I’d like to see a compiler warning for when someth...

# stdlib

[Mark]

I’d like to see a compiler warning for when something like buildString is used but the StringBuilder receiver is never accessed. Same for buildList , buildSet etc. My motivation for this is that after some refactoring, I made this mistake:

buildString {
    items.joinToString(separator = " ") {
        foo(it)
    }
}

[Klitos Kyriacou]

That sounds fairly reasonable, but too specific to me. How about something more general, such as a compiler warning when the this receiver is not used in a lambda? That would include scope functions such as apply, with and run.
Maybe this is the wrong channel as it's about compiler behaviour.

[Mark]

I think there are often legitimate reasons to not (in general) use a receiver in a lambda, but in the examples I gave, there are no reasons not to use it. Perhaps a function that accepts a lambda can be annotated somehow to say that the receiver must be used.

[Jeff Lockhart]

This seems like a similar issue as unused function return values. Sometimes it is completely valid to ignore the value. But other times it should be considered a misuse and result in a warning. Maybe a similar @CheckReceiver annotation for functions/lambdas makes sense?

[Klitos Kyriacou]

There are many cases when the receiver is it and there's no need to check it, but I can't think of any ignorable cases at the moment for a this receiver.

[CLOVIS]

A case where it is safe to ignore the receiver:

runBlocking {
    delay(5000)
    println("Done")
}

It's reasonable to write this code, and I'm never using the CoroutineScope receiver.

[Klitos Kyriacou]

Thanks, and, come to think of it, I do it all the time with KoTest.