lang: Fix declare_program! using non-instruction composite accounts multiple times

[tomjohn1028]

Problem

Described in #3349

Summary of changes

Use a HashSet to enforce unique naming for account definitions

Resolves #3349

[vercel]

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[acheroncrypto]

Thanks for the fix, but I think the solution is insufficient, because we're deriving the name of the accounts struct from the name of the account field. So in the example you've added:

#[derive(Accounts)]
pub struct SecondUseOfNonInstructionComposite<'info> {
    pub non_instruction_update: NonInstructionUpdate<'info>,
}

This works, but if the accounts were different with the same identifier non_instruction_update:

#[derive(Accounts)]
pub struct SecondUseOfNonInstructionComposite<'info> {
    pub non_instruction_update: DifferentCompositeAccount<'info>,
}

In this case, it would use the incorrect NonInstructionUpdate rather than DifferentCompositeAccount. We can't know the name DifferentCompositeAccount (due to this information not being stored in the IDL), so we'd probably need to add some sort of suffix to non_instruction_update to indicate this is a different set of composite accounts.

[acheroncrypto]

The get_non_instruction_composite_accounts should ideally return a HashSet rather than a Vec, but the IDL types don't implement the necessary traits currently. I guess I thought this check would make them unique, but it doesn't cover the initial outer case.

I think it would be better for the get_non_instruction_composite_accounts to return unique accounts rather than sorting them out here.