path: root/compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

package dotty.tools
package dotc
package ast

import core._
import Flags._, Trees._, Types._, Contexts._, Constants._
import Names._, StdNames._, NameOps._, Decorators._, Symbols._
import util.HashSet
import typer.ConstFold

trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
  import TreeInfo._

  // Note: the <: Type constraint looks necessary (and is needed to make the file compile in dotc).
  // But Scalac accepts the program happily without it. Need to find out why.

  def unsplice[T >: Untyped](tree: Trees.Tree[T]): Trees.Tree[T] = tree.asInstanceOf[untpd.Tree] match {
    case untpd.TypedSplice(tree1) => tree1.asInstanceOf[Trees.Tree[T]]
    case _ => tree
  }

  def isDeclarationOrTypeDef(tree: Tree): Boolean = unsplice(tree) match {
    case DefDef(_, _, _, _, EmptyTree)
      | ValDef(_, _, EmptyTree)
      | TypeDef(_, _) => true
    case _ => false
  }

  def isOpAssign(tree: Tree) = unsplice(tree) match {
    case Apply(fn, _ :: _) =>
      unsplice(fn) match {
        case Select(_, name) if name.isOpAssignmentName => true
        case _ => false
      }
    case _ => false
  }

  class MatchingArgs(params: List[Symbol], args: List[Tree])(implicit ctx: Context) {
    def foreach(f: (Symbol, Tree) => Unit): Boolean = {
      def recur(params: List[Symbol], args: List[Tree]): Boolean = params match {
        case Nil => args.isEmpty
        case param :: params1 =>
          if (param.info.isRepeatedParam) {
            for (arg <- args) f(param, arg)
            true
          } else args match {
            case Nil => false
            case arg :: args1 =>
              f(param, args.head)
              recur(params1, args1)
          }
      }
      recur(params, args)
    }
    def zipped: List[(Symbol, Tree)] = map((_, _))
    def map[R](f: (Symbol, Tree) => R): List[R] = {
      val b = List.newBuilder[R]
      foreach(b += f(_, _))
      b.result
    }
  }

  /** The method part of an application node, possibly enclosed in a block
   *  with only valdefs as statements. the reason for also considering blocks
   *  is that named arguments can transform a call into a block, e.g.
   *   <init>(b = foo, a = bar)
   * is transformed to
   *   { val x$1 = foo
   *     val x$2 = bar
   *     <init>(x$2, x$1)
   *   }
   */
  def methPart(tree: Tree): Tree = stripApply(tree) match {
    case TypeApply(fn, _) => methPart(fn)
    case AppliedTypeTree(fn, _) => methPart(fn) // !!! should not be needed
    case Block(stats, expr) => methPart(expr)
    case mp => mp
  }

  /** If this is an application, its function part, stripping all
   *  Apply nodes (but leaving TypeApply nodes in). Otherwise the tree itself.
   */
  def stripApply(tree: Tree): Tree = unsplice(tree) match {
    case Apply(fn, _) => stripApply(fn)
    case _ => tree
  }

  /** The number of arguments in an application */
  def numArgs(tree: Tree): Int = unsplice(tree) match {
    case Apply(fn, args) => numArgs(fn) + args.length
    case TypeApply(fn, _) => numArgs(fn)
    case Block(_, expr) => numArgs(expr)
    case _ => 0
  }

  /** The (last) list of arguments of an application */
  def arguments(tree: Tree): List[Tree] = unsplice(tree) match {
    case Apply(_, args) => args
    case TypeApply(fn, _) => arguments(fn)
    case Block(_, expr) => arguments(expr)
    case _ => Nil
  }

  /** Is tree a self constructor call this(...)? I.e. a call to a constructor of the
   *  same object?
   */
  def isSelfConstrCall(tree: Tree): Boolean = methPart(tree) match {
    case Ident(nme.CONSTRUCTOR) | Select(This(_), nme.CONSTRUCTOR) => true
    case _ => false
  }

