[msbarnett]

It's the Rust equivalent of the CUSTOM_HASH_FUNCTION macro in the C source. The comment from there might help explain things:

    // Define a custom hash function to use instead of khash's default hash
    // function. This custom hash function uses a simpler bit shift and XOR which
    // results in several percent faster performance compared to when khash's
    // default hash function is used.
    #define CUSTOM_HASH_FUNCTION(key) (khint32_t)((key) ^ (key)>>7)