Refactor safe approve method

[gabririgo]

Summary

Refactor the safe approve internal method _safeApprove(...args) to avoid the use of low-level calls.

Motivation

A refactoring of the safe approve method has the scope of:

1. avoiding the use of low-level calls
2. using a set of more explicit try/catch statements to improve readability
3. allowing to exclude edge scenarios without having to make implicit/non-linear assumptions
4. excluding the target token not being a smart contract in an alternative way

Specification

function _safeApprove(address token, address spender, uint256 amount) internal {
  try IToken(token).approve(spender, amount) returns (bool success) {
    assert(success);
  } catch {
    try IToken(token).approve(spender, amount) {

    } catch { revert(); }
  }
}

Rationale

Using a try/catch statement we will be able to have the transaction revert in case the approve call fails silently. While a silent failure does not cause negative side effects to the protocol (the swap transaction would revert with error), we will be able to exclude that the approval target token is a not smart contract or does not implement the method as expected. As a result, we can remove the assertion of the target token being a smart contract, which will result in net gas efficiency (will cost just slightly more in general).

Notes

• An initial implementation has been proposed here.
• As Uniswap is launching v4 sometime in Q1 2024, it would be ideal to have this proposal combined with the future proposal to support the new release.