Where can I find the reference for the keyword "ensure" in solidity?

Question

I see lots of ensure(deadline) in uniswap's code like

function swapExactTokensForTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external virtual override ensure(deadline) returns (uint[] memory amounts) {
omitted
}

But I had been searching for a while and could not find any reference for it.

Answer 1

ensure isn't a keyword, it is a function modifier. As you can guess, the ensure modifier requires that the swap transaction is executed before the given deadline:

modifier ensure(uint deadline) {
require(deadline >= block.timestamp, 'UniswapV2Router: EXPIRED'); 
_;
    }