Coption usage on solana spl-token program

Question

can someone give me a explanation of the usage of repr(C) macro, since in my understanding it simply acts for bridging up the compilation in C, is not wrong?

Further, why does COption keep 4 bytes for only matching the appended tags, shouldn't it be enough in just 1 bit storage?

/// Mint data.
#[repr(C)]
#[derive(Clone, Copy, Debug, Default, PartialEq)]
pub struct Mint {
    /// Optional authority used to mint new tokens. The mint authority may only be provided during
    /// mint creation. If no mint authority is present then the mint has a fixed supply and no
    /// further tokens may be minted.
    pub mint_authority: COption<Pubkey>,
    /// Total supply of tokens.
    pub supply: u64,
    /// Number of base 10 digits to the right of the decimal place.
    pub decimals: u8,
    /// Is `true` if this structure has been initialized
    pub is_initialized: bool,
    /// Optional authority to freeze token accounts.
    pub freeze_authority: COption<Pubkey>,
}


impl Pack for Mint {
    const LEN: usize = 82;
    fn unpack_from_slice(src: &[u8]) -> Result<Self, ProgramError> {
        let src = array_ref![src, 0, 82];
        let (mint_authority, supply, decimals, is_initialized, freeze_authority) =
            array_refs![src, 36, 8, 1, 1, 36];
    ...
}

...

fn pack_coption_u64(src: &COption<u64>, dst: &mut [u8; 12]) {
    let (tag, body) = mut_array_refs![dst, 4, 8];
    match src {
        COption::Some(amount) => {
            *tag = [1, 0, 0, 0];
            *body = amount.to_le_bytes();
        }
        COption::None => {
            *tag = [0; 4];
        }
    }
}
fn unpack_coption_u64(src: &[u8; 12]) -> Result<COption<u64>, ProgramError> {
    let (tag, body) = array_refs![src, 4, 8];
    match *tag {
        [0, 0, 0, 0] => Ok(COption::None),
        [1, 0, 0, 0] => Ok(COption::Some(u64::from_le_bytes(*body))),
        _ => Err(ProgramError::InvalidAccountData),
    }
}

Answer 1

Generally speaking, you are correct about repr(C) however; the rust documentation is your friend.

Regarding COption using 4 vs 1 byte to indicate presence or not. First consider the alignment benefits on memory addressing. Secondly, it does not presume any type of serialization (it is its own).