Function encode_string 

pub fn encode_string(
    dst: &mut [u8],
    algorithm: Algorithm,
    version: Version,
    params: &Params,
    salt: &[u8],
    hash: &[u8],
) -> Result<usize, Error>

encode_string(dst, dst_len, ctx, type).

Writes the PHC string into dst (no NUL terminator) and returns its length. Always emits $v=, as the C does.

dst must hold the string plus one byte, because the C reserves room for its NUL terminator and this port keeps the capacity rule identical: a buffer of exactly encoded_len bytes is what succeeds, in Rust and in C alike.

hash.len() plays the role of ctx->outlen — it is the length that gets encoded, so it, and not Params::tag_len_bytes, is what the leading validate_inputs() checks. For a tag produced from these params the two are the same value.

Errors

Error::EncodingFail if dst is too small, or whatever crate::params::validate_inputs returns — encode_string runs it first, exactly like the C.