CheckUnused.scala

package dotty.tools.dotc.transform

import scala.annotation.tailrec

import dotty.tools.uncheckedNN
import dotty.tools.dotc.ast.tpd
import dotty.tools.dotc.core.Symbols.*
import dotty.tools.dotc.ast.tpd.{Inlined, 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.{em, i}
import dotty.tools.dotc.core.Denotations.SingleDenotation
import dotty.tools.dotc.core.Flags.*
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.reporting.UnusedSymbol as UnusedSymbolMessage
import dotty.tools.dotc.typer.ImportInfo
import dotty.tools.dotc.util.{Property, SrcPos}
import dotty.tools.dotc.core.Mode
import dotty.tools.dotc.core.Types.{AnnotatedType, ConstantType, NoType, TermRef, Type, TypeTraverser}
import dotty.tools.dotc.core.Flags.flagsString
import dotty.tools.dotc.core.Flags
import dotty.tools.dotc.core.Names.Name
import dotty.tools.dotc.core.NameOps.isReplWrapperName
import dotty.tools.dotc.transform.MegaPhase.MiniPhase
import dotty.tools.dotc.core.Annotations
import dotty.tools.dotc.core.Definitions
import dotty.tools.dotc.core.NameKinds.WildcardParamName
import dotty.tools.dotc.core.Symbols.Symbol
import dotty.tools.dotc.core.StdNames.nme
import dotty.tools.dotc.util.Spans.Span
import scala.math.Ordering


/**
 * 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 private (phaseMode: CheckUnused.PhaseMode, suffix: String, _key: Property.Key[CheckUnused.UnusedData]) extends MiniPhase:
  import CheckUnused.*
  import UnusedData.*

  private inline def unusedDataApply[U](inline f: UnusedData => U)(using Context): Context =
    ctx.property(_key) match
      case Some(ud) => f(ud)
      case None     => ()
    ctx

  override def phaseName: String = CheckUnused.phaseNamePrefix + suffix

  override def description: String = CheckUnused.description

  override def isRunnable(using Context): Boolean =
    super.isRunnable &&
    ctx.settings.Wunused.value.nonEmpty &&
    !ctx.isJava

  // ========== SETUP ============

  override def prepareForUnit(tree: tpd.Tree)(using Context): Context =
    val data = UnusedData()
    tree.getAttachment(_key).foreach(oldData =>
      data.unusedAggregate = oldData.unusedAggregate
    )
    val fresh = ctx.fresh.setProperty(_key, data)
    tree.putAttachment(_key, data)
    fresh

  // ========== END + REPORTING ==========

  override def transformUnit(tree: tpd.Tree)(using Context): tpd.Tree =
    unusedDataApply { ud =>
      ud.finishAggregation()
      if(phaseMode == PhaseMode.Report) then
        ud.unusedAggregate.foreach(reportUnused)
    }
    tree

  // ========== MiniPhase Prepare ==========
  override def prepareForOther(tree: tpd.Tree)(using Context): Context =
    // A standard tree traverser covers cases not handled by the Mega/MiniPhase
    traverser.traverse(tree)
    ctx

  override def prepareForInlined(tree: tpd.Inlined)(using Context): Context =
    traverser.traverse(tree.call)
    ctx

  override def prepareForIdent(tree: tpd.Ident)(using Context): Context =
    if tree.symbol.exists then
      unusedDataApply { ud =>
        @tailrec
        def loopOnNormalizedPrefixes(prefix: Type, depth: Int): Unit =
          // limit to 10 as failsafe for the odd case where there is an infinite cycle
          if depth < 10 && prefix.exists then
            ud.registerUsed(prefix.classSymbol, None)
            loopOnNormalizedPrefixes(prefix.normalizedPrefix, depth + 1)

        loopOnNormalizedPrefixes(tree.typeOpt.normalizedPrefix, depth = 0)
        ud.registerUsed(tree.symbol, Some(tree.name))
      }
    else if tree.hasType then
      unusedDataApply(_.registerUsed(tree.tpe.classSymbol, Some(tree.name)))
    else
      ctx

  override def prepareForSelect(tree: tpd.Select)(using Context): Context =
    val name = tree.removeAttachment(OriginalName)
    unusedDataApply(_.registerUsed(tree.symbol, name, includeForImport = tree.qualifier.span.isSynthetic))

