Function normalize_into 

pub fn normalize_into(input: &[u8], out: &mut [u8]) -> Result<usize, NfcError>

Normalize input (well-formed UTF-8) into NFC, writing to out. Returns the number of bytes written.

Errors

• NfcError::InvalidUtf8 — input is not well-formed UTF-8.
• NfcError::OutputOverflow — out is too small to hold the normalized form.
• NfcError::CombiningRunOverflow — a combining run in input exceeds UAX #15’s stream-safe bound (30 non-starters).

Idempotence

normalize_into(normalize_into(x)) == normalize_into(x) byte-for-byte per UAX #15 stability — verified in tests against UCD NormalizationTest.txt.