Function parse_command_into 

pub fn parse_command_into(
    buf: &[u8],
    dst: &mut Argv,
) -> Result<Option<usize>, ProtocolError>

Same as parse_command, but writes into a caller-provided scratch Argv instead of allocating a new one each call. The reactor stores one Argv per shard and reuses it for every cmd on the local hot path; the internal Vec<u8> + Vec<u32> capacities amortise to zero allocations per command after the first few cmds warm them.

dst is cleared at the start of every call; on Ok(None) and Err, dst is left empty (so the caller doesn’t see partial state).