  override def prepareForBlock(tree: tpd.Block)(using Context): Context =
    pushInBlockTemplatePackageDef(tree)

  override def prepareForTemplate(tree: tpd.Template)(using Context): Context =
    pushInBlockTemplatePackageDef(tree)

  override def prepareForPackageDef(tree: tpd.PackageDef)(using Context): Context =
    pushInBlockTemplatePackageDef(tree)

  override def prepareForValDef(tree: tpd.ValDef)(using Context): Context =
    unusedDataApply{ud =>
      // do not register the ValDef generated for `object`
      traverseAnnotations(tree.symbol)
      if !tree.symbol.is(Module) then
        ud.registerDef(tree)
      if tree.name.startsWith("derived$") && tree.typeOpt != NoType then
        ud.registerUsed(tree.typeOpt.typeSymbol, None, isDerived = true)
      ud.addIgnoredUsage(tree.symbol)
    }

  override def prepareForDefDef(tree: tpd.DefDef)(using Context): Context =
    unusedDataApply: ud =>
      if !tree.symbol.is(Private) then
        tree.termParamss.flatten.foreach { p =>
          ud.addIgnoredParam(p.symbol)
        }
      ud.registerTrivial(tree)
      traverseAnnotations(tree.symbol)
      ud.registerDef(tree)
      ud.addIgnoredUsage(tree.symbol)

  override def prepareForTypeDef(tree: tpd.TypeDef)(using Context): Context =
    unusedDataApply: ud =>
      traverseAnnotations(tree.symbol)
      if !tree.symbol.is(Param) then // Ignore type parameter (as Scala 2)
        ud.registerDef(tree)
        ud.addIgnoredUsage(tree.symbol)

  override def prepareForBind(tree: tpd.Bind)(using Context): Context =
    traverseAnnotations(tree.symbol)
    unusedDataApply(_.registerPatVar(tree))

  override def prepareForTypeTree(tree: tpd.TypeTree)(using Context): Context =
    if !tree.isInstanceOf[tpd.InferredTypeTree] then typeTraverser(unusedDataApply).traverse(tree.tpe)
    ctx

  override def prepareForAssign(tree: tpd.Assign)(using Context): Context =
    unusedDataApply{ ud =>
      val sym = tree.lhs.symbol
      if sym.exists then
        ud.registerSetVar(sym)
    }

  // ========== MiniPhase Transform ==========

  override def transformBlock(tree: tpd.Block)(using Context): tpd.Tree =
    popOutBlockTemplatePackageDef()
    tree

  override def transformTemplate(tree: tpd.Template)(using Context): tpd.Tree =
    popOutBlockTemplatePackageDef()
    tree

  override def transformPackageDef(tree: tpd.PackageDef)(using Context): tpd.Tree =
    popOutBlockTemplatePackageDef()
    tree

  override def transformValDef(tree: tpd.ValDef)(using Context): tpd.Tree =
    unusedDataApply(_.removeIgnoredUsage(tree.symbol))
    tree

  override def transformDefDef(tree: tpd.DefDef)(using Context): tpd.Tree =
    unusedDataApply(_.removeIgnoredUsage(tree.symbol))
    tree

  override def transformTypeDef(tree: tpd.TypeDef)(using Context): tpd.Tree =
    unusedDataApply(_.removeIgnoredUsage(tree.symbol))
    tree


  // ---------- MiniPhase HELPERS -----------

  private def pushInBlockTemplatePackageDef(tree: tpd.Block | tpd.Template | tpd.PackageDef)(using Context): Context =
    unusedDataApply { ud =>
      ud.pushScope(UnusedData.ScopeType.fromTree(tree))
    }
    ctx

  private def popOutBlockTemplatePackageDef()(using Context): Context =
    unusedDataApply { ud =>
      ud.popScope()
    }
    ctx

  /**
   * 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.*
    import UnusedData.ScopeType

