Fix translating implicit rest in multi-assignment

Author: andrykonchin

spec/truffle/parsing/fixtures/operators/multi_assignments/when_implicit_rest.yaml

@@ -0,0 +1,29 @@
+subject: "Multi-assignment"
+description: "When implicit rest (a, = [])"
+notes: >
+  Implicit rest is just ignored
+focused_on_node: "org.truffleruby.core.array.MultipleAssignmentNode"
+ruby: |
+  a, = []
+ast: |
+  MultipleAssignmentNode
+      attributes:
+          flags = 1
+      children:
+          preNodes = [
+              WriteFrameSlotNodeGen
+                  attributes:
+                      frameSlot = 2 # a
+          ]
+          rhsNode =
+              ArrayLiteralNode$UninitialisedArrayLiteralNode
+                  attributes:
+                      flags = 0
+                      language = org.truffleruby.RubyLanguage@...
+          splatCastNode =
+              SplatCastNodeGen
+                  attributes:
+                      conversionMethod = :to_ary
+                      copy = true
+                      flags = 0
+                      nilBehavior = ARRAY_WITH_NIL

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

@@ -262,7 +262,6 @@ public RubyNode visitImplicitRestNode(Nodes.ImplicitRestNode node) {
         throw CompilerDirectives.shouldNotReachHere("handled in #translateWithParameters");
     }
 
-
     @Override
     public RubyNode visitKeywordRestParameterNode(Nodes.KeywordRestParameterNode node) {
         final RubyNode readNode = new ReadKeywordRestArgumentNode(language, arity);

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

@@ -9,6 +9,7 @@
  */
 package org.truffleruby.parser;
 
+import com.oracle.truffle.api.CompilerDirectives;
 import org.prism.AbstractNodeVisitor;
 import org.prism.Nodes;
 import org.truffleruby.RubyLanguage;
@@ -50,7 +51,13 @@ public RubyNode translate() {
 
         final AssignableNode restNode;
         if (node.rest != null) {
-            restNode = node.rest.accept(this);
+            if (node.rest instanceof Nodes.ImplicitRestNode) {
+                // a, = []
+                // do nothing
+                restNode = null;
+            } else {
+                restNode = node.rest.accept(this);
+            }
         } else {
             restNode = null;
         }
@@ -99,6 +106,11 @@ public AssignableNode visitGlobalVariableTargetNode(Nodes.GlobalVariableTargetNo
         return ((AssignableNode) rubyNode).toAssignableNode();
     }
 
+    @Override
+    public AssignableNode visitImplicitRestNode(Nodes.ImplicitRestNode node) {
+        throw CompilerDirectives.shouldNotReachHere("handled in #translate");
+    }
+
     @Override
     public AssignableNode visitIndexTargetNode(Nodes.IndexTargetNode node) {
         final RubyNode rubyNode = node.accept(yarpTranslator);