Avoid some boxings of vars

Author: odersky

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -154,14 +154,17 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
     }
   }
 
+  private class LastPosRef:
+    var positioned: Positioned | Null = null
+    var span = NoSpan
+
   /** Check that all positioned items in this tree satisfy the following conditions:
    *  - Parent spans contain child spans
    *  - If item is a non-empty tree, it has a position
    */
   def checkPos(nonOverlapping: Boolean)(using Context): Unit = try {
     import untpd._
-    var lastPositioned: Positioned | Null = null
-    var lastSpan = NoSpan
+    val last = LastPosRef()
     def check(p: Any): Unit = p match {
       case p: Positioned =>
         assert(span contains p.span,
@@ -181,19 +184,19 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Src
             case _: XMLBlock =>
               // FIXME: Trees generated by the XML parser do not satisfy `checkPos`
             case _: WildcardFunction
-            if lastPositioned.isInstanceOf[ValDef] && !p.isInstanceOf[ValDef] =>
+            if last.positioned.isInstanceOf[ValDef] && !p.isInstanceOf[ValDef] =>
               // ignore transition from last wildcard parameter to body
             case _ =>
-              assert(!lastSpan.exists || !p.span.exists || lastSpan.end <= p.span.start,
+              assert(!last.span.exists || !p.span.exists || last.span.end <= p.span.start,
                 i"""position error, child positions overlap or in wrong order
                    |parent         = $this
-                   |1st child      = $lastPositioned
-                   |1st child span = $lastSpan
+                   |1st child      = ${last.positioned}
+                   |1st child span = ${last.span}
                    |2nd child      = $p
                    |2nd child span = ${p.span}""".stripMargin)
           }
-          lastPositioned = p
-          lastSpan = p.span
+          last.positioned = p
+          last.span = p.span
         p.checkPos(nonOverlapping)
       case m: untpd.Modifiers =>
         m.annotations.foreach(check)

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -786,10 +786,12 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
 
   /** The symbols defined locally in a statement list */
   def localSyms(stats: List[Tree])(using Context): List[Symbol] =
-    val locals = new mutable.ListBuffer[Symbol]
-    for stat <- stats do
-      if stat.isDef && stat.symbol.exists then locals += stat.symbol
-    locals.toList
+    if stats.isEmpty then Nil
+    else
+      val locals = new mutable.ListBuffer[Symbol]
+      for stat <- stats do
+        if stat.isDef && stat.symbol.exists then locals += stat.symbol
+      locals.toList
 
   /** If `tree` is a DefTree, the symbol defined by it, otherwise NoSymbol */
   def definedSym(tree: Tree)(using Context): Symbol =

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -553,7 +553,7 @@ object Scanners {
 
       // If nextWidth is an indentation level not yet seen by enclosing indentation
       // region, invoke `handler`.
-      def handleNewIndentWidth(r: Region, handler: Indented => Unit): Unit = r match
+      inline def handleNewIndentWidth(r: Region, inline handler: Indented => Unit): Unit = r match
         case r @ Indented(curWidth, prefix, outer)
         if curWidth < nextWidth && !r.otherIndentWidths.contains(nextWidth) && nextWidth != lastWidth =>
           handler(r)
@@ -571,7 +571,7 @@ object Scanners {
        *     they start with `(`, `[` or `{`, or the last statement ends in a `return`.
        *   The Scala 2 rules apply under source `3.0-migration` or under `-no-indent`.
        */
-      def isContinuing =
+      inline def isContinuing =
         lastWidth < nextWidth
         && (openParensTokens.contains(token) || lastToken == RETURN)
         && !pastBlankLine
@@ -608,10 +608,11 @@ object Scanners {
               case r: Indented =>
                 insert(OUTDENT, offset)
                 handleNewIndentWidth(r.enclosing, ir =>
+                  val lw = lastWidth
                   errorButContinue(
                     em"""The start of this line does not match any of the previous indentation widths.
                         |Indentation width of current line : $nextWidth
-                        |This falls between previous widths: ${ir.width} and $lastWidth"""))
+                        |This falls between previous widths: ${ir.width} and $lw"""))
               case r =>
                 if skipping then
                   if r.enclosing.isClosedByUndentAt(nextWidth) then
@@ -627,7 +628,8 @@ object Scanners {
           else if lastToken == SELFARROW then
             currentRegion.knownWidth = nextWidth
         else if (lastWidth != nextWidth)
-          errorButContinue(spaceTabMismatchMsg(lastWidth, nextWidth))
+          val lw = lastWidth
+          errorButContinue(spaceTabMismatchMsg(lw, nextWidth))
       if token != OUTDENT then
         handleNewIndentWidth(currentRegion, _.otherIndentWidths += nextWidth)
       if next.token == EMPTY then