    /* Register every imports, definition and usage */
    override def traverse(tree: tpd.Tree)(using Context): Unit =
      val newCtx = if tree.symbol.exists then ctx.withOwner(tree.symbol) else ctx
      tree match
        case imp: tpd.Import =>
          unusedDataApply(_.registerImport(imp))
          imp.selectors.filter(_.isGiven).map(_.bound).collect {
            case untpd.TypedSplice(tree1) => tree1
          }.foreach(traverse(_)(using newCtx))
          traverseChildren(tree)(using newCtx)
        case ident: Ident =>
          prepareForIdent(ident)
          traverseChildren(tree)(using newCtx)
        case sel: Select =>
          prepareForSelect(sel)
          traverseChildren(tree)(using newCtx)
        case tree: (tpd.Block | tpd.Template | tpd.PackageDef) =>
          //! DIFFERS FROM MINIPHASE
          pushInBlockTemplatePackageDef(tree)
          traverseChildren(tree)(using newCtx)
          popOutBlockTemplatePackageDef()
        case t: tpd.ValDef =>
          prepareForValDef(t)
          traverseChildren(tree)(using newCtx)
          transformValDef(t)
        case t: tpd.DefDef =>
          prepareForDefDef(t)
          traverseChildren(tree)(using newCtx)
          transformDefDef(t)
        case t: tpd.TypeDef =>
          prepareForTypeDef(t)
          traverseChildren(tree)(using newCtx)
          transformTypeDef(t)
        case t: tpd.Bind =>
          prepareForBind(t)
          traverseChildren(tree)(using newCtx)
        case t:tpd.Assign =>
          prepareForAssign(t)
          traverseChildren(tree)
        case _: tpd.InferredTypeTree =>
        case t@tpd.RefinedTypeTree(tpt, refinements) =>
          //! DIFFERS FROM MINIPHASE
          typeTraverser(unusedDataApply).traverse(t.tpe)
          traverse(tpt)(using newCtx)
        case t@tpd.TypeTree() =>
          //! DIFFERS FROM MINIPHASE
          typeTraverser(unusedDataApply).traverse(t.tpe)
          traverseChildren(tree)(using newCtx)
        case _ =>
          //! DIFFERS FROM MINIPHASE
          traverseChildren(tree)(using newCtx)
    end traverse
  end traverser

  /** This is a type traverser which catch some special Types not traversed by the term traverser above */
  private def typeTraverser(dt: (UnusedData => Any) => Unit)(using Context) = new TypeTraverser:
    override def traverse(tp: Type): Unit =
      if tp.typeSymbol.exists then dt(_.registerUsed(tp.typeSymbol, Some(tp.typeSymbol.name)))
      tp match
        case AnnotatedType(_, annot) =>
          dt(_.registerUsed(annot.symbol, None))
          traverseChildren(tp)
        case _ =>
          traverseChildren(tp)

  /** This traverse the annotations of the symbol */
  private def traverseAnnotations(sym: Symbol)(using Context): Unit =
    sym.denot.annotations.foreach(annot => traverser.traverse(annot.tree))


  /** Do the actual reporting given the result of the anaylsis */
  private def reportUnused(res: UnusedData.UnusedResult)(using Context): Unit =
    res.warnings.toList.sortBy(_.pos.span.point)(using Ordering[Int]).foreach { s =>
      s match
        case UnusedSymbol(t, _, WarnTypes.Imports) =>
          report.warning(UnusedSymbolMessage.imports, t)
        case UnusedSymbol(t, _, WarnTypes.LocalDefs) =>
          report.warning(UnusedSymbolMessage.localDefs, t)
        case UnusedSymbol(t, _, WarnTypes.ExplicitParams) =>
          report.warning(UnusedSymbolMessage.explicitParams, t)
        case UnusedSymbol(t, _, WarnTypes.ImplicitParams) =>
          report.warning(UnusedSymbolMessage.implicitParams, t)
        case UnusedSymbol(t, _, WarnTypes.PrivateMembers) =>
          report.warning(UnusedSymbolMessage.privateMembers, t)
        case UnusedSymbol(t, _, WarnTypes.PatVars) =>
          report.warning(UnusedSymbolMessage.patVars, t)
        case UnusedSymbol(t, _, WarnTypes.UnsetLocals) =>
          report.warning("unset local variable, consider using an immutable val instead", t)
        case UnusedSymbol(t, _, WarnTypes.UnsetPrivates) =>
          report.warning("unset private variable, consider using an immutable val instead", t)
    }

end CheckUnused

object CheckUnused:
  val phaseNamePrefix: String = "checkUnused"
  val description: String = "check for unused elements"

  enum PhaseMode:
    case Aggregate
    case Report

  private enum WarnTypes:
    case Imports
    case LocalDefs
    case ExplicitParams
    case ImplicitParams
    case PrivateMembers
    case PatVars
    case UnsetLocals
    case UnsetPrivates

