[FR]: Tag absl::MutexLock with [[nodiscard]] / ABSL_MUST_USE_RESULT

[ppetr]

It happened a few times to me already that I had

{
   absl::MutexLock(&my_lock);
   // ...
}

in my code, wondering why it's broken. This is quite easy to miss! But adding this tag to the MutexLock constructor would prevent the issue. Namely, https://en.cppreference.com/w/cpp/language/attributes/nodiscard.html has this note:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P1771R1	C++17	[[nodiscard]] on constructors has no effect	can cause a warning if the constructed object is discarded

[derekmauro]

I agree that this is a good feature request, in fact I tried to make this change in 2022. Unfortunately I found a lot of broken code in our internal codebase, but I didn't have time to clean it all up, so I had to put it on the back-burner.

By the way, in the latest version of Abseil, absl::Mutex is compatible with std::lock_guard and friends, which may already be tagged with [[nodiscard]].