Lint request: drop_flags

[SoniEx2]

What it does

Detects usage of drop flags.

Categories (optional)

• Kind: Restriction

Drop flags come with a hidden runtime cost, and can sometimes be an oversight.

Drawbacks

None.

Example

let x;
{
  x = RequiresDrop;
  x.do_something();
}

Could be written as:

{
  let x = RequiresDrop;
  x.do_something();
}

---

This is a restriction lint because there are some cases where drop flags seemingly make sense, such as a lazy or optional lock:

let _x; // _x is a LockGuard
if let Some(lock) = self.lock {
  _x = lock.lock();
}
// protected code here
// drop flags take care of _x here

altho we'd argue this code is confusing and an explicit drop flag would be better:

let _x = self.lock.by_ref().map(Lock::lock); // _x is an Option<LockGuard> i.e. it has a drop flag attached to it in the form of an Option.
// protected code here
// Option takes care of _x, not drop flags

[kvverti]

Your first example does not require runtime conditional drop, since r#use takes ownership of x. The second is a valid demonstration of conditional drop.

I would also suggest this be called conditional_drop, so as not to allude to any particular implementation.

[SoniEx2]

maybe it should be called hidden_runtime_cost then, because Option<T> also provides conditional drop. ;)