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

@@ -698,18 +698,20 @@ object Erasure {
         return tree.asInstanceOf[Tree] // we are re-typing a primitive array op
 
       val owner = mapOwner(origSym)
-      var sym = if (owner eq origSym.maybeOwner) origSym else owner.info.decl(tree.name).symbol
-      if !sym.exists then
-        // We fail the sym.exists test for pos/i15158.scala, where we pass an infinitely
-        // recurring match type to an overloaded constructor. An equivalent test
-        // with regular apply methods succeeds. It's at present unclear whether
-        //  - the program should be rejected, or
-        //  - there is another fix.
-        // Therefore, we apply the fix to use the pre-erasure symbol, but only
-        // for constructors, in order not to mask other possible bugs that would
-        // trigger the assert(sym.exists, ...) below.
-        val prevSym = tree.symbol(using preErasureCtx)
-        if prevSym.isConstructor then sym = prevSym
+      val sym =
+        (if (owner eq origSym.maybeOwner) origSym else owner.info.decl(tree.name).symbol)
+        .orElse {
+          // We fail the sym.exists test for pos/i15158.scala, where we pass an infinitely
+          // recurring match type to an overloaded constructor. An equivalent test
+          // with regular apply methods succeeds. It's at present unclear whether
+          //  - the program should be rejected, or
+          //  - there is another fix.
+          // Therefore, we apply the fix to use the pre-erasure symbol, but only
+          // for constructors, in order not to mask other possible bugs that would
+          // trigger the assert(sym.exists, ...) below.
+          val prevSym = tree.symbol(using preErasureCtx)
+          if prevSym.isConstructor then prevSym else NoSymbol
+        }
 
       assert(sym.exists, i"no owner from $owner/${origSym.showLocated} in $tree")
 

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -250,15 +250,17 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         imp.importSym.info match
           case ImportType(expr) =>
             val pre = expr.tpe
-            var denot = pre.memberBasedOnFlags(name, required, excluded)
+            val denot0 = pre.memberBasedOnFlags(name, required, excluded)
               .accessibleFrom(pre)(using refctx)
             // Pass refctx so that any errors are reported in the context of the
             // reference instead of the context of the import scope
-            if denot.exists then
-              if checkBounds then
-                denot = denot.filterWithPredicate { mbr =>
-                  mbr.matchesImportBound(if mbr.symbol.is(Given) then imp.givenBound else imp.wildcardBound)
-                }
+            if denot0.exists then
+              val denot =
+                if checkBounds then
+                  denot0.filterWithPredicate { mbr =>
+                    mbr.matchesImportBound(if mbr.symbol.is(Given) then imp.givenBound else imp.wildcardBound)
+                  }
+                else denot0
               def isScalaJsPseudoUnion =
                 denot.name == tpnme.raw.BAR && ctx.settings.scalajs.value && denot.symbol == JSDefinitions.jsdefn.PseudoUnionClass
               // Just like Scala2Unpickler reinterprets Scala.js pseudo-unions