[ruby/prism] Prevent optional block parameter from accepting certain prefixed values

This commit prevents the following unary operators from being accepted
as the value prefix of a block's optional parameter:

- `+`
- `-`
- `~`
- `!`

For example, `f { |a = +b| }` will now raise a syntax error.

https://github.com/ruby/prism/commit/3024bee60c

Co-authored-by: Kevin Newton <kddnewton@gmail.com>

3 files changed, 19 insertions, 0 deletions

diff --git a/prism/config.yml b/prism/config.yml
index 52c3982261..321264e686 100644
--- a/prism/config.yml
+++ b/prism/config.yml
@@ -263,6 +263,7 @@ errors:
   - TERNARY_COLON
   - TERNARY_EXPRESSION_FALSE
   - TERNARY_EXPRESSION_TRUE
+  - UNARY_DISALLOWED
   - UNARY_RECEIVER
   - UNDEF_ARGUMENT
   - UNEXPECTED_BLOCK_ARGUMENT
diff --git a/prism/prism.c b/prism/prism.c
index f9d8f84f91..73df294213 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -17355,6 +17355,10 @@ pm_parser_err_prefix(pm_parser_t *parser, pm_diagnostic_id_t diag_id) {
             PM_PARSER_ERR_TOKEN_FORMAT(parser, parser->previous, diag_id, human, parser->previous.start[0]);
             break;
         }
+        case PM_ERR_UNARY_DISALLOWED: {
+            PM_PARSER_ERR_TOKEN_FORMAT(parser, parser->current, diag_id, pm_token_type_human(parser->current.type));
+            break;
+        }
         default:
             pm_parser_err_previous(parser, diag_id);
             break;
@@ -19904,6 +19908,10 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             }
         }
         case PM_TOKEN_BANG: {
+            if (binding_power > PM_BINDING_POWER_UNARY) {
+                pm_parser_err_prefix(parser, PM_ERR_UNARY_DISALLOWED);
+            }
+
             parser_lex(parser);
 
             pm_token_t operator = parser->previous;
@@ -19914,6 +19922,9 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             return (pm_node_t *) node;
         }
         case PM_TOKEN_TILDE: {
+            if (binding_power > PM_BINDING_POWER_UNARY) {
+                pm_parser_err_prefix(parser, PM_ERR_UNARY_DISALLOWED);
+            }
             parser_lex(parser);
 
             pm_token_t operator = parser->previous;
@@ -19923,6 +19934,9 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             return (pm_node_t *) node;
         }
         case PM_TOKEN_UMINUS: {
+            if (binding_power > PM_BINDING_POWER_UNARY) {
+                pm_parser_err_prefix(parser, PM_ERR_UNARY_DISALLOWED);
+            }
             parser_lex(parser);
 
             pm_token_t operator = parser->previous;
@@ -20039,6 +20053,9 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             return (pm_node_t *) pm_lambda_node_create(parser, &locals, &operator, &opening, &parser->previous, parameters, body);
         }
         case PM_TOKEN_UPLUS: {
+            if (binding_power > PM_BINDING_POWER_UNARY) {
+                pm_parser_err_prefix(parser, PM_ERR_UNARY_DISALLOWED);
+            }
             parser_lex(parser);
 
             pm_token_t operator = parser->previous;
diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index b09aca1c23..064063d9f4 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -346,6 +346,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_TERNARY_EXPRESSION_TRUE]            = { "expected an expression after `?` in the ternary operator", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNDEF_ARGUMENT]                     = { "invalid argument being passed to `undef`; expected a bare word, constant, or symbol argument", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_UNARY_RECEIVER]                     = { "unexpected %s, expected a receiver for unary `%c`", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_UNARY_DISALLOWED]                   = { "unexpected %s; unary calls are not allowed in this context", PM_ERROR_LEVEL_SYNTAX },
     [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 },