Symbol#to_s now returns a frozen string.

[Feature #22137]

It has returned a chilled string, hence emitting deprecation
warnings for two versions.

NEWS.md

@@ -58,6 +58,10 @@ Note: We're only listing outstanding class updates.
     * A deprecated behavior, `Set#to_set`, `Range#to_set`, and
       `Enumerable#to_set` accepting arguments, was removed.  [[Feature #21390]]
 
+* Symbol
+
+    * `Symbol#to_s` now returns a frozen string. [[Feature #22137]]
+
 ## Stdlib updates
 
 ### The following bundled gems are added.
@@ -211,6 +215,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 [Feature #21861]: https://bugs.ruby-lang.org/issues/21861
 [Feature #21932]: https://bugs.ruby-lang.org/issues/21932
 [Feature #21981]: https://bugs.ruby-lang.org/issues/21981
+[Feature #22137]: https://bugs.ruby-lang.org/issues/22137
 [Feature #22139]: https://bugs.ruby-lang.org/issues/22139
 [PR #17201]: https://github.com/ruby/ruby/pull/17201
 [RubyGems-v4.0.4]: https://github.com/rubygems/rubygems/releases/tag/v4.0.4

error.c

@@ -4271,15 +4271,6 @@ rb_warn_unchilled_literal(VALUE obj)
     }
 }
 
-void
-rb_warn_unchilled_symbol_to_s(VALUE obj)
-{
-    rb_category_warn(
-        RB_WARN_CATEGORY_DEPRECATED,
-        "string returned by :%s.to_s will be frozen in the future", RSTRING_PTR(obj)
-    );
-}
-
 #undef rb_check_frozen
 void
 rb_check_frozen(VALUE obj)

ext/-test-/string/capacity.c

@@ -11,8 +11,15 @@ bug_str_capacity(VALUE klass, VALUE str)
     return LONG2FIX(rb_str_capacity(str));
 }
 
+static VALUE
+bug_str_new_shared(VALUE klass, VALUE str)
+{
+    return rb_str_new_shared(str);
+}
+
 void
 Init_string_capacity(VALUE klass)
 {
     rb_define_singleton_method(klass, "capacity", bug_str_capacity, 1);
+    rb_define_singleton_method(klass, "rb_str_new_shared", bug_str_new_shared, 1);
 }

include/ruby/internal/intern/error.h

@@ -258,7 +258,7 @@ rb_check_frozen_inline(VALUE obj)
 
     /* ref: internal CHILLED_STRING_P()
        This is an implementation detail subject to change. */
-    if (RB_UNLIKELY(RB_TYPE_P(obj, T_STRING) && FL_TEST_RAW(obj, RUBY_FL_USER2 | RUBY_FL_USER3))) { // STR_CHILLED
+    if (RB_UNLIKELY(RB_TYPE_P(obj, T_STRING) && FL_TEST_RAW(obj, RUBY_FL_USER2))) { // STR_CHILLED
         rb_str_modify(obj);
     }
 }

internal/string.h

@@ -18,9 +18,7 @@
 
 #define STR_SHARED                  FL_USER0 /* = ELTS_SHARED */
 #define STR_NOEMBED                 FL_USER1
-#define STR_CHILLED                 (FL_USER2 | FL_USER3)
-#define STR_CHILLED_LITERAL         FL_USER2
-#define STR_CHILLED_SYMBOL_TO_S     FL_USER3
+#define STR_CHILLED         FL_USER2
 #define STR_FAKESTR                 FL_USER19
 
 enum ruby_rstring_private_flags {
@@ -103,7 +101,6 @@ VALUE rb_str_casecmp(VALUE str1, VALUE str2);
 
 /* error.c */
 void rb_warn_unchilled_literal(VALUE str);
-void rb_warn_unchilled_symbol_to_s(VALUE str);
 
 static inline bool STR_EMBED_P(VALUE str);
 static inline bool STR_SHARED_P(VALUE str);
@@ -173,17 +170,9 @@ CHILLED_STRING_P(VALUE obj)
 static inline void
 CHILLED_STRING_MUTATED(VALUE str)
 {
-    VALUE chilled_reason = RB_FL_TEST_RAW(str, STR_CHILLED);
-    FL_UNSET_RAW(str, STR_CHILLED);
-    switch (chilled_reason) {
-      case STR_CHILLED_SYMBOL_TO_S:
-        rb_warn_unchilled_symbol_to_s(str);
-        break;
-      case STR_CHILLED_LITERAL:
+    if (RB_FL_TEST_RAW(str, STR_CHILLED)) {
+        FL_UNSET_RAW(str, STR_CHILLED);
         rb_warn_unchilled_literal(str);
-        break;
-      default:
-        rb_bug("RString was chilled for multiple reasons");
     }
 }
 

spec/ruby/core/string/chilled_string_spec.rb

@@ -78,7 +78,9 @@ describe "chilled String" do
         end
       end
     end
+  end
 
