Defer "pure expression .. in statement position" warnings until refchecks.

[retronym]

This allows a macro to transform code that used to warn into something that
does not.

[retronym]

/rebuild

[som-snytt]

The macro use case corresponds to the linting mode. It's impossible to know in advance, for a given macro, whether before or after expansion should look "clean."

The "pure expr" check is narrow enough that post-expansion is clearly an error; but if the user writes

m { 42 ; 27 }

it depends on m. Alternatively, it's the responsibility of the macro to issue warnings?

The unused warnings are emitted after typer only because imports are tracked then.

$ scalac -Wmacros:help
Usage: -Wmacros:<mode> where <mode> choices are none, before, after, both (default: before).
  none    Do not inspect expansions or their original trees when generating unused symbol warnings.
  before  Only inspect unexpanded user-written code for unused symbols.
  after   Only inspect expanded trees when generating unused symbol warnings.
  both    Inspect both user-written code and expanded trees when generating unused symbol warnings.

[retronym]

My bias is removing false warnings (especially ones that users can't suppress), so I'm not overly worried about the subset of macros that transform warnable input into non-warnable output trees.

But, with this warning in refchecks, it would possible to hitch it to the -Wmacros option. However -Wmacro, is coarse grained, the right answer might depend on the type of warning (unused vs pure vs ...) and the macro involved.

An example of such a macro where we'd lose the warning is reify:

➜  scala-ref 2.12.x
Welcome to Scala 2.12.12-20200513-094353-8d19363 (OpenJDK 64-Bit Server VM, Java 1.8.0-adoptopenjdk).
Type in expressions for evaluation. Or try :help.

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> reify { 1; 1 }
<console>:15: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
       reify { 1; 1 }
               ^
res0: reflect.runtime.universe.Expr[Int] =
Expr[Int]({
  1;
  1
})

scala> :quit
➜  scala-pr 8995
Welcome to Scala 2.12.12-20200520-011129-58407ea (OpenJDK 64-Bit Server VM, Java 1.8.0-adoptopenjdk).
Type in expressions for evaluation. Or try :help.

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> reify { 1; 1 }
res0: reflect.runtime.universe.Expr[Int] =
Expr[Int]({
  1;
  1
})

A macro implementation that uses a warning reify, like above, would still result in warnings in client code.

➜  scala-ref 2.12.x
scala> import scala.language.experimental.macros; import scala.reflect.macros.blackbox.Context; def impl(c: Context): c.Tree = c.universe.reify { 1; 1 }.tree; def m: Int = macro impl
<console>:11: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
       import scala.language.experimental.macros; import scala.reflect.macros.blackbox.Context; def impl(c: Context): c.Tree = c.universe.reify { 1; 1 }.tree; def m: Int = macro impl
                                                                                                                                                  ^
import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
impl: (c: scala.reflect.macros.blackbox.Context)c.Tree
defined term macro m: Int

scala> m
<console>:15: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
       m
       ^

➜   scala-pr 8995
scala> import scala.language.experimental.macros; import scala.reflect.macros.blackbox.Context; def impl(c: Context): c.Tree = c.universe.reify { 1; 1 }.tree; def m: Int = macro impl
import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
impl: (c: scala.reflect.macros.blackbox.Context)c.Tree
defined term macro m: Int

scala> m
<console>:15: warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
       m
       ^

[som-snytt]

-Wmacros might work better as a -Wconf condition. Sorry to spam your PR. Under lockdown, I have to conserve post-it notes in case they run out at the stationer's.

[retronym]

I appreciate the discussion! This is an aspect of the macro system that didn't have any up-front design so its hard to figure out the answers that balance strictness of warnings vs false positives and have good defaults that avoid the need for users to tweak compiler options.

[lrytz]

I think this is likely to be an improvement for most use cases, so let's book it, even if we're lacking a principled approach.