  /** Is tree a super constructor call?
   */
  def isSuperConstrCall(tree: Tree): Boolean = methPart(tree) match {
    case Select(Super(_, _), nme.CONSTRUCTOR) => true
    case _ => false
  }

  def isSuperSelection(tree: untpd.Tree) = unsplice(tree) match {
    case Select(Super(_, _), _) => true
    case _ => false
  }

  def isSelfOrSuperConstrCall(tree: Tree): Boolean = methPart(tree) match {
    case Ident(nme.CONSTRUCTOR)
       | Select(This(_), nme.CONSTRUCTOR)
       | Select(Super(_, _), nme.CONSTRUCTOR) => true
    case _ => false
  }

  /** Is tree a variable pattern? */
  def isVarPattern(pat: untpd.Tree): Boolean = unsplice(pat) match {
    case x: BackquotedIdent => false
    case x: Ident => x.name.isVariableName
    case _  => false
  }

  /** The first constructor definition in `stats` */
  def firstConstructor(stats: List[Tree]): Tree = stats match {
    case (meth: DefDef) :: _ if meth.name.isConstructorName => meth
    case stat :: stats => firstConstructor(stats)
    case nil => EmptyTree
  }

  /** The arguments to the first constructor in `stats`. */
  def firstConstructorArgs(stats: List[Tree]): List[Tree] = firstConstructor(stats) match {
    case DefDef(_, _, args :: _, _, _) => args
    case _                                => Nil
  }

  /** Is tpt a vararg type of the form T* or => T*? */
  def isRepeatedParamType(tpt: Tree)(implicit ctx: Context): Boolean = tpt match {
    case ByNameTypeTree(tpt1) => isRepeatedParamType(tpt1)
    case tpt: TypeTree => tpt.typeOpt.isRepeatedParam
    case AppliedTypeTree(Select(_, tpnme.REPEATED_PARAM_CLASS), _) => true
    case _ => false
  }

  /** Is name a left-associative operator? */
  def isLeftAssoc(operator: Name) = !operator.isEmpty && (operator.toSimpleName.last != ':')

  /** can this type be a type pattern? */
  def mayBeTypePat(tree: untpd.Tree): Boolean = unsplice(tree) match {
    case AndTypeTree(tpt1, tpt2) => mayBeTypePat(tpt1) || mayBeTypePat(tpt2)
    case OrTypeTree(tpt1, tpt2) => mayBeTypePat(tpt1) || mayBeTypePat(tpt2)
    case RefinedTypeTree(tpt, refinements) => mayBeTypePat(tpt) || refinements.exists(_.isInstanceOf[Bind])
    case AppliedTypeTree(tpt, args) => mayBeTypePat(tpt) || args.exists(_.isInstanceOf[Bind])
    case Select(tpt, _) => mayBeTypePat(tpt)
    case Annotated(tpt, _) => mayBeTypePat(tpt)
    case _ => false
  }

  /** Is this argument node of the form <expr> : _* ?
   */
  def isWildcardStarArg(tree: Tree)(implicit ctx: Context): Boolean = unbind(tree) match {
    case Typed(Ident(nme.WILDCARD_STAR), _) => true
    case Typed(_, Ident(tpnme.WILDCARD_STAR)) => true
    case Typed(_, tpt: TypeTree) => tpt.hasType && tpt.tpe.isRepeatedParam
    case _ => false
  }

  /** If this tree has type parameters, those.  Otherwise Nil.
  def typeParameters(tree: Tree): List[TypeDef] = tree match {
    case DefDef(_, _, tparams, _, _, _) => tparams
    case ClassDef(_, _, tparams, _)     => tparams
    case TypeDef(_, _, tparams, _)      => tparams
    case _                              => Nil
  }*/

  /** Does this argument list end with an argument of the form <expr> : _* ? */
  def isWildcardStarArgList(trees: List[Tree])(implicit ctx: Context) =
    trees.nonEmpty && isWildcardStarArg(trees.last)

