Backport "Ignore warnings when compiletime.testing is imported" to 3.7.4 (#24229)

Backports #24036 to the 3.7.4.

PR submitted by the release tooling.

Author: WojciechMazur

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

@@ -433,6 +433,7 @@ object CheckUnused:
         if inliners == 0
           && languageImport(imp.expr).isEmpty
           && !imp.isGeneratedByEnum
+          && !imp.isCompiletimeTesting
           && !ctx.owner.name.isReplWrapperName
         then
           imps.put(imp, ())
@@ -990,6 +991,10 @@ object CheckUnused:
       then imp.expr.tpe.allMembers.exists(_.symbol.isCanEqual)
       else imp.expr.tpe.member(sel.name.toTermName).hasAltWith(_.symbol.isCanEqual)
 
+    /** 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:imports
+
+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.* // warn should be nowarn
+  import p.* // warn should be 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)