Lazy Rent Sysvar Fetching for Pre-funded Accounts

[spellsaif]

Use case

• Minimizing required syscalls and saving CUs during account initialization.

What are you trying to do?

Currently, the #[derive(Accounts)] macro uses RentPlan::FetchOnce for instructions involving init or realloc. This unconditionally executes the sol_get_sysvar syscall at the start of the instruction, costing ~100 to 150 CUs.
During initialization in lang/src/cpi/system.rs, the framework uses Rent strictly to check if an account requires additional lamports:

let lamports = rent.try_minimum_balance(space as usize)?;
if account.lamports() >= lamports { 
    // Allocate + Assign
} else {
    // CreateAccount (transfers lamports)
}

If an account is pre-funded, the framework doesn't actually need to calculate the minimum balance. The Solana runtime natively enforces rent-exemption at the end of the transaction anyway. Fetching the Rent sysvar for pre-funded accounts wastes ~150 CUs for no reason.

Proposed solution

We can move the Rent sysvar fetch to be completely lazy, executing only if an on-chain transfer is strictly required (when account.lamports() == 0).

1. Modify RentPlan::FetchOnce in the derive macro so that it passes a lazy closure or Option<Rent> to the initialization context instead of eagerly fetching the sysvar.
2. Update init_account_with_rent (in lang/src/cpi/system.rs) to check if account.lamports() > 0 before attempting to evaluate the minimum rent balance.
3. If true, safely skip the Rent syscall and proceed directly with Allocate and Assign (the SVM guarantees rent-exemption safety at the end of the transaction).
4. If false, invoke the lazy fetch (sol_get_sysvar) to determine the exact lamports required for the System Program transfer.
   This saves ~150 CUs for pre-funded initialization paths automatically.

[L0STE]

Took a concrete pass and I do not think the default behavior should be this. The reality is that even if the account is pre-funded we still need to fetch the rent to make sure that the pre-funded amount is > then the minimum amount required for an account to exist.