'Closeable' return value ignored

[cpurdy]

Gene and I discussed having a compiler warning for methods returning a Closeable value that is ignored by the caller. This seemed reasonable at first, but having implemented it, it turns out that we have many, many places that naturally return a Closeable value that is ignored. For example, various buffers (supporting a fluent style interface) are both Closeable and typically return this. So buf.add(val); would generate this warning, and we have that in thousands of places.

Thoughts?

[markandrewfalco]

Regarding the buffer example, I imagine the reason it is ok to ignore is because the closeable is already handled in some outer scope?

[cpurdy]

The challenge is that we don't know (at compile time) what instance is being returned; we only know the type. So reporting a warning based on the type causes a billion false positives :(

[markandrewfalco]

Perhaps we need an annotation to say that a return value cannot be ignored

[cpurdy]

Instead of just looking for Closeable? i.e. "you should warn any time that someone calls this method and doesn't capture this return value"?

[ggleyzer]

It appears that we need the opposite: "don't warn if someone calls this method and doesn't capture..."

[markandrewfalco]

I think there will be far too many places where things which happen to implement Closeable are returned for opting-out to be practical, I think you have to opt-in. This really becomes a transfer of ownership annotation saying that the receiver is taking over the responsibility for the eventual close. This same annotation could be applied to parameters too, thus indicating to a caller that they are giving up that responsibility. @transferownership, or perhaps @adopt.