  /** Is the argument a wildcard argument of the form `_` or `x @ _`?
   */
  def isWildcardArg(tree: Tree): Boolean = unbind(tree) match {
    case Ident(nme.WILDCARD) => true
    case _                   => false
  }

  /** Does this list contain a named argument tree? */
  def hasNamedArg(args: List[Any]) = args exists isNamedArg
  val isNamedArg = (arg: Any) => arg.isInstanceOf[Trees.NamedArg[_]]

  /** Is this pattern node a catch-all (wildcard or variable) pattern? */
  def isDefaultCase(cdef: CaseDef) = cdef match {
    case CaseDef(pat, EmptyTree, _) => isWildcardArg(pat)
    case _                            => false
  }

  /** Is this pattern node a synthetic catch-all case, added during PartialFuction synthesis before we know
    * whether the user provided cases are exhaustive. */
  def isSyntheticDefaultCase(cdef: CaseDef) = unsplice(cdef) match {
    case CaseDef(Bind(nme.DEFAULT_CASE, _), EmptyTree, _) => true
    case _                                                  => false
  }

  /** Does this CaseDef catch Throwable? */
  def catchesThrowable(cdef: CaseDef)(implicit ctx: Context) =
    catchesAllOf(cdef, defn.ThrowableType)

  /** Does this CaseDef catch everything of a certain Type? */
  def catchesAllOf(cdef: CaseDef, threshold: Type)(implicit ctx: Context) =
    isDefaultCase(cdef) ||
    cdef.guard.isEmpty && {
      unbind(cdef.pat) match {
        case Typed(Ident(nme.WILDCARD), tpt) => threshold <:< tpt.typeOpt
        case _                               => false
      }
    }

  /** Is this case guarded? */
  def isGuardedCase(cdef: CaseDef) = cdef.guard ne EmptyTree

  /** The underlying pattern ignoring any bindings */
  def unbind(x: Tree): Tree = unsplice(x) match {
    case Bind(_, y) => unbind(y)
    case y          => y
  }

  /** Checks whether predicate `p` is true for all result parts of this expression,
   *  where we zoom into Ifs, Matches, and Blocks.
   */
  def forallResults(tree: Tree, p: Tree => Boolean): Boolean = tree match {
    case If(_, thenp, elsep) => forallResults(thenp, p) && forallResults(elsep, p)
    case Match(_, cases) => cases forall (c => forallResults(c.body, p))
    case Block(_, expr) => forallResults(expr, p)
    case _ => p(tree)
  }
}

trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped] =>
  import TreeInfo._
  import untpd._

  /** True iff definition is a val or def with no right-hand-side, or it
   *  is an abstract typoe declaration
   */
  def lacksDefinition(mdef: MemberDef)(implicit ctx: Context) = mdef match {
    case mdef: ValOrDefDef =>
      mdef.unforcedRhs == EmptyTree && !mdef.name.isConstructorName && !mdef.mods.is(ParamAccessor)
    case mdef: TypeDef =>
      def isBounds(rhs: Tree): Boolean = rhs match {
        case _: TypeBoundsTree => true
        case LambdaTypeTree(_, body) => isBounds(body)
        case _ => false
      }
      mdef.rhs.isEmpty || isBounds(mdef.rhs)
    case _ => false
  }

  def isFunctionWithUnknownParamType(tree: Tree) = tree match {
    case Function(args, _) =>
      args.exists {
        case ValDef(_, tpt, _) => tpt.isEmpty
        case _ => false
      }
    case Match(EmptyTree, _) =>
      true
    case _ => false
  }

  /** Is `tree` an implicit function or closure, possibly nested in a block? */
  def isImplicitClosure(tree: Tree)(implicit ctx: Context): Boolean = unsplice(tree) match {
    case Function((param: untpd.ValDef) :: _, _) => param.mods.is(Implicit)
    case Closure(_, meth, _) => true
    case Block(Nil, expr) => isImplicitClosure(expr)
    case Block(DefDef(nme.ANON_FUN, _, (param :: _) :: _, _, _) :: Nil, _: Closure) =>
      param.mods.is(Implicit)
    case _ => false
  }