  /**
   * The key used to retrieve the "unused entity" analysis metadata,
   * from the compilation `Context`
   */
  private val _key = Property.StickyKey[UnusedData]

  val OriginalName = Property.StickyKey[Name]

  class PostTyper extends CheckUnused(PhaseMode.Aggregate, "PostTyper", _key)

  class PostInlining extends CheckUnused(PhaseMode.Report, "PostInlining", _key)

  /**
   * A stateful class gathering the infos on :
   * - imports
   * - definitions
   * - usage
   */
  private class UnusedData:
    import collection.mutable.{Set => MutSet, Map => MutMap, Stack => MutStack, ListBuffer => MutList}
    import UnusedData.*

    /** The current scope during the tree traversal */
    val currScopeType: MutStack[ScopeType] = MutStack(ScopeType.Other)

    var unusedAggregate: Option[UnusedResult] = None

    /* IMPORTS */
    private val impInScope = MutStack(MutList[ImportSelectorData]())
    /**
     * We store the symbol along with their accessibility without import.
     * Accessibility to their definition in outer context/scope
     *
     * See the `isAccessibleAsIdent` extension method below in the file
     */
    private val usedInScope = MutStack(MutSet[(Symbol, Option[Name], Boolean)]())
    private val usedInPosition = MutMap.empty[Name, MutSet[Symbol]]
    /* unused import collected during traversal */
    private val unusedImport = MutList.empty[ImportSelectorData]

    /* LOCAL DEF OR VAL / Private Def or Val / Pattern variables */
    private val localDefInScope = MutList.empty[tpd.MemberDef]
    private val privateDefInScope = MutList.empty[tpd.MemberDef]
    private val explicitParamInScope = MutList.empty[tpd.MemberDef]
    private val implicitParamInScope = MutList.empty[tpd.MemberDef]
    private val patVarsInScope = MutList.empty[tpd.Bind]

    /** All variables sets*/
    private val setVars = MutSet[Symbol]()

    /** All used symbols */
    private val usedDef = MutSet[Symbol]()
    /** Do not register as used */
    private val doNotRegister = MutSet[Symbol]()

    /** Trivial definitions, avoid registering params */
    private val trivialDefs = MutSet[Symbol]()

    private val paramsToSkip = MutSet[Symbol]()


    def finishAggregation(using Context)(): Unit =
      val unusedInThisStage = this.getUnused
      this.unusedAggregate match {
        case None =>
          this.unusedAggregate = Some(unusedInThisStage)
        case Some(prevUnused) =>
          val intersection = unusedInThisStage.warnings.intersect(prevUnused.warnings)
          this.unusedAggregate = Some(UnusedResult(intersection))
      }


    /**
     * Register a found (used) symbol along with its name
     *
     * The optional name will be used to target the right import
     * as the same element can be imported with different renaming
     */
    def registerUsed(sym: Symbol, name: Option[Name], includeForImport: Boolean = true, isDerived: Boolean = false)(using Context): Unit =
      if sym.exists && !isConstructorOfSynth(sym) && !doNotRegister(sym) then
        if sym.isConstructor then
          registerUsed(sym.owner, None, includeForImport) // constructor are "implicitly" imported with the class
        else
          // If the symbol is accessible in this scope without an import, do not register it for unused import analysis
          val includeForImport1 =
            includeForImport
              && (name.exists(_.toTermName != sym.name.toTermName) || !sym.isAccessibleAsIdent)

          def addIfExists(sym: Symbol): Unit =
            if sym.exists then
              usedDef += sym
              if includeForImport1 then
                usedInScope.top += ((sym, name, isDerived))
          addIfExists(sym)
          addIfExists(sym.companionModule)
          addIfExists(sym.companionClass)
          if sym.sourcePos.exists then
            for n <- name do
              usedInPosition.getOrElseUpdate(n, MutSet.empty) += sym

    /** Register a symbol that should be ignored */
    def addIgnoredUsage(sym: Symbol)(using Context): Unit =
      doNotRegister ++= sym.everySymbol

