[Semester Project] Add new front-end phase for unused entities and add support for unused imports

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -35,6 +35,7 @@ class Compiler {
   protected def frontendPhases: List[List[Phase]] =
     List(new Parser) ::             // Compiler frontend: scanner, parser
     List(new TyperPhase) ::         // Compiler frontend: namer, typer
+    List(new CheckUnused) ::        // Check for unused elements
     List(new YCheckPositions) ::    // YCheck positions
     List(new sbt.ExtractDependencies) :: // Sends information on classes' dependencies to sbt via callbacks
     List(new semanticdb.ExtractSemanticDB) :: // Extract info into .semanticdb files

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -162,12 +162,13 @@ private sealed trait WarningSettings:
     name = "-Wunused",
     helpArg = "warning",
     descr = "Enable or disable specific `unused` warnings",
-    choices = List("nowarn", "all"),
+    choices = List("nowarn", "all", "imports"),
     default = Nil
   )
   object WunusedHas:
     def allOr(s: String)(using Context) = Wunused.value.pipe(us => us.contains("all") || us.contains(s))
     def nowarn(using Context) = allOr("nowarn")
+    def imports(using Context) = allOr("imports")
 
   val Wconf: Setting[List[String]] = MultiStringSetting(
     "-Wconf",

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

@@ -0,0 +1,163 @@
+package dotty.tools.dotc.transform
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.ast.tpd.TreeTraverser
+import dotty.tools.dotc.ast.untpd
+import dotty.tools.dotc.ast.untpd.ImportSelector
+import dotty.tools.dotc.config.ScalaSettings
+import dotty.tools.dotc.core.Contexts._
+import dotty.tools.dotc.core.Decorators.i
+import dotty.tools.dotc.core.Flags.Given
+import dotty.tools.dotc.core.Phases.Phase
+import dotty.tools.dotc.core.StdNames
+import dotty.tools.dotc.report
+import dotty.tools.dotc.reporting.Message
+import dotty.tools.dotc.typer.ImportInfo
+import dotty.tools.dotc.util.Property
+
+/**
+ * A compiler phase that checks for unused imports or definitions
+ *
+ * Basically, it gathers definition/imports and their usage. If a
+ * definition/imports does not have any usage, then it is reported.
+ */
+class CheckUnused extends Phase:
+  import CheckUnused.UnusedData
+
+  private val _key = Property.Key[UnusedData]
+
+  override def phaseName: String = CheckUnused.phaseName
+
+  override def description: String = CheckUnused.description
+
+  override def isRunnable(using Context): Boolean =
+    ctx.settings.WunusedHas.imports
+
+  override def run(using Context): Unit =
+    val tree = ctx.compilationUnit.tpdTree
+    val data = UnusedData(ctx)
+    val fresh = ctx.fresh.setProperty(_key, data)
+    traverser.traverse(tree)(using fresh)
+    reportUnusedImport(data.getUnused)
+
+  /**
+   * This traverse is the **main** component of this phase
+   *
+   * It traverse the tree the tree and gather the data in the
+   * corresponding context property
+   */
+  private def traverser = new TreeTraverser {
+    import tpd._
+
+    override def traverse(tree: tpd.Tree)(using Context): Unit = tree match
+      case imp@Import(_, sels) => sels.foreach { s =>
+          ctx.property(_key).foreach(_.registerImport(imp))
+        }
+      case ident: Ident =>
+        val id = ident.symbol.id
+        ctx.property(_key).foreach(_.registerUsed(id))
+        traverseChildren(tree)
+      case sel: Select =>
+        val id = sel.symbol.id
+        ctx.property(_key).foreach(_.registerUsed(id))
+        traverseChildren(tree)
+      case tpd.Block(_,_) | tpd.Template(_,_,_,_) =>
+        ctx.property(_key).foreach(_.pushScope())
+        traverseChildren(tree)
+        ctx.property(_key).foreach(_.popScope())
+      case _ => traverseChildren(tree)
+
+  }
+
+  private def reportUnusedImport(sels: List[ImportSelector])(using Context) =
+    if ctx.settings.WunusedHas.imports then
+      sels.foreach { s =>
+        report.warning(i"unused import", s.srcPos)
+      }
+end CheckUnused
+
+object CheckUnused:
+  val phaseName: String = "check unused"
+  val description: String = "check for unused elements"
+
+  /**
+   * A stateful class gathering the infos on :
+   * - imports
+   * - definitions
+   * - usage
+   */
+  private class UnusedData(initctx: Context):
+    import collection.mutable.{Set => MutSet, Map => MutMap, Stack, ListBuffer}
+
+
+    private val used = Stack(MutSet[Int]())
+    private val impInScope = Stack(MutMap[Int, ListBuffer[ImportSelector]]())
+    private val unused = ListBuffer[ImportSelector]()
+
+    private def isImportExclusion(sel: ImportSelector): Boolean = sel.renamed match
+      case [email protected](name) => name == StdNames.nme.WILDCARD
+      case _ => false
+
+    /** Register the id of a found (used) symbol */
+    def registerUsed(id: Int): Unit = used.top += id
+
+    /** Register an import */
+    def registerImport(imp: tpd.Import)(using Context): Unit =
+      val tpd.Import(tree, sels) = imp
+      val map = impInScope.top
+      val entries = sels.flatMap{ s =>
+        if s.isWildcard then
+          tree.tpe.allMembers
+            .filter(m => m.symbol.is(Given) == s.isGiven) // given imports
+            .map(_.symbol.id -> s)
+        else
+          val id = tree.tpe.member(s.name.toTermName).symbol.id
+          val typeId = tree.tpe.member(s.name.toTypeName).symbol.id
+          List(id -> s, typeId -> s)
+      }
+      entries.foreach{(id, sel) =>
+        map.get(id) match
+          case None => map.put(id, ListBuffer(sel))
+          case Some(value) => value += sel
+      }
+
+    /** enter a new scope */
+    def pushScope(): Unit =
+      used.push(MutSet())
+      impInScope.push(MutMap())
+
+    /** leave the current scope */
+    def popScope(): Unit =
+      val usedImp = MutSet[ImportSelector]()
+      val poppedImp = impInScope.pop()
+      val notDefined = used.pop().filter{id =>
+        poppedImp.remove(id) match
+          case None => true
+          case Some(value) =>
+            usedImp.addAll(value)
+            false
+      }
+      if used.size > 0 then
+        used.top.addAll(notDefined)
+      poppedImp.values.flatten.foreach{ sel =>
+        // If **any** of the entities used by the import is used,
+        // do not add to the `unused` Set
+        if !usedImp(sel) then
+          unused += sel
+      }
+
+    /**
+     * Leave the scope and return a `List` of unused `ImportSelector`s
+     *
+     * The given `List` is sorted by line and then column of the position
+     */
+    def getUnused(using Context): List[ImportSelector] =
+      popScope()
+      unused.toList.sortBy{ sel =>
+        val pos = sel.srcPos.sourcePos
+        (pos.line, pos.column)
+      }
+
+  end UnusedData
+end CheckUnused
+