  // todo: fill with other methods from TreeInfo that only apply to untpd.Tree's
}

trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
  import TreeInfo._
  import tpd._

  /** The purity level of this statement.
   *  @return   pure        if statement has no side effects
   *            idempotent  if running the statement a second time has no side effects
   *            impure      otherwise
   */
  private def statPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
    case EmptyTree
       | TypeDef(_, _)
       | Import(_, _)
       | DefDef(_, _, _, _, _) =>
      Pure
    case vdef @ ValDef(_, _, _) =>
      if (vdef.symbol.flags is Mutable) Impure else exprPurity(vdef.rhs)
    case _ =>
      Impure
      // TODO: It seem like this should be exprPurity(tree)
      // But if we do that the repl/vars test break. Need to figure out why that's the case.
  }

  /** The purity level of this expression.
   *  @return   pure        if expression has no side effects
   *            idempotent  if running the expression a second time has no side effects
   *            impure      otherwise
   *
   *  Note that purity and idempotency are different. References to modules and lazy
   *  vals are impure (side-effecting) both because side-effecting code may be executed and because the first reference
   *  takes a different code path than all to follow; but they are idempotent
   *  because running the expression a second time gives the cached result.
   */
  private def exprPurity(tree: Tree)(implicit ctx: Context): PurityLevel = unsplice(tree) match {
    case EmptyTree
       | This(_)
       | Super(_, _)
       | Literal(_)
       | Closure(_, _, _) =>
      Pure
    case Ident(_) =>
      refPurity(tree)
    case Select(qual, _) =>
      refPurity(tree).min(exprPurity(qual))
    case TypeApply(fn, _) =>
      exprPurity(fn)
/*
 * Not sure we'll need that. Comment out until we find out
    case Apply(Select(free @ Ident(_), nme.apply), _) if free.symbol.name endsWith nme.REIFY_FREE_VALUE_SUFFIX =>
      // see a detailed explanation of this trick in `GenSymbols.reifyFreeTerm`
      free.symbol.hasStableFlag && isIdempotentExpr(free)
*/
    case Apply(fn, args) =>
      def isKnownPureOp(sym: Symbol) =
        sym.owner.isPrimitiveValueClass || sym.owner == defn.StringClass
      // Note: After uncurry, field accesses are represented as Apply(getter, Nil),
      // so an Apply can also be pure.
      if (args.isEmpty && fn.symbol.is(Stable)) exprPurity(fn)
      else if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol))
        // A constant expression with pure arguments is pure.
        minOf(exprPurity(fn), args.map(exprPurity))
      else Impure
    case Typed(expr, _) =>
      exprPurity(expr)
    case Block(stats, expr) =>
      minOf(exprPurity(expr), stats.map(statPurity))
    case _ =>
      Impure
  }

  private def minOf(l0: PurityLevel, ls: List[PurityLevel]) = (l0 /: ls)(_ min _)

  def isPureExpr(tree: Tree)(implicit ctx: Context) = exprPurity(tree) == Pure
  def isIdempotentExpr(tree: Tree)(implicit ctx: Context) = exprPurity(tree) >= Idempotent

  /** The purity level of this reference.
   *  @return
   *    pure        if reference is (nonlazy and stable) or to a parameterized function
   *    idempotent  if reference is lazy and stable
   *    impure      otherwise
   *  @DarkDimius: need to make sure that lazy accessor methods have Lazy and Stable
   *               flags set.
   */
  private def refPurity(tree: Tree)(implicit ctx: Context): PurityLevel =
    if (!tree.tpe.widen.isParameterless) Pure
    else if (!tree.symbol.isStable) Impure
    else if (tree.symbol.is(Lazy)) Idempotent // TODO add Module flag, sinxce Module vals or not Lazy from the start.
    else Pure

  def isPureRef(tree: Tree)(implicit ctx: Context) =
    refPurity(tree) == Pure
  def isIdempotentRef(tree: Tree)(implicit ctx: Context) =
    refPurity(tree) >= Idempotent

