[GR-18163] Fix proc's rest arguments

PullRequest: truffleruby/3717

Author: andrykonchin

CHANGELOG.md

@@ -42,6 +42,7 @@ Bug fixes:
 * Don't trigger the `method_added` event when changing a method's visibility or calling `module_function` (@paracycle, @nirvdrum).
 * Fix `rb_time_timespec_new` function to not call `Time.at` method directly (@andrykonchin).
 * Fix `StringIO#write` to transcode strings with encodings that don't match the `StringIO`'s `external_encoding`. (#2839, @flavorjones)
+* Fix processing of proc rest arguments located at the beginning if there are no actual arguments (#2921, @andrykonchin).
 
 Compatibility:
 

spec/ruby/language/proc_spec.rb

@@ -161,6 +161,18 @@
     end
   end
 
+  describe "taking |*a, b| arguments" do
+    it "assigns [] to the argument when passed no values" do
+      proc { |*a, b| [a, b] }.call.should == [[], nil]
+    end
+  end
+
+  describe "taking |a, *b, c| arguments" do
+    it "assigns [] to the argument when passed no values" do
+      proc { |a, *b, c| [a, b, c] }.call.should == [nil, [], nil]
+    end
+  end
+
   describe "taking |a, | arguments" do
     before :each do
       @l = lambda { |a, | a }

src/main/java/org/truffleruby/language/arguments/ReadRestArgumentNode.java

@@ -76,16 +76,12 @@ public Object execute(VirtualFrame frame) {
 
         final int length = endIndex - startIndex;
 
-        if (startIndex == 0) {
-            return createArray(RubyArguments.getRawArguments(frame, startIndex, length), length);
+        if (startIndex >= endIndex) {
+            noArgumentsLeftProfile.enter();
+            return createEmptyArray();
         } else {
-            if (startIndex >= endIndex) {
-                noArgumentsLeftProfile.enter();
-                return createEmptyArray();
-            } else {
-                subsetOfArgumentsProfile.enter();
-                return createArray(RubyArguments.getRawArguments(frame, startIndex, length), length);
-            }
+            subsetOfArgumentsProfile.enter();
+            return createArray(RubyArguments.getRawArguments(frame, startIndex, length), length);
         }
     }