Show where mutated chilled strings were allocated

[Feature #20205]

The warning now suggests running with --debug-frozen-string-literal:

```
test.rb:3: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

When using --debug-frozen-string-literal, the location where the string
was created is shown:

```
test.rb:3: warning: literal string will be frozen in the future
test.rb:1: the string was created here
```

When resurrecting strings and debug mode is not enabled, the overhead is a simple FL_TEST_RAW.
When mutating chilled strings and deprecation warnings are not enabled,
the overhead is a simple warning category enabled check.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

bootstraptest/test_yjit.rb

@@ -4726,7 +4726,7 @@ def test(klass, args)
 }
 
 # Chilled string setivar trigger warning
-assert_equal 'literal string will be frozen in the future', %q{
+assert_match(/literal string will be frozen in the future/, %q{
   Warning[:deprecated] = true
   $VERBOSE = true
   $warning = "no-warning"
@@ -4754,7 +4754,7 @@ def setivar!(str)
 
   setivar!("chilled") # Emit warning
   $warning
-}
+})
 
 # arity=-2 cfuncs
 assert_equal '["", "1/2", [0, [:ok, 1]]]', %q{

internal/string.h

@@ -14,6 +14,7 @@
 #include "ruby/internal/stdbool.h"     /* for bool */
 #include "ruby/encoding.h"      /* for rb_encoding */
 #include "ruby/ruby.h"          /* for VALUE */
+#include "id.h"                 /* for id_debug_created_info */
 
 #define STR_NOEMBED      FL_USER1
 #define STR_SHARED       FL_USER2 /* = ELTS_SHARED */
@@ -123,8 +124,28 @@ CHILLED_STRING_P(VALUE obj)
 static inline void
 CHILLED_STRING_MUTATED(VALUE str)
 {
+    bool rb_warning_category_enabled_p(rb_warning_category_t category);
+
     FL_UNSET_RAW(str, STR_CHILLED);
-    rb_category_warn(RB_WARN_CATEGORY_DEPRECATED, "literal string will be frozen in the future");
+
+    if (RB_UNLIKELY(rb_warning_category_enabled_p(RB_WARN_CATEGORY_DEPRECATED))) {
+        VALUE debug_info = rb_attr_get(str, id_debug_created_info);
+        if (NIL_P(debug_info)) {
+            rb_category_warn(
+                RB_WARN_CATEGORY_DEPRECATED,
+                "literal string will be frozen in the future "
+                "(run with --debug-frozen-string-literal for more information)");
+        } else {
+            VALUE path = rb_ary_entry(debug_info, 0);
+            VALUE line = rb_ary_entry(debug_info, 1);
+
+            rb_category_warn(
+               RB_WARN_CATEGORY_DEPRECATED,
+               "literal string will be frozen in the future\n%"PRIsVALUE":%"PRIsVALUE": the string was created here",
+               path,
+               line);
+        }
+    }
 }
 
 static inline void

spec/ruby/command_line/fixtures/debug_info.rb

@@ -1,4 +1,3 @@
-# frozen_string_literal: true
 a = 'string'
 b = a
 c = b

spec/ruby/command_line/frozen_strings_spec.rb

@@ -57,9 +57,16 @@
 
 describe "The --debug flag produces" do
   it "debugging info on attempted frozen string modification" do
-    error_str = ruby_exe(fixture(__FILE__, 'debug_info.rb'), options: '--debug',  args: "2>&1")
+    error_str = ruby_exe(fixture(__FILE__, 'debug_info.rb'), options: '--enable-frozen-string-literal --debug',  args: "2>&1")
     error_str.should include("can't modify frozen String")
     error_str.should include("created at")
-    error_str.should include("command_line/fixtures/debug_info.rb:2")
+    error_str.should include("command_line/fixtures/debug_info.rb:1")
+  end
+
+  it "debugging info on mutating chilled string" do
+    error_str = ruby_exe(fixture(__FILE__, 'debug_info.rb'), options: '-w --debug',  args: "2>&1")
+    error_str.should include("literal string will be frozen in the future")
+    error_str.should include("the string was created here")
+    error_str.should include("command_line/fixtures/debug_info.rb:1")
   end
 end

string.c

@@ -1914,6 +1914,12 @@ rb_ec_str_resurrect(struct rb_execution_context_struct *ec, VALUE str, bool chil
     RUBY_DTRACE_CREATE_HOOK(STRING, RSTRING_LEN(str));
     VALUE new_str = ec_str_duplicate(ec, rb_cString, str);
     if (chilled) {
+        if (RB_UNLIKELY(FL_TEST_RAW(str, FL_EXIVAR))) {
+            VALUE debug_info = rb_ivar_get(str, id_debug_created_info);
+            if (!NIL_P(debug_info)) {
+                rb_ivar_set(new_str, id_debug_created_info, debug_info);
+            }
+        }
         STR_CHILL_RAW(new_str);
     }
     return new_str;