Supply Management

A fungible token’s total supply must equal the sum of its balances. Transfers preserve that sum; minting and burning change it.

Minting

A mint operation should check authorization and reject a zero recipient, increase total supply and the recipient’s balance by the same amount, then emit Transfer from the zero address. If there is a cap, enforce it before changing balances.

This helper calculates the next total supply for a capped token:

fn supply_after_mint(supply: u256, amount: u256, cap: u256) -> u256 {
    assert!(supply <= cap, "supply exceeds cap")
    assert!(amount <= cap - supply, "mint exceeds cap")
    supply + amount
}

#[test]
fn mints_up_to_cap() {
    assert!(supply_after_mint(supply: 90, amount: 10, cap: 100) == 100)
}

#[test(should_revert)]
fn rejects_above_cap() {
    let _ = supply_after_mint(supply: 90, amount: 11, cap: 100)
}

Checking against cap - supply avoids overflowing an addition while testing the cap. Fe’s ordinary integer arithmetic is also checked for overflow by default, but an ingot can opt out with arithmetic = "unchecked" in its fe.toml (under [ingot] or a [profiles.<name>] table). The explicit cap check keeps the rule independent of that setting.

Burning

A burn operation checks that the source balance covers the amount, decreases both balance and total supply, and emits Transfer to the zero address. A caller may burn its own tokens; burning someone else’s tokens needs an explicit policy, such as consuming an allowance.

Keeping the Ledger Consistent

Put these updates in shared helpers so that constructors and message handlers follow the same rules. Do not expose a setter that changes total supply independently of balances. See CoolCoin for storage-backed mint and burn helpers.

Test zero amounts, exact-balance burns, minting at the cap, rejected unauthorized mints, and delegated burns with insufficient allowance. After successful transfers, check that total supply is unchanged.