, and sometimes even the authors of the backends struggle because the 128 bit routines are a stress test for the register allocator. Another resolved issue is that u128::MAX is not representable as f32, and thus the cast was UB: https://github.com/rust-lang/rust/issues/41799.

The routines have been rewritten/refactored multiple times since, and I had zero involvement in the LLVM work, but I'm still amazed that I was able to hack on code as fundamental as this and being part of shipping a new width for numbers in a "big" language (certainly big now).