Fix Performance/Squeeze cop error on frozen AST string node value

Since sometimes AST string nodes might have their values frozen,
we should call `dup` explicitly to make sure `to_string_literal`
(which does not work with frozen strings) does not raise. Maybe
we should also apply a patch on RuboCop's core side to fix the
`to_string_literal` itself (it now uses `force_encoding` on its argument).

See [1] and [2] for details.

[1] ruby/prism#3309
[2] rubocop/rubocop-ast#342

changelog/fix_performance_squeeze_cop_error_on_frozen_ast_string_node_value.md

@@ -0,0 +1 @@
+* [#480](https://github.com/rubocop/rubocop-performance/pull/480): Fix `Performance/Squeeze` cop error on frozen AST string node value. ([@viralpraxis][])

lib/rubocop/cop/performance/squeeze.rb

@@ -46,7 +46,7 @@ def on_send(node)
             message = format(MSG, current: bad_method, prefer: good_method)
 
             add_offense(node.loc.selector, message: message) do |corrector|
-              string_literal = to_string_literal(replace_str)
+              string_literal = to_string_literal(replace_str.dup)
               new_code = "#{receiver.source}#{node.loc.dot.source}#{good_method}(#{string_literal})"
 
               corrector.replace(node, new_code)

spec/rubocop/cop/performance/squeeze_spec.rb

@@ -51,4 +51,15 @@
       str.gsub(/a+/, 'b')
     RUBY
   end
+
+  it 'registers an offense when AST string literal might be frozen' do
+    expect_offense(<<~'RUBY')
+      str.gsub(/\n+/, ?\n)
+          ^^^^ Use `squeeze` instead of `gsub`.
+    RUBY
+
+    expect_correction(<<~'RUBY')
+      str.squeeze("\n")
+    RUBY
+  end
 end