tests/neg-custom-args/fatal-warnings/i15503a.scala

@@ -0,0 +1,87 @@
+// scalac: -Wunused:imports
+
+
+object FooUnused:
+  import collection.mutable.Set // error
+  import collection.mutable.{Map => MutMap} // error
+  import collection.mutable._ // error
+
+object FooWildcardUnused:
+  import collection.mutable._ // error
+
+object Foo:
+  import collection.mutable.Set // OK
+  import collection.mutable.{Map => MutMap} // OK
+
+  val bar = Set() // OK
+  val baz = MutMap() // OK
+
+object FooWildcard:
+  import collection.mutable._ // OK
+
+  val bar = Set() // OK
+
+object FooNestedUnused:
+  import collection.mutable.Set // error
+  object Nested:
+    def hello = 1
+
+object FooNested:
+  import collection.mutable.Set // OK
+  object Nested:
+    def hello = Set()
+
+object FooGivenUnused:
+  import SomeGivenImports.given // error
+
+object FooGiven:
+  import SomeGivenImports.given // OK
+  import SomeGivenImports._ // error
+
+  val foo = summon[Int]
+
+/**
+ * Import used as type name are considered
+ * as used.
+ *
+ * Import here are only used as types, not as
+ * Term
+ */
+object FooTypeName:
+  import collection.mutable.Set // OK
+  import collection.mutable.Map // OK
+  import collection.mutable.Seq // OK
+  import collection.mutable.ArrayBuilder // OK
+  import collection.mutable.ListBuffer // error
+
+  def checkImplicit[A](using Set[A]) = ()
+  def checkParamType[B](a: Map[B,B]): Seq[B] = ???
+  def checkTypeParam[A] = ()
+
+  checkTypeParam[ArrayBuilder[Int]]
+
+
+object InlineChecks:
+  object InlineFoo:
+    import collection.mutable.Set // OK
+    import collection.mutable.Map // error
+    inline def getSet = Set(1)
+
+  object InlinedBar:
+    import collection.mutable.Set // error
+    import collection.mutable.Map // error
+    val a = InlineFoo.getSet
+
+object MacroChecks:
+  object StringInterpol:
+    import collection.mutable.Set // OK
+    import collection.mutable.Map // OK
+    println(s"This is a mutableSet : ${Set[Map[Int,Int]]()}")
+
+/**
+  * Some given values for the test
+  */
+object SomeGivenImports:
+  given Int = 0
+  given String = "foo"
+