Translate "ab" "cd" as a single StringLiteralNode

Author: eregon

spec/truffle/parsing/fixtures/strings/concatenated_literal.yaml

@@ -0,0 +1,11 @@
+subject: "String"
+description: "2 string literals concatenated at parse/translation time"
+focused_on_node: "org.truffleruby.language.literal.StringLiteralNode"
+ruby: |
+  "foo" "bar"
+ast: |
+  StringLiteralNode
+      attributes:
+          encoding = UTF-8
+          flags = 0
+          tstring = foobar

src/main/java/org/truffleruby/parser/YARPTranslator.java

@@ -1564,6 +1564,10 @@ public RubyNode visitInterpolatedRegularExpressionNode(Nodes.InterpolatedRegular
 
     @Override
     public RubyNode visitInterpolatedStringNode(Nodes.InterpolatedStringNode node) {
+        if (allPartsAreStringNodes(node)) {
+            return visitStringNode(concatStringNodes(node));
+        }
+
         final ToSNode[] children = translateInterpolatedParts(node.parts);
 
         final RubyNode rubyNode = new InterpolatedStringNode(children, sourceEncoding.jcoding);
@@ -1588,6 +1592,38 @@ public RubyNode visitInterpolatedXStringNode(Nodes.InterpolatedXStringNode node)
         return assignPositionAndFlags(node, rubyNode);
     }
 
+    private static boolean allPartsAreStringNodes(Nodes.InterpolatedStringNode node) {
+        for (var part : node.parts) {
+            if (!(part instanceof Nodes.StringNode)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private static Nodes.StringNode concatStringNodes(Nodes.InterpolatedStringNode node) {
+        Nodes.Node[] parts = node.parts;
+        assert parts.length > 0;
+
+        int totalSize = 0;
+        for (var part : parts) {
+            totalSize += ((Nodes.StringNode) part).unescaped.length;
+        }
+
+        byte[] concatenated = new byte[totalSize];
+        int i = 0;
+        for (var part : parts) {
+            byte[] bytes = ((Nodes.StringNode) part).unescaped;
+            System.arraycopy(bytes, 0, concatenated, i, bytes.length);
+            i += bytes.length;
+        }
+
+        int start = parts[0].startOffset;
+        var last = parts[parts.length - 1];
+        int length = last.endOffset() - start;
+        return new Nodes.StringNode(NO_FLAGS, concatenated, start, length);
+    }
+
     private ToSNode[] translateInterpolatedParts(Nodes.Node[] parts) {
         final ToSNode[] children = new ToSNode[parts.length];
 
@@ -2661,7 +2697,7 @@ private void assignPositionOnly(Nodes.Node[] nodes, RubyNode rubyNode) {
         final Nodes.Node first = nodes[0];
         final Nodes.Node last = nodes[nodes.length - 1];
 
-        final int length = last.startOffset - first.startOffset + last.length;
+        final int length = last.endOffset() - first.startOffset;
         rubyNode.unsafeSetSourceSection(first.startOffset, length);
     }