+  guard -> { ruby_version_is "3.4"..."4.1" and !:test.to_s.equal?(:test.to_s) } do
     describe "chilled strings returned by Symbol#to_s" do
 
       describe "#frozen?" do

spec/ruby/core/symbol/to_s_spec.rb

@@ -16,7 +16,7 @@ describe "Symbol#to_s" do
     symbol.to_s.encoding.should == Encoding::US_ASCII
   end
 
-  ruby_version_is "3.4" do
+  ruby_version_is "3.4"..."4.1" do
     it "warns about mutating returned string" do
       -> { :bad!.to_s.upcase! }.should complain(/warning: string returned by :bad!.to_s will be frozen in the future/)
     end
@@ -29,4 +29,10 @@ describe "Symbol#to_s" do
       Warning[:deprecated] = deprecated
     end
   end
+
+  ruby_version_is "4.1" do
+    it "returns a frozen string" do
+      :test.to_s.frozen?.should == true
+    end
+  end
 end

string.c

@@ -90,12 +90,9 @@ VALUE rb_cSymbol;
  *            The string is not embedded. When a string is embedded, the contents
  *            follow the header. When a string is not embedded, the contents is
  *            on a separately allocated buffer.
- * 2:     STR_CHILLED_LITERAL (will be frozen in a future version)
+ * 2:     STR_CHILLED (will be frozen in a future version)
  *            The string was allocated as a literal in a file without an explicit `frozen_string_literal` comment.
  *            It emits a deprecation warning when mutated for the first time.
- * 3:     STR_CHILLED_SYMBOL_TO_S (will be frozen in a future version)
- *            The string was allocated by the `Symbol#to_s` method.
- *            It emits a deprecation warning when mutated for the first time.
  * 4:     STR_PRECOMPUTED_HASH
  *            The string is embedded and has its precomputed hashcode stored
  *            after the terminator.
@@ -2035,7 +2032,7 @@ rb_ec_str_resurrect(struct rb_execution_context_struct *ec, VALUE str, bool chil
         str_duplicate_setup_heap(klass, str, new_str);
     }
     if (chilled) {
-        FL_SET_RAW(new_str, STR_CHILLED_LITERAL);
+        FL_SET_RAW(new_str, STR_CHILLED);
     }
     return new_str;
 }
@@ -2046,7 +2043,7 @@ rb_str_with_debug_created_info(VALUE str, VALUE path, int line)
     VALUE debug_info = rb_ary_new_from_args(2, path, INT2FIX(line));
     if (OBJ_FROZEN_RAW(str)) str = rb_str_dup(str);
     rb_ivar_set(str, id_debug_created_info, rb_ary_freeze(debug_info));
-    FL_SET_RAW(str, STR_CHILLED_LITERAL);
+    FL_SET_RAW(str, STR_CHILLED);
     return rb_str_freeze(str);
 }
 
@@ -12441,9 +12438,7 @@ sym_inspect(VALUE sym)
 VALUE
 rb_sym_to_s(VALUE sym)
 {
-    VALUE str = str_new_shared(rb_cString, rb_sym2str(sym));
-    FL_SET_RAW(str, STR_CHILLED_SYMBOL_TO_S);
-    return str;
+    return rb_sym2str(sym);
 }
 
 VALUE

symbol.rb

@@ -2,32 +2,20 @@ class Symbol
   # call-seq:
   #   to_s -> string
   #
-  # Returns a string representation of +self+ (not including the leading colon):
-  #
-  #   :foo.to_s # => "foo"
-  #
-  # Related: Symbol#inspect, Symbol#name.
-  def to_s
-    Primitive.attr! :leaf
-    Primitive.cexpr! 'rb_sym_to_s(self)'
-  end
-
-  alias id2name to_s
-
-  # call-seq:
-  #   name -> string
-  #
   # Returns a frozen string representation of +self+ (not including the leading colon):
   #
-  #   :foo.name         # => "foo"
+  #   :foo.to_s # => "foo"
   #   :foo.name.frozen? # => true
   #
-  # Related: Symbol#to_s, Symbol#inspect.
-  def name
+  # Related: Symbol#inspect
+  def to_s
     Primitive.attr! :leaf
     Primitive.cexpr! 'rb_sym2str(self)'
   end
 