  /** If `tree` is a constant expression, its value as a Literal,
   *  or `tree` itself otherwise.
   *
   *  Note: Demanding idempotency instead of purity in literalize is strictly speaking too loose.
   *  Example
   *
   *    object O { final val x = 42; println("43") }
   *    O.x
   *
   *  Strictly speaking we can't replace `O.x` with `42`.  But this would make
   *  most expressions non-constant. Maybe we can change the spec to accept this
   *  kind of eliding behavior. Or else enforce true purity in the compiler.
   *  The choice will be affected by what we will do with `inline` and with
   *  Singleton type bounds (see SIP 23). Presumably
   *
   *     object O1 { val x: Singleton = 42; println("43") }
   *     object O2 { inline val x = 42; println("43") }
   *
   *  should behave differently.
   *
   *     O1.x  should have the same effect as   { println("43"); 42 }
   *
   *  whereas
   *
   *     O2.x = 42
   *
   *  Revisit this issue once we have implemented `inline`. Then we can demand
   *  purity of the prefix unless the selection goes to an inline val.
   *
   *  Note: This method should be applied to all term tree nodes that are not literals,
   *        that can be idempotent, and that can have constant types. So far, only nodes
   *        of the following classes qualify:
   *
   *        Ident
   *        Select
   *        TypeApply
   */
  def constToLiteral(tree: Tree)(implicit ctx: Context): Tree = {
    val tree1 = ConstFold(tree)
    def canInlineConstant(value: Constant): Boolean = {
      val sym = tree1.symbol
      isIdempotentExpr(tree1) && // see note in documentation
      // lazy value must be initialized (would not be needed with isPureExpr)
      !sym.is(Lazy) &&
      // could hide initialization order issues (ex. val with constant type read before initialized)
      (!ctx.owner.isLocalDummy || (!sym.is(Method) && !sym.is(Lazy) && value.isZero) ||
        ctx.scala2Mode // ignore in Scala 2 because of inlined `final val` values
      )
    }
    tree1.tpe.widenTermRefExpr match {
      case ConstantType(value) if canInlineConstant(value) => Literal(value)
      case _ => tree1
    }
  }

  /** Is symbol potentially a getter of a mutable variable?
   */
  def mayBeVarGetter(sym: Symbol)(implicit ctx: Context): Boolean = {
    def maybeGetterType(tpe: Type): Boolean = tpe match {
      case _: ExprType | _: ImplicitMethodType => true
      case tpe: PolyType => maybeGetterType(tpe.resultType)
      case _ => false
    }
    sym.owner.isClass && !sym.isStable && maybeGetterType(sym.info)
  }

  /** Is tree a reference to a mutable variable, or to a potential getter
   *  that has a setter in the same class?
   */
  def isVariableOrGetter(tree: Tree)(implicit ctx: Context) = {
    def sym = tree.symbol
    def isVar    = sym is Mutable
    def isGetter =
      mayBeVarGetter(sym) && sym.owner.info.member(sym.name.asTermName.setterName).exists

    unsplice(tree) match {
      case Ident(_) => isVar
      case Select(_, _) => isVar || isGetter
      case Apply(_, _) =>
        methPart(tree) match {
          case Select(qual, nme.apply) => qual.tpe.member(nme.update).exists
          case _ => false
        }
      case _ => false
    }
  }

  /** Is tree a `this` node which belongs to `enclClass`? */
  def isSelf(tree: Tree, enclClass: Symbol)(implicit ctx: Context): Boolean = unsplice(tree) match {
    case This(_) => tree.symbol == enclClass
    case _ => false
  }

  /** Strips layers of `.asInstanceOf[T]` / `_.$asInstanceOf[T]()` from an expression */
  def stripCast(tree: Tree)(implicit ctx: Context): Tree = {
    def isCast(sel: Tree) = sel.symbol == defn.Any_asInstanceOf
    unsplice(tree) match {
      case TypeApply(sel @ Select(inner, _), _) if isCast(sel) =>
        stripCast(inner)
      case Apply(TypeApply(sel @ Select(inner, _), _), Nil) if isCast(sel) =>
        stripCast(inner)
      case t =>
        t
    }
  }

