Ignore compiletime.testing for now

[Cherry-picked 4d28ffd]

Author: som-snytt

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

@@ -473,6 +473,7 @@ object CheckUnused:
         if inliners == 0
           && languageImport(imp.expr).isEmpty
           && !imp.isGeneratedByEnum
+          && !imp.isCompiletimeTesting
           && !ctx.owner.name.isReplWrapperName
         then
           imps.put(imp, ())
@@ -1003,6 +1004,10 @@ 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

@@ -2,7 +2,7 @@
 
 def i23967: Boolean = {
   //import scala.compiletime.testing.typeCheckErrors
-  import scala.compiletime.testing.* // warn
+  import scala.compiletime.testing.* // nowarn
   typeChecks("2 + 2")
 }
 
@@ -21,10 +21,10 @@ package q:
 
 package i23967b:
   package ok:
-    import scala.compiletime.testing.* // warn
+    import scala.compiletime.testing.* // nowarn
     def test() = typeChecks("42 + 27")
   package nok:
-    import scala.compiletime.testing.typeChecks // warn
+    import scala.compiletime.testing.typeChecks // nowarn
     def test() = typeChecks("42 + 27")
 
 @main def Test = println: