Interpolated string literals are no longer frozen

Since Ruby 3.0 or higher interpolated strings are not frozen
automatically anymore.

Therefore, two changes must happen to how we treat interpolated literals

* Style/MutableConstant should enforce freezing them
* Style/RedundantFreeze should not consider it redundant to freeze them

Strongly inspired by #8743

changelog/change_interpolated_string_literals_are_no.md

@@ -0,0 +1 @@
+* [#10006](https://github.com/rubocop/rubocop/pull/10006): Interpolated string literals are no longer frozen since Ruby 3.0. ([@splattael][])

lib/rubocop/cop/mixin/frozen_string_literal.rb

@@ -8,9 +8,10 @@ module FrozenStringLiteral
 
       FROZEN_STRING_LITERAL = '# frozen_string_literal:'
       FROZEN_STRING_LITERAL_ENABLED = '# frozen_string_literal: true'
-      FROZEN_STRING_LITERAL_TYPES = %i[str dstr].freeze
+      FROZEN_STRING_LITERAL_TYPES_RUBY27 = %i[str dstr].freeze
+      FROZEN_STRING_LITERAL_TYPES_RUBY30 = %i[str].freeze
 
-      private_constant :FROZEN_STRING_LITERAL_TYPES
+      private_constant :FROZEN_STRING_LITERAL_TYPES_RUBY27, :FROZEN_STRING_LITERAL_TYPES_RUBY30
 
       def frozen_string_literal_comment_exists?
         leading_comment_lines.any? { |line| MagicComment.parse(line).valid_literal_value? }
@@ -19,7 +20,13 @@ def frozen_string_literal_comment_exists?
       private
 
       def frozen_string_literal?(node)
-        FROZEN_STRING_LITERAL_TYPES.include?(node.type) && frozen_string_literals_enabled?
+        literal_types = if target_ruby_version >= 3.0
+                          FROZEN_STRING_LITERAL_TYPES_RUBY30
+                        else
+                          FROZEN_STRING_LITERAL_TYPES_RUBY27
+                        end
+
+        literal_types.include?(node.type) && frozen_string_literals_enabled?
       end
 
       def frozen_string_literals_enabled?

lib/rubocop/cop/style/mutable_constant.rb

@@ -21,6 +21,9 @@ module Style
       #
       # NOTE: Regexp and Range literals are frozen objects since Ruby 3.0.
       #
+      # NOTE: From Ruby 3.0, this cop allows explicit freezing of interpolated
+      # string literals when `# frozen-string-literal: true` is used.
+      #
       # @example EnforcedStyle: literals (default)
       #   # bad
       #   CONST = [1, 2, 3]

lib/rubocop/cop/style/redundant_freeze.rb

@@ -7,6 +7,9 @@ module Style
       #
       # NOTE: Regexp and Range literals are frozen objects since Ruby 3.0.
       #
+      # NOTE: From Ruby 3.0, this cop allows explicit freezing of interpolated
+      # string literals when `# frozen-string-literal: true` is used.
+      #
       # @example
       #   # bad
       #   CONST = 1.freeze

spec/rubocop/cop/style/mutable_constant_spec.rb

@@ -106,20 +106,40 @@
       end
     end
 
-    context 'when the frozen string literal comment is missing' do
-      it_behaves_like 'mutable objects', '"#{a}"'
-    end
+    context 'Ruby 3.0 or higher', :ruby30 do
+      context 'when the frozen string literal comment is missing' do
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
 
-    context 'when the frozen string literal comment is true' do
-      let(:prefix) { '# frozen_string_literal: true' }
+      context 'when the frozen string literal comment is true' do
+        let(:prefix) { '# frozen_string_literal: true' }
 
-      it_behaves_like 'immutable objects', '"#{a}"'
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is false' do
+        let(:prefix) { '# frozen_string_literal: false' }
+
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
     end
 
-    context 'when the frozen string literal comment is false' do
-      let(:prefix) { '# frozen_string_literal: false' }
+    context 'Ruby 2.7 or lower', :ruby27 do
+      context 'when the frozen string literal comment is missing' do
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
 
-      it_behaves_like 'mutable objects', '"#{a}"'
+      context 'when the frozen string literal comment is true' do
+        let(:prefix) { '# frozen_string_literal: true' }
+
+        it_behaves_like 'immutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is false' do
+        let(:prefix) { '# frozen_string_literal: false' }
+
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
     end
   end
 

spec/rubocop/cop/style/redundant_freeze_spec.rb

@@ -75,20 +75,40 @@
       end
     end
 
-    context 'when the frozen string literal comment is missing' do
-      it_behaves_like 'mutable objects', '"#{a}"'
-    end
+    context 'Ruby 3.0 or higher', :ruby30 do
+      context 'when the frozen string literal comment is missing' do
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is true' do
+        let(:prefix) { '# frozen_string_literal: true' }
 
-    context 'when the frozen string literal comment is true' do
-      let(:prefix) { '# frozen_string_literal: true' }
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is false' do
+        let(:prefix) { '# frozen_string_literal: false' }
 
-      it_behaves_like 'immutable objects', '"#{a}"'
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
     end
 
-    context 'when the frozen string literal comment is false' do
-      let(:prefix) { '# frozen_string_literal: false' }
+    context 'Ruby 2.7 or lower', :ruby27 do
+      context 'when the frozen string literal comment is missing' do
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is true' do
+        let(:prefix) { '# frozen_string_literal: true' }
+
+        it_behaves_like 'immutable objects', '"#{a}"'
+      end
+
+      context 'when the frozen string literal comment is false' do
+        let(:prefix) { '# frozen_string_literal: false' }
 
-      it_behaves_like 'mutable objects', '"#{a}"'
+        it_behaves_like 'mutable objects', '"#{a}"'
+      end
     end
 
     describe 'Regexp and Range literals' do