  /** Decompose a call fn[targs](vargs_1)...(vargs_n)
   *  into its constituents (where targs, vargss may be empty)
   */
  def decomposeCall(tree: Tree): (Tree, List[Tree], List[List[Tree]]) = tree match {
    case Apply(fn, args) =>
      val (meth, targs, argss) = decomposeCall(fn)
      (meth, targs, argss :+ args)
    case TypeApply(fn, targs) =>
      val (meth, Nil, Nil) = decomposeCall(fn)
      (meth, targs, Nil)
    case _ =>
      (tree, Nil, Nil)
  }

  /** An extractor for closures, either contained in a block or standalone.
   */
  object closure {
    def unapply(tree: Tree): Option[(List[Tree], Tree, Tree)] = tree match {
      case Block(_, expr) => unapply(expr)
      case Closure(env, meth, tpt) => Some(env, meth, tpt)
      case _ => None
    }
  }

  /** If tree is a closure, its body, otherwise tree itself */
  def closureBody(tree: Tree)(implicit ctx: Context): Tree = tree match {
    case Block((meth @ DefDef(nme.ANON_FUN, _, _, _, _)) :: Nil, Closure(_, _, _)) => meth.rhs
    case _ => tree
  }

  /** The variables defined by a pattern, in reverse order of their appearance. */
  def patVars(tree: Tree)(implicit ctx: Context): List[Symbol] = {
    val acc = new TreeAccumulator[List[Symbol]] {
      def apply(syms: List[Symbol], tree: Tree)(implicit ctx: Context) = tree match {
        case Bind(_, body) => apply(tree.symbol :: syms, body)
        case _ => foldOver(syms, tree)
      }
    }
    acc(Nil, tree)
  }

  /** Is this pattern node a catch-all or type-test pattern? */
  def isCatchCase(cdef: CaseDef)(implicit ctx: Context) = cdef match {
    case CaseDef(Typed(Ident(nme.WILDCARD), tpt), EmptyTree, _) =>
      isSimpleThrowable(tpt.tpe)
    case CaseDef(Bind(_, Typed(Ident(nme.WILDCARD), tpt)), EmptyTree, _) =>
      isSimpleThrowable(tpt.tpe)
    case _ =>
      isDefaultCase(cdef)
  }

  private def isSimpleThrowable(tp: Type)(implicit ctx: Context): Boolean = tp match {
    case tp @ TypeRef(pre, _) =>
      (pre == NoPrefix || pre.widen.typeSymbol.isStatic) &&
      (tp.symbol derivesFrom defn.ThrowableClass) && !(tp.symbol is Trait)
    case _ =>
      false
  }

  /** The symbols defined locally in a statement list */
  def localSyms(stats: List[Tree])(implicit ctx: Context): List[Symbol] =
    for (stat <- stats if stat.isDef && stat.symbol.exists) yield stat.symbol

  /** If `tree` is a DefTree, the symbol defined by it, otherwise NoSymbol */
  def definedSym(tree: Tree)(implicit ctx: Context): Symbol =
    if (tree.isDef) tree.symbol else NoSymbol

  /** Going from child to parent, the path of tree nodes that starts
   *  with a definition of symbol `sym` and ends with `root`, or Nil
   *  if no such path exists.
   *  Pre: `sym` must have a position.
   */
  def defPath(sym: Symbol, root: Tree)(implicit ctx: Context): List[Tree] = ctx.debugTraceIndented(s"defpath($sym with position ${sym.pos}, ${root.show})") {
    require(sym.pos.exists)
    object accum extends TreeAccumulator[List[Tree]] {
      def apply(x: List[Tree], tree: Tree)(implicit ctx: Context): List[Tree] = {
        if (tree.pos.contains(sym.pos))
          if (definedSym(tree) == sym) tree :: x
          else {
            val x1 = foldOver(x, tree)
            if (x1 ne x) tree :: x1 else x1
          }
        else x
      }
    }
    accum(Nil, root)
  }

