Constant MAX_LABEL_LEN 

const MAX_LABEL_LEN: u64 = _; // 2_305_843_009_213_693_952u64

Maximum label size (2^64 bits) for SHA-1 and SHA-256 hash functions.

In theory, other hash functions (e.g. SHA-512 and SHA-3) can process longer labels, but such huge inputs are practically impossible on one machine, so we use this limit for all hash functions.