You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: poly1305.rs
    path of file: ./repos/ring/src/aead
    
    the code of the file until where you have to start completion: // Copyright 2015-2016 Brian Smith.
// Portions Copyright (c) 2014, 2015, Google Inc.
//
// Permission to use, copy, modify
    
    pub(super) fn from_key(Key { key_and_nonce }: Key, cpu_features: cpu::Features) -> Self {
        let mut ctx = Self {
            state: poly1305_state([0u8; OPAQUE_LEN]),
            cpu_features,
        };

        dispatch!(
            cpu_features =>
            (CRYPTO_poly1305_init | CRYPTO_poly1305_init_neon)
            (statep: &mut poly1305_state, key: &[u8; KEY_LEN])
            (&mut ctx.state, &key_and_nonce));

        ctx
    }

    #[inline(always)]
    pub fn update(&mut self, input: &[u8]) {
        dispatch!(
            self.cpu_features =>
            (CRYPTO_poly1305_update | CRYPTO_poly1305_update_neon)
            (statep: &mut poly1305_state, input: *const u8, in_len: c::size_t)
            (&mut self.state, input.as_ptr(), input.len()));
    }

    pub(super) fn finish(mut self) -> Tag {
        let mut tag = Tag([0u8; TAG_LEN]);
        dispatch!(
            self.cpu_features =>
            (CRYPTO_poly1305_finish | CRYPTO_poly1305_finish_neon)
            (statep: &mut poly1305_state, mac: &mut [u8; TAG_LEN])
            (&mut self.state, &mut tag.0));
        tag
    }
}