  /**  The largest subset of {NoInits, PureInterface} that a
   *   trait enclosing this statement can have as flags.
   */
  def defKind(tree: Tree): FlagSet = unsplice(tree) match {
    case EmptyTree | _: Import => NoInitsInterface
    case tree: TypeDef => if (tree.isClassDef) NoInits else NoInitsInterface
    case tree: DefDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else NoInits
    case tree: ValDef => if (tree.unforcedRhs == EmptyTree) NoInitsInterface else EmptyFlags
    case _ => EmptyFlags
  }

  /** The top level classes in this tree, including only those module classes that
   *  are not a linked class of some other class in the result.
   */
  def topLevelClasses(tree: Tree)(implicit ctx: Context): List[ClassSymbol] = tree match {
    case PackageDef(_, stats) => stats.flatMap(topLevelClasses)
    case tdef: TypeDef if tdef.symbol.isClass => tdef.symbol.asClass :: Nil
    case _ => Nil
  }

  /** The tree containing only the top-level classes and objects matching either `cls` or its companion object */
  def sliceTopLevel(tree: Tree, cls: ClassSymbol)(implicit ctx: Context): List[Tree] = tree match {
    case PackageDef(pid, stats) =>
      cpy.PackageDef(tree)(pid, stats.flatMap(sliceTopLevel(_, cls))) :: Nil
    case tdef: TypeDef =>
      val sym = tdef.symbol
      assert(sym.isClass)
      if (cls == sym || cls == sym.linkedClass) tdef :: Nil
      else Nil
    case vdef: ValDef =>
      val sym = vdef.symbol
      assert(sym is Module)
      if (cls == sym.companionClass || cls == sym.moduleClass) vdef :: Nil
      else Nil
    case tree =>
      tree :: Nil
  }

  /** The statement sequence that contains a definition of `sym`, or Nil
   *  if none was found.
   *  For a tree to be found, The symbol must have a position and its definition
   *  tree must be reachable from come tree stored in an enclosing context.
   */
  def definingStats(sym: Symbol)(implicit ctx: Context): List[Tree] =
    if (!sym.pos.exists || (ctx eq NoContext) || ctx.compilationUnit == null) Nil
    else defPath(sym, ctx.compilationUnit.tpdTree) match {
      case defn :: encl :: _ =>
        def verify(stats: List[Tree]) =
          if (stats exists (definedSym(_) == sym)) stats else Nil
        encl match {
          case Block(stats, _) => verify(stats)
          case encl: Template => verify(encl.body)
          case PackageDef(_, stats) => verify(stats)
          case _ => Nil
        }
      case nil =>
        Nil
    }

  /** Is this a selection of a member of a structural type that is not a member
   *  of an underlying class or trait?
   */
  def isStructuralTermSelect(tree: Tree)(implicit ctx: Context) = tree match {
    case tree: Select =>
      def hasRefinement(qualtpe: Type): Boolean = qualtpe.dealias match {
        case RefinedType(parent, rname, rinfo) =>
          rname == tree.name || hasRefinement(parent)
        case tp: TypeProxy =>
          hasRefinement(tp.underlying)
        case tp: OrType =>
          hasRefinement(tp.tp1) || hasRefinement(tp.tp2)
        case tp: AndType =>
          hasRefinement(tp.tp1) && hasRefinement(tp.tp2)
        case _ =>
          false
      }
      !tree.symbol.exists && tree.isTerm && hasRefinement(tree.qualifier.tpe)
    case _ =>
      false
  }
}

object TreeInfo {
  class PurityLevel(val x: Int) extends AnyVal {
    def >= (that: PurityLevel) = x >= that.x
    def min(that: PurityLevel) = new PurityLevel(x min that.x)
  }

  val Pure = new PurityLevel(2)
  val Idempotent = new PurityLevel(1)
  val Impure = new PurityLevel(0)
}

