Why is token_program needed when initiating an associated token account?

Question

In the code below if I don't have token_program defined, I get compile time error. However, I am only initializing the ATA but not minting, transferring, approving, or burning the token mint which is handled by token program.

 #[account(
        init_if_needed,
        payer = buyer,
        associated_token::mint = mint,
        associated_token::authority = buyer
    )]
    pub buyer_mint: InterfaceAccount<'info,TokenAccount>,

    pub associated_token_program: Program<'info, AssociatedToken>
    pub token_program: Interface<'info, TokenInterface>,

But if I assume the ATA is already initialized below, I don't get any compile time errors if I omit both token program and atp which makes sense because the program assumes its an existing ata.

 #[account(
        mut,
        associated_token::mint = mint,
        associated_token::authority = buyer
    )]
    pub buyer_mint: InterfaceAccount<'info,TokenAccount>,

My question is what does token_program has to do in the first code base ?

Answer 1

TL;DR, solana needs to know all accounts that an instruction will interact with beforehand. That is why your program will panic if an account required by the #[derive(Accounts)] struct isn't provided

In your case,

   #[account(
        init_if_needed,
        payer = buyer,
        associated_token::mint = mint,
        associated_token::authority = buyer
    )]

this attribute macro will create the ATA if it doesn't exist because of the init_if_needed clause. As you've already guessed, you'll also need to pass in the associated_token_program responsible for creating this account.

In the second snippet

 #[account(
        mut,
        associated_token::mint = mint,
        associated_token::authority = buyer
    )]
    pub buyer_mint: InterfaceAccount<'info,TokenAccount>,

An ATA is simply a token account that's mapped to a particular address and that's why this is still correct.

I think that maybe an error might have been in the snippet because the account name is buyer_mint

Finally, specify the token_program because it's responsible for all SPL token operations.