Comment by cogman10

Encoder and decoder writers frequently need extremely fine grain control over SIMD instructions in order to get good performance.

The way they weave these instructions can be very hard to express with a high level language.

Further, there's a ton of work with arrays and importantly parts of arrays. They can, for example, need to extract every other element up to 1/2 the array. Unfortunately, rust has runtime array bounds checks which make writing that sort of code slower. The compiler can elade those checks, but usually only in simple cases.

The authors would be writing a bunch of unsafe rust to get the performance they want and rust makes that more painful on purpose.

I like rust, but C/ASM really is the right choice here. This is one of the few cases where rust's safety is a major detriment.