Ignore warnings when compiletime.testing is imported

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -503,7 +503,10 @@ object CheckUnused:
           && !imp.isGeneratedByEnum
           && !ctx.owner.name.isReplWrapperName
         then
-          imps.put(imp, ())
+          if imp.isCompiletimeTesting then
+            isNullified = true
+          else
+            imps.put(imp, ())
       case tree: Bind =>
         if !tree.name.isInstanceOf[DerivedName] && !tree.name.is(WildcardParamName) then
           if tree.hasAttachment(NoWarn) then
@@ -533,6 +536,9 @@ object CheckUnused:
         asss.addOne(sym)
       else
         refs.addOne(sym)
+
+    // currently compiletime.testing is completely erased, so ignore the unit
+    var isNullified = false
   end RefInfos
 
   // Names are resolved by definitions and imports, which have four precedence levels:
@@ -547,7 +553,7 @@ object CheckUnused:
       inline def weakerThan(q: Precedence): Boolean = p > q
       inline def isNone: Boolean = p == NoPrecedence
 
-  def reportUnused()(using Context): Unit =
+  def reportUnused()(using Context): Unit = if !refInfos.isNullified then
     for (msg, pos, origin) <- warnings do
       if origin.isEmpty then report.warning(msg, pos)
       else report.warning(msg, pos, origin)
@@ -1031,6 +1037,10 @@ object CheckUnused:
     def isGeneratedByEnum: Boolean =
       imp.symbol.exists && imp.symbol.owner.is(Enum, butNot = Case)
 
+    /** No mechanism for detection yet. */
+    def isCompiletimeTesting: Boolean =
+      imp.expr.symbol == defn.CompiletimeTestingPackage//.moduleClass
+
   extension (pos: SrcPos)
     def isZeroExtentSynthetic: Boolean = pos.span.isSynthetic && pos.span.isZeroExtent
     def isSynthetic: Boolean = pos.span.isSynthetic && pos.span.exists

tests/warn/i21805.scala

@@ -0,0 +1,31 @@
+//> using options -Wunused:all
+
+def i23967: Boolean = {
+  //import scala.compiletime.testing.typeCheckErrors
+  import scala.compiletime.testing.* // nowarn
+  typeChecks("2 + 2")
+}
+
+package p:
+  val code = """"hello, world""""
+package c:
+  class C(i: Int)
+
+package q:
+  import c.* // nowarn, unit is nullified
+  import p.* // nowarn
+  import scala.compiletime.testing.*
+  def test() = typeCheckErrors("""println(C("hello, world"))""")
+  def ok() = typeChecks("println(code)")
+  inline def f(inline i: Int) = 42 + i
+
+package i23967b:
+  package ok:
+    import scala.compiletime.testing.* // nowarn
+    def test() = typeChecks("42 + 27")
+  package nok:
+    import scala.compiletime.testing.typeChecks // nowarn
+    def test() = typeChecks("42 + 27")
+
+@main def Test = println:
+  q.f(27)