  /** a Match(Typed(_, tpt), _) must be translated into a switch if isSwitchAnnotation(tpt.tpe)
  def isSwitchAnnotation(tpe: Type) = tpe hasAnnotation defn.SwitchClass
  */

  /** Does list of trees start with a definition of
   *  a class of module with given name (ignoring imports)
  def firstDefinesClassOrObject(trees: List[Tree], name: Name): Boolean = trees match {
      case Import(_, _) :: xs               => firstDefinesClassOrObject(xs, name)
      case Annotated(_, tree1) :: Nil       => firstDefinesClassOrObject(List(tree1), name)
      case ModuleDef(_, `name`, _) :: Nil   => true
      case ClassDef(_, `name`, _, _) :: Nil => true
      case _                                => false
    }


  /** Is this file the body of a compilation unit which should not
   *  have Predef imported?
   */
  def noPredefImportForUnit(body: Tree) = {
    // Top-level definition whose leading imports include Predef.
    def isLeadingPredefImport(defn: Tree): Boolean = defn match {
      case PackageDef(_, defs1) => defs1 exists isLeadingPredefImport
      case Import(expr, _)      => isReferenceToPredef(expr)
      case _                    => false
    }
    // Compilation unit is class or object 'name' in package 'scala'
    def isUnitInScala(tree: Tree, name: Name) = tree match {
      case PackageDef(Ident(nme.scala_), defs) => firstDefinesClassOrObject(defs, name)
      case _                                   => false
    }

    isUnitInScala(body, nme.Predef) || isLeadingPredefImport(body)
  }
   */

  /*
  def isAbsTypeDef(tree: Tree) = tree match {
    case TypeDef(_, _, _, TypeBoundsTree(_, _)) => true
    case TypeDef(_, _, _, rhs) => rhs.tpe.isInstanceOf[TypeBounds]
    case _ => false
  }

  def isAliasTypeDef(tree: Tree) = tree match {
    case TypeDef(_, _, _, _) => !isAbsTypeDef(tree)
    case _ => false
  }

  /** Some handy extractors for spotting trees through the
   *  the haze of irrelevant braces: i.e. Block(Nil, SomeTree)
   *  should not keep us from seeing SomeTree.
   */
  abstract class SeeThroughBlocks[T] {
    protected def unapplyImpl(x: Tree): T
    def unapply(x: Tree): T = x match {
      case Block(Nil, expr)         => unapply(expr)
      case _                        => unapplyImpl(x)
    }
  }
  object IsTrue extends SeeThroughBlocks[Boolean] {
    protected def unapplyImpl(x: Tree): Boolean = x match {
      case Literal(Constant(true)) => true
      case _                       => false
    }
  }
  object IsFalse extends SeeThroughBlocks[Boolean] {
    protected def unapplyImpl(x: Tree): Boolean = x match {
      case Literal(Constant(false)) => true
      case _                        => false
    }
  }
  object IsIf extends SeeThroughBlocks[Option[(Tree, Tree, Tree)]] {
    protected def unapplyImpl(x: Tree) = x match {
      case If(cond, thenp, elsep) => Some((cond, thenp, elsep))
      case _                      => None
    }
  }

  object MacroImplReference {
    private def refPart(tree: Tree): Tree = tree match {
      case TypeApply(fun, _) => refPart(fun)
      case ref: RefTree => ref
      case _ => EmptyTree()
    }

    def unapply(tree: Tree) = refPart(tree) match {
      case ref: RefTree => Some((ref.qualifier.symbol, ref.symbol, dissectApplied(tree).targs))
      case _            => None
    }
  }

  def isNullaryInvocation(tree: Tree): Boolean =
    tree.symbol != null && tree.symbol.isMethod && (tree match {
      case TypeApply(fun, _) => isNullaryInvocation(fun)
      case tree: RefTree => true
      case _ => false
    })*/