Interesting, but mostly just sounds like Rust issues, and requiring some nonsense to fix issues that shouldn't have existed in the first place.

Leading me to the conclusion that Rust is a dubious choice for highly optimized SIMD code.

The "comparing a pair of i16s" thing is clearly not just a Rust issue since the C version had the same hack.

transpiled code is rarely good. Rust is often better than C for SIMD code (it actually has useful SIMD instructions exposed, and aliasing guarantees make it a lot easier for the compiler to figure out obvious optimization. By transpiling, however you loose most of the structure of an idiomatic project and generally make a bit of a mess of things.