pub fn casefold<'a>(input: &'a str, mode: CaseFoldMode) -> Cow<'a, str>Expand description
Fold a string using simple case folding.
Returns Cow::Borrowed if the string is already fully case-folded
(no characters changed).
In CaseFoldMode::Standard, the implementation runs an ASCII fast path:
64-byte chunks are scanned via the dispatched SIMD scanner with bound
0x80. Chunks with zero non-ASCII bytes get a scalar mask | 0x20-style
lowercase pass (no per-byte trie lookup), which dominates throughput on
ASCII / Latin-1 inputs. Non-ASCII chunks fall back to the per-codepoint
trie-driven path. Other modes (Turkish, etc.) skip the fast path because
their override rules apply within the ASCII range.