    /** Remove a symbol that shouldn't be ignored anymore */
    def removeIgnoredUsage(sym: Symbol)(using Context): Unit =
      doNotRegister --= sym.everySymbol

    def addIgnoredParam(sym: Symbol)(using Context): Unit =
      paramsToSkip += sym

    /** Register an import */
    def registerImport(imp: tpd.Import)(using Context): Unit =
      if
        !tpd.languageImport(imp.expr).nonEmpty
          && !imp.isGeneratedByEnum
          && !isTransparentAndInline(imp)
          && currScopeType.top != ScopeType.ReplWrapper // #18383 Do not report top-level import's in the repl as unused
      then
        val qualTpe = imp.expr.tpe

        // Put wildcard imports at the end, because they have lower priority within one Import
        val reorderdSelectors =
          val (wildcardSels, nonWildcardSels) = imp.selectors.partition(_.isWildcard)
          nonWildcardSels ::: wildcardSels

        val newDataInScope =
          for sel <- reorderdSelectors yield
            val data = new ImportSelectorData(qualTpe, sel)
            if shouldSelectorBeReported(imp, sel) || isImportExclusion(sel) || isImportIgnored(imp, sel) then
              // Immediately mark the selector as used
              data.markUsed()
            data
        impInScope.top.prependAll(newDataInScope)
    end registerImport

    /** Register (or not) some `val` or `def` according to the context, scope and flags */
    def registerDef(memDef: tpd.MemberDef)(using Context): Unit =
      if memDef.isValidMemberDef && !isDefIgnored(memDef) then
        if memDef.isValidParam then
          if memDef.symbol.isOneOf(GivenOrImplicit) then
            if !paramsToSkip.contains(memDef.symbol) then
              implicitParamInScope += memDef
          else if !paramsToSkip.contains(memDef.symbol) then
            explicitParamInScope += memDef
        else if currScopeType.top == ScopeType.Local then
          localDefInScope += memDef
        else if memDef.shouldReportPrivateDef then
          privateDefInScope += memDef

    /** Register pattern variable */
    def registerPatVar(patvar: tpd.Bind)(using Context): Unit =
      if !patvar.symbol.isUnusedAnnot then
        patVarsInScope += patvar

    /** enter a new scope */
    def pushScope(newScopeType: ScopeType): Unit =
      // unused imports :
      currScopeType.push(newScopeType)
      impInScope.push(MutList())
      usedInScope.push(MutSet())

    def registerSetVar(sym: Symbol): Unit =
      setVars += sym

    /**
     * leave the current scope and do :
     *
     * - If there are imports in this scope check for unused ones
     */
    def popScope()(using Context): Unit =
      currScopeType.pop()
      val usedInfos = usedInScope.pop()
      val selDatas = impInScope.pop()

      for usedInfo <- usedInfos do
        val (sym, optName, isDerived) = usedInfo
        val usedData = selDatas.find { selData =>
          sym.isInImport(selData, optName, isDerived)
        }
        usedData match
          case Some(data) =>
            data.markUsed()
          case None =>
            // Propagate the symbol one level up
            if usedInScope.nonEmpty then
              usedInScope.top += usedInfo
      end for // each in `used`

      for selData <- selDatas do
        if !selData.isUsed then
          unusedImport += selData
    end popScope

    /**
     * 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): UnusedResult =
      popScope()

      def isUsedInPosition(name: Name, span: Span): Boolean =
        usedInPosition.get(name) match
          case Some(syms) => syms.exists(sym => span.contains(sym.span))
          case None       => false

      val sortedImp =
        if ctx.settings.WunusedHas.imports || ctx.settings.WunusedHas.strictNoImplicitWarn then
          unusedImport.toList
            .map(d => UnusedSymbol(d.selector.srcPos, d.selector.name, WarnTypes.Imports))
        else
          Nil
      // Partition to extract unset local variables from usedLocalDefs
      val (usedLocalDefs, unusedLocalDefs) =
        if ctx.settings.WunusedHas.locals then
          localDefInScope.toList.partition(d => d.symbol.usedDefContains)
        else
          (Nil, Nil)
      val sortedLocalDefs =
        unusedLocalDefs
          .filterNot(d => isUsedInPosition(d.symbol.name, d.span))
          .filterNot(d => containsSyntheticSuffix(d.symbol))
          .map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.LocalDefs))
      val unsetLocalDefs = usedLocalDefs.filter(isUnsetVarDef).map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.UnsetLocals)).toList

