Unused warning with transparent inline

som-snytt · March 30, 2025, 5:48am

I neglected to reference the open ticket:

Some warnings in inlined code show up at the callsite

opened 09:47AM - 13 Mar 25 UTC

itype:bug itype:question area:reporting area:inline

Some warnings in inlined code show up at the callsite, some don't. It depends on… the compiler phase in which the warning is issued. That seems arbitrary. For example, this definition triggers two warnings: ```scala scala> inline def foo = { 1; Stream(1).head } 2 warnings found -- [E129] Potential Issue Warning: --------------------------------------------- 1 |inline def foo = { 1; Stream(1).head } | ^ | A pure expression does nothing in statement position | | longer explanation available when compiling with `-explain` -- Deprecation Warning: -------------------------------------------------------- 1 |inline def foo = { 1; Stream(1).head } | ^^^^^^ |value Stream in package scala is deprecated since 2.13.0: Use LazyList instead of Stream def foo: Int ``` One of them shows up when using the inline method: ```scala scala> foo 1 warning found -- Deprecation Warning: -------------------------------------------------------- 1 |inline def foo = { 1; Stream(1).head } | ^^^^^^ |value Stream in package scala is deprecated since 2.13.0: Use LazyList instead of Stream val res0: Int = 1 ``` The recent https://github.com/scala/scala3/pull/22682 makes sure that messages filtered by `@nowarn` in the inline def don't leak to the callsite. But does it ever make sense to see warnings in inlined code? Should that be configurable with a flag?

I assume that will be addressed under the aegis of “how to warn about Inlined expansions”. (What knob to press or pull, what to report.)

For student use of 3.6.4, -Wconf:msg=regex, but the message doesn’t have enough information.

Perhaps in future, the origin of the diagnostic will specify what was inlined.

I wonder if msg matches the whole “explanation” when -explain is turned on. Often a message will report that there was code inlined from a file.

Adding @nowarn annotations to inline code, as in your item 2, works only recently. Adding @unused may already work in 3.6, where it issues a warning only if a problem is detected both times the check runs (after typer and again a few phases later).