[ruby/prism] Change frozen_string_literal to be a tri-state

An explicit `false` is not equivalent to the comment being missing,
because the default can be switched with a runtime flag:

```bash
$ ruby --enable-frozen-string-literal -e 'p "foo".frozen?'
true
```

https://github.com/ruby/prism/commit/4660f58775

prism/config.yml

@@ -673,7 +673,9 @@ flags:
       - name: FORCED_BINARY_ENCODING
         comment: "internal bytes forced the encoding to binary"
       - name: FROZEN
-        comment: "frozen by virtue of a `frozen_string_literal` comment"
+        comment: "frozen by virtue of a `frozen_string_literal: true` comment or `--enable-frozen-string-literal`"
+      - name: MUTABLE
+        comment: "mutable by virtue of a `frozen_string_literal: false` comment or `--disable-frozen-string-literal`"
     comment: Flags for string nodes.
   - name: SymbolFlags
     values:

prism/extension.c

@@ -142,7 +142,7 @@ build_options_i(VALUE key, VALUE value, VALUE argument) {
     } else if (key_id == rb_option_id_offset) {
         if (!NIL_P(value)) pm_options_offset_set(options, NUM2UINT(value));
     } else if (key_id == rb_option_id_frozen_string_literal) {
-        if (!NIL_P(value)) pm_options_frozen_string_literal_set(options, value == Qtrue);
+        if (!NIL_P(value)) pm_options_frozen_string_literal_set(options, RTEST(value));
     } else if (key_id == rb_option_id_version) {
         if (!NIL_P(value)) {
             const char *version = check_string(value);

prism/options.c

@@ -37,7 +37,7 @@ pm_options_offset_set(pm_options_t *options, uint32_t offset) {
  */
 PRISM_EXPORTED_FUNCTION void
 pm_options_frozen_string_literal_set(pm_options_t *options, bool frozen_string_literal) {
-    options->frozen_string_literal = frozen_string_literal;
+    options->frozen_string_literal = frozen_string_literal ? 1 : -1;
 }
 
 /**
@@ -212,7 +212,7 @@ pm_options_read(pm_options_t *options, const char *data) {
         data += encoding_length;
     }
 
-    options->frozen_string_literal = (*data++) ? true : false;
+    options->frozen_string_literal = (int8_t) *data++;
     options->command_line = (uint8_t) *data++;
     options->version = (pm_options_version_t) *data++;
 

prism/options.h

@@ -13,6 +13,22 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/**
+ * String literals should be made frozen.
+ */
+#define PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED   ((int8_t) -1)
+
+/**
+ * String literals may be frozen or mutable depending on the implementation
+ * default.
+ */
+#define PM_OPTIONS_FROZEN_STRING_LITERAL_UNSET      ((int8_t)  0)
+
+/**
+ * String literals should be made mutable.
+ */
+#define PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED    ((int8_t)  1)
+
 /**
  * A scope of locals surrounding the code that is being parsed.
  */
@@ -85,8 +101,14 @@ typedef struct {
     /** A bitset of the various options that were set on the command line. */
     uint8_t command_line;
 
-    /** Whether or not the frozen string literal option has been set. */
+    /**
-    bool frozen_string_literal;
+    * Whether or not the frozen string literal option has been set.
+    * May be:
+    *  - PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED
+    *  - PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED
+    *  - PM_OPTIONS_FROZEN_STRING_LITERAL_UNSET
+    */
+    int8_t frozen_string_literal;
 } pm_options_t;
 
 /**

prism/parser.h

@@ -709,6 +709,16 @@ struct pm_parser {
     /** The command line flags given from the options. */
     uint8_t command_line;
 
+    /**
+     * Whether or not we have found a frozen_string_literal magic comment with
+     * a true or false value.
+     * May be:
+     *  - PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED
+     *  - PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED
+     *  - PM_OPTIONS_FROZEN_STRING_LITERAL_UNSET
+     */
+    int8_t frozen_string_literal;
+
     /** Whether or not we're at the beginning of a command. */
     bool command_start;
 
@@ -737,12 +747,6 @@ struct pm_parser {
      */
     bool semantic_token_seen;
 
-    /**
-     * Whether or not we have found a frozen_string_literal magic comment with
-     * a true value.
-     */
-    bool frozen_string_literal;
-
     /**
      * True if the current regular expression being lexed contains only ASCII
      * characters.

prism/prism.c

@@ -5898,8 +5898,13 @@ pm_string_node_create_unescaped(pm_parser_t *parser, const pm_token_t *opening,
     pm_string_node_t *node = PM_ALLOC_NODE(parser, pm_string_node_t);
     pm_node_flags_t flags = 0;
 
-    if (parser->frozen_string_literal) {
+    switch (parser->frozen_string_literal) {
-        flags = PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN;
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED:
+            flags = PM_STRING_FLAGS_MUTABLE;
+            break;
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED:
+            flags = PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN;
+            break;
     }
 
     *node = (pm_string_node_t) {
@@ -6298,8 +6303,13 @@ pm_symbol_node_to_string_node(pm_parser_t *parser, pm_symbol_node_t *node) {
     pm_string_node_t *new_node = PM_ALLOC_NODE(parser, pm_string_node_t);
     pm_node_flags_t flags = 0;
 
-    if (parser->frozen_string_literal) {
+    switch (parser->frozen_string_literal) {
-        flags = PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN;
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_DISABLED:
+            flags = PM_STRING_FLAGS_MUTABLE;
+            break;
+        case PM_OPTIONS_FROZEN_STRING_LITERAL_ENABLED:
+            flags = PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN;
+            break;
     }
 
     *new_node = (pm_string_node_t) {
@@ -7173,9 +7183,9 @@ parser_lex_magic_comment_encoding(pm_parser_t *parser) {
 static void
 parser_lex_magic_comment_frozen_string_literal_value(pm_parser_t *parser, const uint8_t *start, const uint8_t *end) {
     if ((start + 4 <= end) && pm_strncasecmp(start, (const uint8_t *) "true", 4) == 0) {
-        parser->frozen_string_literal = true;
+        parser->frozen_string_literal = 1;
     } else if ((start + 5 <= end) && pm_strncasecmp(start, (const uint8_t *) "false", 5) == 0) {
-        parser->frozen_string_literal = false;
+        parser->frozen_string_literal = -1;
     }
 }
 
@@ -18912,7 +18922,7 @@ pm_parser_init(pm_parser_t *parser, const uint8_t *source, size_t size, const pm
         .in_keyword_arg = false,
         .current_param_name = 0,
         .semantic_token_seen = false,
-        .frozen_string_literal = false,
+        .frozen_string_literal = 0,
         .current_regular_expression_ascii_only = false
     };
 
@@ -18971,9 +18981,7 @@ pm_parser_init(pm_parser_t *parser, const uint8_t *source, size_t size, const pm
         }
 
         // frozen_string_literal option
-        if (options->frozen_string_literal) {
+        parser->frozen_string_literal = options->frozen_string_literal;
-            parser->frozen_string_literal = true;
-        }
 
         // command_line option
         parser->command_line = options->command_line;