Work around "a pure expression does nothing" warning, take 2

In https://github.com/sbt/sbt/pull/5981 I tried to work around the spruious post-macro "a pure expression does nothing" warning (https://github.com/scala/bug/issues/12112) by trying to remove some pure-looking expressions out of the tree.

This quickly backfired when it was reported that sbt 1.4.3 was not evaluating some code. This backs out the macro-level manipulation, and instead try to silence the warning at the reporter level. This feels safer, and it seems to work just as well.

core-macros/src/main/scala/sbt/internal/util/appmacro/Instance.scala

@@ -187,9 +187,7 @@ object Instance {
       qual.foreach(checkQual)
       val vd = util.freshValDef(tpe, qual.pos, functionSym)
       inputs ::= new Input(tpe, qual, vd)
-      // try to workaround https://github.com/scala/bug/issues/12112 by calling Predef.identity(...)
-      val rv = util.refVal(selection, vd)
-      q"scala.Predef.identity[$tpe]($rv: $tpe)"
+      util.refVal(selection, vd)
     }
     def sub(name: String, tpe: Type, qual: Tree, replace: Tree): Converted[c.type] = {
       val tag = c.WeakTypeTag[T](tpe)

main/src/main/scala/sbt/internal/server/BuildServerProtocol.scala

@@ -204,10 +204,11 @@ object BuildServerProtocol {
       val converter = fileConverter.value
       val underlying = (Keys.compile / compilerReporter).value
       val logger = streams.value.log
+      val meta = isMetaBuild.value
       if (bspEnabled.value) {
-        new BuildServerReporterImpl(targetId, converter, logger, underlying)
+        new BuildServerReporterImpl(targetId, converter, meta, logger, underlying)
       } else {
-        new BuildServerForwarder(logger, underlying)
+        new BuildServerForwarder(meta, logger, underlying)
       }
     }
   )

main/src/main/scala/sbt/internal/server/BuildServerReporter.scala

@@ -28,6 +28,9 @@ import scala.collection.mutable
 
 sealed trait BuildServerReporter extends Reporter {
   private final val sigFilesWritten = "[sig files written]"
+  private final val pureExpression = "a pure expression does nothing in statement position"
+
+  protected def isMetaBuild: Boolean
 
   protected def logger: ManagedLogger
 
@@ -52,6 +55,9 @@ sealed trait BuildServerReporter extends Reporter {
   override def log(problem: Problem): Unit = {
     if (problem.message == sigFilesWritten) {
       logger.debug(sigFilesWritten)
+    } else if (isMetaBuild && problem.message.startsWith(pureExpression)) {
+      // work around https://github.com/scala/bug/issues/12112 by ignoring it in the reporter
+      logger.debug(problem.message)
     } else {
       publishDiagnostic(problem)
       underlying.log(problem)
@@ -64,6 +70,7 @@ sealed trait BuildServerReporter extends Reporter {
 final class BuildServerReporterImpl(
     buildTarget: BuildTargetIdentifier,
     converter: FileConverter,
+    protected override val isMetaBuild: Boolean,
     protected override val logger: ManagedLogger,
     protected override val underlying: Reporter
 ) extends BuildServerReporter {
@@ -159,6 +166,7 @@ final class BuildServerReporterImpl(
 }
 
 final class BuildServerForwarder(
+    protected override val isMetaBuild: Boolean,
     protected override val logger: ManagedLogger,
     protected override val underlying: Reporter
 ) extends BuildServerReporter {