Rust's String requires allocation, but str (typically seen as the fat pointer type &str) does not. "The cat sat on the mat".split('a') is an iterator over sub-s

Rust's String requires allocation, but str (typically seen as the fat pointer type &str) does not. "The cat sat on the mat".split('a') is an iterator over sub-strings, no more allocation needed here than for 105u32.leading_zeros() A lot of the Rust string API lives in str, not String.