Stream.withdraw amount subtraction can be unchecked

[code423n4]

Handle

hyh

Vulnerability details

Impact

Gas is overspent on check (compiler is 0.8.0). As withdraw is a core user facing function even small optimizations are recommended.

Proof of Concept

withdraw function:
https://github.com/code-423n4/2021-11-streaming/blob/main/Streaming/src/Locke.sol#L455

Also, depositTokenAmount variable always changes along with ts.tokens one, with the only difference that ts.tokens can be reduced by updateStreamInternal logic:
https://github.com/code-423n4/2021-11-streaming/blob/main/Streaming/src/Locke.sol#L229

Recommended Mitigation Steps

This way depositTokenAmount is always not less than ts.tokens and can be decreased unchecked.

Now:

require(ts.tokens >= amount, "amt");
ts.tokens -= amount;
...
depositTokenAmount -= amount;

To be:

require(ts.tokens >= amount, "amt");
unchecked {
		ts.tokens -= amount;
		depositTokenAmount -= amount;
}

[0xean]

dupe of #238