Remove deprecated advice

[som-snytt]

Scala 3 won't mitigate warning value discard under expr: Unit syntax, so don't advertise it.

The mitigation is not removed, for convenience, and because no one writes normal code like that.

To see how to suppress warnings, use -Wconf:any:warning-verbose in Scala 2 and -Wconf:any:verbose in Scala 3.

Extracted from stalled #10476

[SethTisue]

Is there a different recommendation we could substitute?

[som-snytt]

Every warning is nowarnable. Unfortunately, Scala 3 has different syntax and categories and messages, so there is no specific advice that cross-compiles.

[SethTisue]

What is the recommended mitigation in Scala 3?

I'm asking for a couple reasons, one is that I'd like the PR description to document the current situation for users, the other is that maybe we could do something on the Scala 3 side.

[som-snytt]

scala -Wvalue-discard -Wnonunit-statement -Wconf:any:warning-verbose
Welcome to Scala 2.13.12 (OpenJDK 64-Bit Server VM, Java 21.0.2).
Type in expressions for evaluation. Or try :help.

scala> def f = 42
def f: Int

scala> def g: Unit = f
                     ^
       warning: discarded non-Unit value of type Int
       Applicable -Wconf / @nowarn filters for this warning: msg=<part of the message>, cat=w-flag-value-discard, site=g
                     ^
       warning: unused value of type Int (add `: Unit` to discard silently)
       Applicable -Wconf / @nowarn filters for this warning: msg=<part of the message>, cat=other-pure-statement, site=g
def g: Unit

scala>

and

scala -Wvalue-discard -Wnonunit-statement -Wconf:any:verbose
Welcome to Scala 3.3.1 (21.0.2, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> def f = 42
def f: Int

scala> def g: Unit = f
1 warning found
-- [E175] Potential Issue Warning: -------------------------------------------------------------------------------------
1 |def g: Unit = f
  |              ^
  |              discarded non-Unit value of type (f : => Int)
Matching filters for @nowarn or -Wconf:
  - id=E175
  - name=ValueDiscarding
def g: Unit

TIL dotty doesn't warn nonunit-statement for value discard. (The conversion moves a result expression to statement position, even the user did not "write it like that".)

[lrytz]

Would be good if we could limit it to one warning. -Wvalue-discard is in typer, -Wnonunit-statement in refchecks. Every discarded value will lead to nonunit statement, or?

[som-snytt]

Well, that may be true now. IIRC they used to have slightly different triggers.

scala -Wvalue-discard -Wnonunit-statement
Welcome to Scala 2.13.12 (OpenJDK 64-Bit Server VM, Java 21.0.2).
Type in expressions for evaluation. Or try :help.

scala> def f(): Unit = 42
                       ^
       warning: discarded non-Unit value of type Int(42)
                       ^
       warning: unused value of type Int(42) (add `: Unit` to discard silently)
def f(): Unit

scala> val b = collection.mutable.ListBuffer.empty[Int]
val b: scala.collection.mutable.ListBuffer[Int] = ListBuffer()

scala> def f(): Unit = b += 42
def f(): Unit

scala>

[SethTisue]

I don't mean to be difficult, but I don't feel comfortable merging this unless we can, at the same time, provide users extremely clear guidance about how to proceed, including in cross-built projects. Changing the behavior without providing that seems like churn to me.

[som-snytt]

@SethTisue they will have to supply version-specific -Wconf as shown above. But that is already true now.

This PR doesn't change behavior, only the "advice".