object DiscardedFuture extends WartTraverser
Flags statements that return a scala.concurrent.Future. Typically, we should not
discard scala.concurrent.Future because exceptions inside the future may not get logged.
Use FutureUtil.doNotAwait to log exceptions and discard the future where necessary.
Also detects discarded cats.data.EitherT[scala.concurrent.Future, ..., ...]
and cats.data.OptionT[scala.concurrent.Future, ...] and arbitrary nestings of those.
Custom type constructors can be registered to take the same role as scala.concurrent.Future
by annotating the type definition with DoNotDiscardLikeFuture.
Also flags usages of scala.collection.IterableOnceOps.foreach and scala.collection.IterableOnceOps.tapEach where the function returns a future-like type.
This wart is a special case of the warts NonUnitStatements and NonUnitForEach
and scalac's -Wnonunit-statement flag,
in that it warns only if the return type is future-like.
Additionally, this wart uses a different set of exceptions when no warning is issued.
We keep this specialized wart for two reasons:
1. It is not practically feasible to use -Wnonunit-statement and NonUnitForEach in scalatest
because it would flag many of the assertions of the form x should be >= y in statement positions.
Yet, it is important to check for discarded futures in tests because a discarded future may hide an exception.
2. In some production code, it is convenient to suppress the warnings coming from -Wnonunit-statement and NonUnitForEach,
just due to how the code is written. In such places, we still want to benefit from the explicit checks
against discarded futures.
This wart does not look at futures that are discarded at the end of a unit-type expression.
These cases are caught by -Ywarn-value-discard. We do not implement a specialized version
for future-like values because we do not expect to suppress the warnings coming from -Ywarn-value-discard.
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