std::str::from_bytes is that API.
Rust’s current implementation of full validation: https://github.com/rust-lang/rust/blob/2a3f5367a23a769a068c3...
I have a vague feeling there’s an even faster path for probably-ASCII out there, but I can’t immediately recall where and am going to bed. Doubtless someone else will answer before I get up.
The core team will be amenable to replacing this algorithm with something faster presuming it’s still correct.