[ruby/prism] Disallow multi writes when not in statement level

https://github.com/ruby/prism/commit/fe34a77df2

prism/config.yml

@@ -266,6 +266,7 @@ errors:
   - UNEXPECTED_BLOCK_ARGUMENT
   - UNEXPECTED_INDEX_BLOCK
   - UNEXPECTED_INDEX_KEYWORDS
+  - UNEXPECTED_MULTI_WRITE
   - UNEXPECTED_SAFE_NAVIGATION
   - UNEXPECTED_TOKEN_CLOSE_CONTEXT
   - UNEXPECTED_TOKEN_IGNORE

prism/prism.c

@@ -19901,10 +19901,15 @@ parse_assignment_value_local(pm_parser_t *parser, const pm_node_t *node) {
  */
 static pm_node_t *
 parse_assignment_values(pm_parser_t *parser, pm_binding_power_t previous_binding_power, pm_binding_power_t binding_power, bool accepts_command_call, pm_diagnostic_id_t diag_id) {
-    pm_node_t *value = parse_starred_expression(parser, binding_power, previous_binding_power == PM_BINDING_POWER_ASSIGNMENT ? accepts_command_call : previous_binding_power < PM_BINDING_POWER_MATCH, diag_id);
-    parse_assignment_value_local(parser, value);
+    bool permitted = true;
+    if (previous_binding_power != PM_BINDING_POWER_STATEMENT && match1(parser, PM_TOKEN_USTAR)) permitted = false;
 
+    pm_node_t *value = parse_starred_expression(parser, binding_power, previous_binding_power == PM_BINDING_POWER_ASSIGNMENT ? accepts_command_call : previous_binding_power < PM_BINDING_POWER_MATCH, diag_id);
+    if (!permitted) pm_parser_err_node(parser, value, PM_ERR_UNEXPECTED_MULTI_WRITE);
+
+    parse_assignment_value_local(parser, value);
     bool single_value = true;
+
     if (previous_binding_power == PM_BINDING_POWER_STATEMENT && (PM_NODE_TYPE_P(value, PM_SPLAT_NODE) || match1(parser, PM_TOKEN_COMMA))) {
         single_value = false;
 

prism/templates/src/diagnostic.c.erb

@@ -347,6 +347,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_UNEXPECTED_BLOCK_ARGUMENT]          = { "block argument should not be given", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNEXPECTED_INDEX_BLOCK]             = { "unexpected block arg given in index assignment; blocks are not allowed in index assignment expressions", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNEXPECTED_INDEX_KEYWORDS]          = { "unexpected keyword arg given in index assignment; keywords are not allowed in index assignment expressions", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_UNEXPECTED_MULTI_WRITE]             = { "unexpected multiple assignment; multiple assignment is not allowed in this context", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNEXPECTED_SAFE_NAVIGATION]         = { "&. inside multiple assignment destination", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNEXPECTED_TOKEN_CLOSE_CONTEXT]     = { "unexpected %s, assuming it is closing the parent %s", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNEXPECTED_TOKEN_IGNORE]            = { "unexpected %s, ignoring it", PM_ERROR_LEVEL_SYNTAX },