Eliminate some hot allocations

Again, these might be eliminated by escape analysis, but it does not
hurt to not allocate in the first place.

Author: odersky

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

@@ -51,8 +51,12 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
     else {
       val newpd: this.type =
         if (mySpan.isSynthetic) {
-          if (!mySpan.exists && span.exists)
+          if (!mySpan.exists && span.exists) {
             envelope(source, span.startPos) // fill in children spans
+            () // Note: the `()` is there to prevent some inefficient code from being generated.
+               // Without it we get an allocation of a span here since the result type of the `if`
+               // is `Any`, the lub of `Span` and `Unit`.
+          }
           this
         }
         else cloneIn(source)

compiler/src/dotty/tools/dotc/core/Names.scala

@@ -570,7 +570,7 @@ object Names {
    */
   def termName(cs: Array[Char], offset: Int, len: Int): SimpleName = synchronized {
     util.Stats.record("termName")
-    val h = hashValue(cs, offset, len) & (table.size - 1)
+    val h = hashValue(cs, offset, len) & (table.length - 1)
 
     /** Make sure the capacity of the character array is at least `n` */
     def ensureCapacity(n: Int) =