+  alias id2name to_s
+  alias name to_s
+
   # call-seq:
   #   empty? -> true or false
   #

test/-ext-/string/test_capacity.rb

@@ -12,8 +12,8 @@ class Test_StringCapacity < Test::Unit::TestCase
   end
 
   def test_capacity_shared
-    sym = ("a" * pool_slot_size(0)).to_sym
-    assert_equal 0, capa(sym.to_s)
+    source = ("a" * pool_slot_size(0)).freeze
+    assert_equal 0, capa(Bug::String.rb_str_new_shared(source))
   end
 
   def test_capacity_normal

tool/timeline/lib/converter_defs.rb

@@ -91,8 +91,7 @@ module RubyTimelineTool
   StringFlags = FlagsConverter.new({
     STR_SHARED:               FL_USER_N(0),
     RSTRING_NOEMBED:          FL_USER_N(1),
-    STR_CHILLED_LITERAL:      FL_USER_N(2),
-    STR_CHILLED_SYMBOL_TO_S:  FL_USER_N(3),
+    STR_CHILLED:      FL_USER_N(2),
     RSTRING_FSTR:             FL_USER_N(17),
   })
 

variable.c

@@ -2008,7 +2008,7 @@ rb_obj_freeze_inline(VALUE x)
     if (RB_FL_ABLE(x)) {
         RB_FL_SET_RAW(x, RUBY_FL_FREEZE);
         if (TYPE(x) == T_STRING) {
-            RB_FL_UNSET_RAW(x, FL_USER2 | FL_USER3); // STR_CHILLED
+            RB_FL_UNSET_RAW(x, FL_USER2); // STR_CHILLED
         }
 
         // rb_obj_freeze_inline(String)

vm_insnhelper.c

@@ -6188,9 +6188,6 @@ vm_objtostring(struct rb_control_frame_struct *reg_cfp, VALUE recv, CALL_DATA cd
     switch (type) {
       case T_SYMBOL:
         if (check_method_basic_definition(cme)) {
-            // rb_sym_to_s() allocates a mutable string, but since we are only
-            // going to use this string for interpolation, it's fine to use the
-            // frozen string.
             return rb_sym2str(recv);
         }
         break;

yjit/src/cruby_bindings.inc.rs

@@ -679,7 +679,7 @@ pub struct rb_call_data {
     pub ci: *const rb_callinfo,
     pub cc: *const rb_callcache,
 }
-pub const RSTRING_CHILLED: ruby_rstring_private_flags = 49152;
+pub const RSTRING_CHILLED: ruby_rstring_private_flags = 16384;
 pub type ruby_rstring_private_flags = u32;
 pub const RHASH_PASS_AS_KEYWORDS: ruby_rhash_flags = 8192;
 pub const RHASH_PROC_DEFAULT: ruby_rhash_flags = 16384;

zjit/src/cruby_bindings.inc.rs

@@ -1577,7 +1577,7 @@ pub struct rb_call_data {
     pub ci: *const rb_callinfo,
     pub cc: *const rb_callcache,
 }
-pub const RSTRING_CHILLED: ruby_rstring_private_flags = 49152;
+pub const RSTRING_CHILLED: ruby_rstring_private_flags = 16384;
 pub type ruby_rstring_private_flags = u32;
 pub const RHASH_PASS_AS_KEYWORDS: ruby_rhash_flags = 8192;
 pub const RHASH_PROC_DEFAULT: ruby_rhash_flags = 16384;

zjit/src/hir/tests.rs

@@ -5230,7 +5230,7 @@ pub(crate) mod hir_build_tests {
         let iseq = crate::cruby::with_rubyvm(|| get_instance_method_iseq("Symbol", "name"));
         let function = iseq_to_hir(iseq).unwrap();
         assert_snapshot!(hir_string_function(&function), @"
-        fn name@<internal:symbol>:
+        fn to_s@<internal:symbol>:
         bb1():
           EntryPoint interpreter
           v1:BasicObject = LoadSelf