It's a network driver for Asix PHYs[2]. It's provided as an alternative to an existing driver written in C. The features are equivalent. For details see https://git.kernel.org/torvalds/c/d6beb085e8ff3d9547df8a5a55f15ccc7552c5d0 and https://lwn.net/Articles/949270/
[1] reminder, until now the #Linux #kernel's Rust support was not used for anything practical: https://lwn.net/Articles/952029/
[2] https://git.kernel.org/torvalds/c/cbe0e415089636170aa6eb540ca4af5dc9842a60
