Optimize ActiveSupport::SafeBuffer

For normal objects, eagerly defining ivars is good to avoid trashing
inline caches. But for subclasses of core types like String, ivars
are stored in a global hash table, hence initializing and accessing
them is rather costly. That cost is even higher if a ractor has been
spawned, because looking up the table will require synchronizing the
entire VM.

Based on the assumption that the overwhelming majority of `SafeBuffer`
instances are never mutated, hence never end up unsafe, we can optimize
by marking the unsafe status rather than the opposite.

This way we save on eagerly allocating the external buffer in the
global ivar table, save from having to free it when the buffer is
collected, and also save from having to lookup the table when
accessing the ivar because the VM is smart enough to see the object
has the default shape, hence doesn't have any ivar.

```ruby

require "bundler/inline"

gemfile do
  gem "rails", path: "."
  gem "benchmark-ips"
end

require "active_support/all"

Benchmark.ips do |x|
  x.report("String#html_safe") { "test".html_safe }
end
```

Before:

```
ruby 3.5.0dev (2025-07-17T14:01:57Z master a46309d19a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
    String#html_safe   575.727k i/100ms
Calculating -------------------------------------
    String#html_safe      6.421M (± 1.6%) i/s  (155.75 ns/i) -     32.241M in   5.022802s
```

After:

```
ruby 3.5.0dev (2025-07-17T14:01:57Z master a46309d19a) +YJIT +PRISM [arm64-darwin24]
Warming up --------------------------------------
    String#html_safe     1.070M i/100ms
Calculating -------------------------------------
    String#html_safe     12.470M (± 0.8%) i/s   (80.19 ns/i) -     63.140M in   5.063698s
```

Author: byroot

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -67,14 +67,13 @@ def safe_concat(value)
       original_concat(value)
     end
 
-    def initialize(str = "")
-      @html_safe = true
+    def initialize(_str = "")
       super
     end
 
     def initialize_copy(other)
       super
-      @html_safe = other.html_safe?
+      @html_unsafe = true unless other.html_safe?
     end
 
     def concat(value)
@@ -116,7 +115,9 @@ def +(other)
     def *(_)
       new_string = super
       new_safe_buffer = new_string.is_a?(SafeBuffer) ? new_string : SafeBuffer.new(new_string)
-      new_safe_buffer.instance_variable_set(:@html_safe, @html_safe)
+      if @html_unsafe
+        new_safe_buffer.instance_variable_set(:@html_unsafe, true)
+      end
       new_safe_buffer
     end
 
@@ -131,9 +132,9 @@ def %(args)
       self.class.new(super(escaped_args))
     end
 
-    attr_reader :html_safe
-    alias_method :html_safe?, :html_safe
-    remove_method :html_safe
+    def html_safe?
+      @html_unsafe.nil?
+    end
 
     def to_s
       self
@@ -159,7 +160,7 @@ def #{unsafe_method}(*args, &block)       # def capitalize(*args, &block)
           end                                       # end
 
           def #{unsafe_method}!(*args)              # def capitalize!(*args)
-            @html_safe = false                      #   @html_safe = false
+            @html_unsafe = true                     #   @html_unsafe = true
             super                                   #   super
           end                                       # end
         EOT
@@ -180,7 +181,7 @@ def #{unsafe_method}(*args, &block)             # def gsub(*args, &block)
         end                                             # end
 
         def #{unsafe_method}!(*args, &block)            # def gsub!(*args, &block)
-          @html_safe = false                            #   @html_safe = false
+          @html_unsafe = true                           #   @html_unsafe = false
           if block                                      #   if block
             super(*args) { |*params|                    #     super(*args) { |*params|
               set_block_back_references(block, $~)      #       set_block_back_references(block, $~)
@@ -214,7 +215,9 @@ def set_block_back_references(block, match_data)
 
       def string_into_safe_buffer(new_string, is_html_safe)
         new_safe_buffer = new_string.is_a?(SafeBuffer) ? new_string : SafeBuffer.new(new_string)
-        new_safe_buffer.instance_variable_set :@html_safe, is_html_safe
+        unless is_html_safe
+          new_safe_buffer.instance_variable_set :@html_unsafe, true
+        end
         new_safe_buffer
       end
   end