Lint request: drop_flags

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

Start by locating existing Clippy restriction lints and the tests that exercise them, then investigate how Rust exposes variables requiring drop across branches. The work is done when the new lint detects the described drop-flag pattern, permits the documented optional-lock case, and has coverage for both examples.