[Bug #21439] Fix PM_SPLAT_NODE compilation error in for loops

This commit fixes a crash that occurred when using splat nodes (*) as
the index variable in for loops. The error "Unexpected node type for
index in for node: PM_SPLAT_NODE" was thrown because the compiler
didn't know how to handle splat nodes in this context.

The fix allows code like `for *x in [[1,2], [3,4]]` to compile and
execute correctly, where the splat collects each sub-array.

Author: paracycle

prism_compile.c

@@ -5164,6 +5164,20 @@ pm_compile_target_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *cons
 
         break;
       }
+      case PM_SPLAT_NODE: {
+        // Splat nodes capture all values into an array. They can be used
+        // as targets in assignments or for loops.
+        //
+        //     for *x in []; end
+        //
+        const pm_splat_node_t *cast = (const pm_splat_node_t *) node;
+
+        if (cast->expression != NULL) {
+            pm_compile_target_node(iseq, cast->expression, parents, writes, cleanup, scope_node, state);
+        }
+
+        break;
+      }
       default:
         rb_bug("Unexpected node type: %s", pm_node_type_to_str(PM_NODE_TYPE(node)));
         break;
@@ -5277,7 +5291,8 @@ pm_compile_for_node_index(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *c
       case PM_INSTANCE_VARIABLE_TARGET_NODE:
       case PM_CONSTANT_PATH_TARGET_NODE:
       case PM_CALL_TARGET_NODE:
-      case PM_INDEX_TARGET_NODE: {
+      case PM_INDEX_TARGET_NODE:
+      case PM_SPLAT_NODE: {
         // For other targets, we need to potentially compile the parent or
         // owning expression of this target, then retrieve the value, expand it,
         // and then compile the necessary writes.

test/ruby/test_compile_prism.rb

@@ -1053,6 +1053,9 @@ def test_ForNode
       assert_prism_eval("for foo, in  [1,2,3] do end")
 
       assert_prism_eval("for i, j in {a: 'b'} do; i; j; end")
+
+      # Test splat node as index in for loop
+      assert_prism_eval("for *x in [[1,2], [3,4]] do; x; end")
     end
 
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