Module rent 

Rent-exemption helpers.

Solana’s rent model charges accounts for storage on a per-byte-year basis. An account that holds at least (data_len + ACCOUNT_STORAGE_OVERHEAD) * LAMPORTS_PER_BYTE_YEAR * EXEMPTION_THRESHOLD lamports is rent-exempt and never loses balance to rent collection.

This module exposes two things:

1. minimum_balance — a pure function computing the rent-exempt threshold for a given data_len, using the cluster constants that have been in effect on Solana mainnet since launch (lamports_per_byte_year = 3480, exemption_threshold = 2 years, account_storage_overhead = 128 bytes). These values are governed on-chain but have never been changed. If the cluster ever re-governs them, the check will be conservative (strictly requiring at least the pre-governance threshold) — still safe, just not tight.

2. check_rent_exempt — the runtime guard backing the #[account(rent_exempt = enforce)] field keyword emitted by #[hopper::context]. Compares account.lamports() to minimum_balance(account.data_len()) and returns ProgramError::AccountNotRentExempt (Solana’s canonical error code, routed through ProgramError::Custom) on failure.

Why not use sol_get_rent_sysvar?

The syscall is ~100 CU and returns the same values this module hard-codes. Using the constants inline lets the check run at zero additional CU beyond the comparison. Programs that need to read the live Rent sysvar for other reasons (rent-collection scheduling, etc.) can still invoke the syscall directly; this helper is specifically for the rent-exemption gate where the constants suffice.

Constants

ACCOUNT_STORAGE_OVERHEAD

Fixed per-account storage overhead the cluster charges on top of user data. 128 bytes (header + metadata).

EXEMPTION_THRESHOLD_YEARS

Years of rent an account must prepay to be exempt.

LAMPORTS_PER_BYTE_YEAR

Lamports charged per byte of account storage per year.

Functions

check_rent_exempt

Assert that account holds enough lamports to be rent-exempt for its current data length. Used by the #[account(rent_exempt = enforce)] constraint lowering in hopper-macros-proc.

minimum_balance

Minimum lamport balance for an account with data_len bytes of data to be rent-exempt under the current Solana cluster constants.