      val sortedExplicitParams =
        if ctx.settings.WunusedHas.explicits then
          explicitParamInScope.toList
            .filterNot(d => d.symbol.usedDefContains)
            .filterNot(d => isUsedInPosition(d.symbol.name, d.span))
            .filterNot(d => containsSyntheticSuffix(d.symbol))
            .map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.ExplicitParams))
        else
          Nil
      val sortedImplicitParams =
        if ctx.settings.WunusedHas.implicits then
          implicitParamInScope.toList
            .filterNot(d => d.symbol.usedDefContains)
            .filterNot(d => containsSyntheticSuffix(d.symbol))
            .map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.ImplicitParams))
        else
          Nil
      // Partition to extract unset private variables from usedPrivates
      val (usedPrivates, unusedPrivates) =
        if ctx.settings.WunusedHas.privates then
          privateDefInScope.toList.partition(d => d.symbol.usedDefContains)
        else
          (Nil, Nil)
      val sortedPrivateDefs = unusedPrivates.filterNot(d => containsSyntheticSuffix(d.symbol)).map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.PrivateMembers))
      val unsetPrivateDefs = usedPrivates.filter(isUnsetVarDef).map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.UnsetPrivates))
      val sortedPatVars =
        if ctx.settings.WunusedHas.patvars then
          patVarsInScope.toList
            .filterNot(d => d.symbol.usedDefContains)
            .filterNot(d => containsSyntheticSuffix(d.symbol))
            .filterNot(d => isUsedInPosition(d.symbol.name, d.span))
            .map(d => UnusedSymbol(d.namePos, d.name, WarnTypes.PatVars))
        else
          Nil
      val warnings =
        sortedImp :::
        sortedLocalDefs :::
        sortedExplicitParams :::
        sortedImplicitParams :::
        sortedPrivateDefs :::
        sortedPatVars :::
        unsetLocalDefs :::
        unsetPrivateDefs
      UnusedResult(warnings.toSet)
    end getUnused
    //============================ HELPERS ====================================


    /**
     * Checks if import selects a def that is transparent and inline
     */
    private def isTransparentAndInline(imp: tpd.Import)(using Context): Boolean =
      imp.selectors.exists { sel =>
        val qual = imp.expr
        val importedMembers = qual.tpe.member(sel.name).alternatives.map(_.symbol)
        importedMembers.exists(s => s.is(Transparent) && s.is(Inline))
      }

    /**
     * Heuristic to detect synthetic suffixes in names of symbols
     */
    private def containsSyntheticSuffix(symbol: Symbol)(using Context): Boolean =
      symbol.name.mangledString.contains("$")

    /**
     * Is the constructor of synthetic package object
     * Should be ignored as it is always imported/used in package
     * Trigger false negative on used import
     *
     * Without this check example:
     *
     * --- WITH PACKAGE : WRONG ---
     * {{{
     * package a:
     *   val x: Int = 0
     * package b:
     *   import a.* // no warning
     * }}}
     * --- WITH OBJECT : OK ---
     * {{{
     * object a:
     *   val x: Int = 0
     * object b:
     *   import a.* // unused warning
     * }}}
     */
    private def isConstructorOfSynth(sym: Symbol)(using Context): Boolean =
      sym.exists && sym.isConstructor && sym.owner.isPackageObject && sym.owner.is(Synthetic)

    /**
     * This is used to avoid reporting the parameters of the synthetic main method
     * generated by `@main`
     */
    private def isSyntheticMainParam(sym: Symbol)(using Context): Boolean =
      sym.exists && ctx.platform.isMainMethod(sym.owner) && sym.owner.is(Synthetic)

    /**
     * This is used to ignore exclusion imports (i.e. import `qual`.{`member` => _})
     */
    private def isImportExclusion(sel: ImportSelector): Boolean = sel.renamed match
      case untpd.Ident(name) => name == StdNames.nme.WILDCARD
      case _ => false

