simd support in Rust was only recently accepted and is being implemented so it currently relies on the vectorisation abilities on the compiler (it might get revisited soon-ish I guess).
As for the assumption of mostly-ascii, the validation function has a "striding" fast path for ascii which checks 2 words at a time (so 128 bits per iteration on 64b platforms) until it finds a non-ascii byte: https://doc.rust-lang.org/src/core/str/mod.rs.html#1541
As for the assumption of mostly-ascii, the validation function has a "striding" fast path for ascii which checks 2 words at a time (so 128 bits per iteration on 64b platforms) until it finds a non-ascii byte: https://doc.rust-lang.org/src/core/str/mod.rs.html#1541