lang: Require zero accounts to be unique

[acheroncrypto]

Problem

Anchor doesn't guarantee uniqueness of mutable accounts.

Requiring all mutable accounts to be unique could solve the problem, but this would also be redundant in most cases because the uniqueness of the account is usually achieved through other means, e.g. via the seeds constraint. It's not so straightforward to figure out all the cases where it makes sense to require uniqueness, especially when composite and optional accounts are involved.

A subclass of this problem is being able to pass the same account to accounts that have the zero constraint, which was reported by OtterSec during our v0.31 audit. Given it will take some time before we can solve the main problem, this specific problem can be solved in isolation in the meantime.

Summary of changes

Require all accounts that have the zero constraint to be unique.

Note: This change covers regular accounts and composite accounts, but not if they are separate. For example:

#[derive(Accounts)]
pub struct Outer<'info> {
    #[account(zero)]
    pub one: Account<'info, One>,
    #[account(zero)]
    pub two: Account<'info, Two>,
    pub inner: Inner<'info>,
}

#[derive(Accounts)]
pub struct Inner<'info> {
    #[account(zero)]
    pub one: Account<'info, One>,
    #[account(zero)]
    pub two: Account<'info, Two>,
}

In this example,

• outer.one and outer.two
• inner.one and inner.two

is guaranteed to be unique, but outer.one and inner.one can still be the same. This is because proc macros (#[derive(Accounts)]) only have access to the code underneath them, meaning it's not possible to get information about Inner from Outer. For this specific case, we may be able to implement a custom trait that we can call internally during try_accounts, but the potential benefits don't seem to be enough to justify the increased complexity in my opinion.

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