    /**
     * If -Wunused:strict-no-implicit-warn import and this import selector could potentially import implicit.
     * return true
     */
    private def shouldSelectorBeReported(imp: tpd.Import, sel: ImportSelector)(using Context): Boolean =
      ctx.settings.WunusedHas.strictNoImplicitWarn && (
        sel.isWildcard ||
        imp.expr.tpe.member(sel.name.toTermName).alternatives.exists(_.symbol.isOneOf(GivenOrImplicit)) ||
        imp.expr.tpe.member(sel.name.toTypeName).alternatives.exists(_.symbol.isOneOf(GivenOrImplicit))
      )

    /**
     * Ignore CanEqual imports
     */
    private def isImportIgnored(imp: tpd.Import, sel: ImportSelector)(using Context): Boolean =
      (sel.isWildcard && sel.isGiven && imp.expr.tpe.allMembers.exists(p => p.symbol.typeRef.baseClasses.exists(_.derivesFrom(defn.CanEqualClass)) && p.symbol.isOneOf(GivenOrImplicit))) ||
      (imp.expr.tpe.member(sel.name.toTermName).alternatives
        .exists(p => p.symbol.isOneOf(GivenOrImplicit) && p.symbol.typeRef.baseClasses.exists(_.derivesFrom(defn.CanEqualClass))))

    /**
     * Ignore definitions of CanEqual given
     */
    private def isDefIgnored(memDef: tpd.MemberDef)(using Context): Boolean =
      memDef.symbol.isOneOf(GivenOrImplicit) && memDef.symbol.typeRef.baseClasses.exists(_.derivesFrom(defn.CanEqualClass))

    extension (tree: ImportSelector)
      def boundTpe: Type = tree.bound match {
        case untpd.TypedSplice(tree1) => tree1.tpe
        case _ => NoType
      }

    extension (sym: Symbol)
      /** is accessible without import in current context */
      private def isAccessibleAsIdent(using Context): Boolean =
        ctx.outersIterator.exists{ c =>
          c.owner == sym.owner
          || sym.owner.isClass && c.owner.isClass
              && c.owner.thisType.baseClasses.contains(sym.owner)
              && c.owner.thisType.member(sym.name).alternatives.contains(sym)
        }

      /** Given an import and accessibility, return selector that matches import<->symbol */
      private def isInImport(selData: ImportSelectorData, altName: Option[Name], isDerived: Boolean)(using Context): Boolean =
        assert(sym.exists)

        val selector = selData.selector

        if !selector.isWildcard then
          if altName.exists(explicitName => selector.rename != explicitName.toTermName) then
            // if there is an explicit name, it must match
            false
          else
            if isDerived then
              // See i15503i.scala, grep for "package foo.test.i17156"
              selData.allSymbolsDealiasedForNamed.contains(dealias(sym))
            else
              selData.allSymbolsForNamed.contains(sym)
        else
          // Wildcard
          if !selData.qualTpe.member(sym.name).hasAltWith(_.symbol == sym) then
            // The qualifier does not have the target symbol as a member
            false
          else
            if selector.isGiven then
              // Further check that the symbol is a given or implicit and conforms to the bound
              sym.isOneOf(Given | Implicit)
                && (selector.bound.isEmpty || sym.info.finalResultType <:< selector.boundTpe)
            else
              // Normal wildcard, check that the symbol is not a given (but can be implicit)
              !sym.is(Given)
        end if
      end isInImport

      /** Annotated with @unused */
      private def isUnusedAnnot(using Context): Boolean =
        sym.annotations.exists(a => a.symbol == ctx.definitions.UnusedAnnot)

      private def shouldNotReportParamOwner(using Context): Boolean =
        if sym.exists then
          val owner = sym.owner
          trivialDefs(owner) || // is a trivial def
          owner.isPrimaryConstructor ||
          owner.annotations.exists ( // @depreacated
            _.symbol == ctx.definitions.DeprecatedAnnot
          ) ||
          owner.isAllOf(Synthetic | PrivateLocal) ||
          owner.is(Accessor) ||
          owner.isOverriden
        else
          false

      private def usedDefContains(using Context): Boolean =
        sym.everySymbol.exists(usedDef.apply)

      private def everySymbol(using Context): List[Symbol] =
        List(sym, sym.companionClass, sym.companionModule, sym.moduleClass).filter(_.exists)

