token: require all token accounts be rent-exempt and restore total supply field in the mint

[mvines]

We lost the total supply field in the mint when we realized that if token accounts aren't created rent-exempt, they could just disappear and cause the total supply to be inflated.

However we've failed to discover a compelling reason to permit non-rent-exempt token accounts. Additionally with the new CloseAccount instruction, you can reclaim your SOL from a token account at any time.

Requiring rent exemption also simplifies wallets and other token clients, as they no longer need to be considered about a token account disappearing because it was underfunded, either by accident or maliciously.

[CriesofCarrots]

Mints too

[mvines]

and multisigs!

[CriesofCarrots]

One effect of this change is that wrapped SOL accounts cannot hold less wrapped SOL than the account rent-exempt minimum balance. Any concerns about this?

[mvines]

Seems fine to me. Technically not necessary but the consistency is nice. Here's a fun edge case:

1. Wrap the rent-exempt amount of SOL
2. Transfer 1 lamport to another token account
3. Now the source account is no longer rent-exempt.

[mvines]

I think we need a rent_exempt_reserve field, like Stake accounts:
https://github.com/solana-labs/solana/blob/57174cdabed9821b529ed359eab759f7b28f52c4/programs/stake/src/stake_state.rs#L130

[CriesofCarrots]

Technically not necessary but the consistency is nice

Allowing non-rent-exempt wrapped SOL accounts requires a bit of special casing, since rent debits will not be reflected in the account's amount field. So yay, glad that seems fine to you.

I think we need a rent_exempt_reserve field, like Stake accounts:

Agreed, we'll need that for wrapped SOL, so Account structs. I think we can leave that out of Mint and Multisig, since there's no way to withdraw lamports from those accounts.