Avoid closure creation in Thicket.span

Author: odersky

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

@@ -855,7 +855,11 @@ object Trees {
     override def isEmpty: Boolean = trees.isEmpty
     override def toList: List[Tree[T]] = flatten(trees)
     override def toString: String = if (isEmpty) "EmptyTree" else "Thicket(" + trees.mkString(", ") + ")"
-    override def span: Span = trees.foldLeft(NoSpan) ((span, t) => span union t.span)
+    override def span: Span =
+      def combine(s: Span, ts: List[Tree[T]]): Span = ts match
+        case t :: ts1 => combine(s.union(t.span), ts1)
+        case nil => s
+      combine(NoSpan, trees)
 
     override def withSpan(span: Span): this.type =
       mapElems(_.withSpan(span)).asInstanceOf[this.type]