      /** A function is overriden. Either has `override flags` or parent has a matching member (type and name) */
      private def isOverriden(using Context): Boolean =
        sym.is(Flags.Override) || (sym.exists && sym.owner.thisType.parents.exists(p => sym.matchingMember(p).exists))

    end extension

    extension (defdef: tpd.DefDef)
      // so trivial that it never consumes params
      private def isTrivial(using Context): Boolean =
        val rhs = defdef.rhs
        rhs.symbol == ctx.definitions.Predef_undefined ||
        rhs.tpe =:= ctx.definitions.NothingType ||
        defdef.symbol.is(Deferred) ||
        (rhs match {
          case _: tpd.Literal => true
          case _ => rhs.tpe match
            case ConstantType(_) => true
            case tp: TermRef =>
              // Detect Scala 2 SingleType
              tp.underlying.classSymbol.is(Flags.Module)
            case _ =>
              false
        })
      def registerTrivial(using Context): Unit =
        if defdef.isTrivial then
          trivialDefs += defdef.symbol

    extension (memDef: tpd.MemberDef)
      private def isValidMemberDef(using Context): Boolean =
        memDef.symbol.exists
          && !memDef.symbol.isUnusedAnnot
          && !memDef.symbol.isAllOf(Flags.AccessorCreationFlags)
          && !memDef.name.isWildcard
          && !memDef.symbol.owner.is(ExtensionMethod)

      private def isValidParam(using Context): Boolean =
        val sym = memDef.symbol
        (sym.is(Param) || sym.isAllOf(PrivateParamAccessor | Local, butNot = CaseAccessor)) &&
        !isSyntheticMainParam(sym) &&
        !sym.shouldNotReportParamOwner

      private def shouldReportPrivateDef(using Context): Boolean =
        currScopeType.top == ScopeType.Template && !memDef.symbol.isConstructor && memDef.symbol.is(Private, butNot = SelfName | Synthetic | CaseAccessor)

      private def isUnsetVarDef(using Context): Boolean =
        val sym = memDef.symbol
        sym.is(Mutable) && !setVars(sym)

    extension (imp: tpd.Import)
      /** Enum generate an import for its cases (but outside them), which should be ignored */
      def isGeneratedByEnum(using Context): Boolean =
        imp.symbol.exists && imp.symbol.owner.is(Flags.Enum, butNot = Flags.Case)

    extension (thisName: Name)
      private def isWildcard: Boolean =
        thisName == StdNames.nme.WILDCARD || thisName.is(WildcardParamName)

  end UnusedData

  private object UnusedData:
    enum ScopeType:
      case Local
      case Template
      case ReplWrapper
      case Other

    object ScopeType:
      /** return the scope corresponding to the enclosing scope of the given tree */
      def fromTree(tree: tpd.Tree)(using Context): ScopeType = tree match
        case tree: tpd.Template => if tree.symbol.name.isReplWrapperName then ReplWrapper else Template
        case _:tpd.Block => Local
        case _ => Other

    final class ImportSelectorData(val qualTpe: Type, val selector: ImportSelector):
      private var myUsed: Boolean = false

      def markUsed(): Unit = myUsed = true

      def isUsed: Boolean = myUsed

      private var myAllSymbols: Set[Symbol] | Null = null

      def allSymbolsForNamed(using Context): Set[Symbol] =
        if myAllSymbols == null then
          val allDenots = qualTpe.member(selector.name).alternatives ::: qualTpe.member(selector.name.toTypeName).alternatives
          myAllSymbols = allDenots.map(_.symbol).toSet
        myAllSymbols.uncheckedNN

      private var myAllSymbolsDealiased: Set[Symbol] | Null = null

      def allSymbolsDealiasedForNamed(using Context): Set[Symbol] =
        if myAllSymbolsDealiased == null then
          myAllSymbolsDealiased = allSymbolsForNamed.map(sym => dealias(sym))
        myAllSymbolsDealiased.uncheckedNN
    end ImportSelectorData

    case class UnusedSymbol(pos: SrcPos, name: Name, warnType: WarnTypes)
    /** A container for the results of the used elements analysis */
    case class UnusedResult(warnings: Set[UnusedSymbol])
    object UnusedResult:
      val Empty = UnusedResult(Set.empty)
  end UnusedData

  private def dealias(symbol: Symbol)(using Context): Symbol =
    if symbol.isType && symbol.asType.denot.isAliasType then
      symbol.asType.typeRef.dealias.typeSymbol
    else
      symbol

end CheckUnused