[Fix #240] Disable Performance/Casecmp cop

This cop suggests an incorrect fix when code involves non-ASCII
characters. Disabling it by default makes this risky behavior opt-in
rather than opt-out.

changelog/change_disalbe_performance_casecmp_by_default.md

@@ -0,0 +1 @@
+* [#240](https://github.com/rubocop/rubocop-performance/issues/240): Disable `Performance/Casecmp` cop by default. ([@parkerfinch][])

config/default.yml

@@ -55,9 +55,10 @@ Performance/Casecmp:
   Description: >-
              Use `casecmp` rather than `downcase ==`, `upcase ==`, `== downcase`, or `== upcase`..
   Reference: 'https://github.com/fastruby/fast-ruby#stringcasecmp-vs--stringcasecmp-vs-stringdowncase---code'
-  Enabled: true
+  Enabled: false
   Safe: false
   VersionAdded: '0.36'
+  VersionChanged: '<<next>>'
 
 Performance/ChainArrayAllocation:
   Description: >-

lib/rubocop/cop/performance/casecmp.rb

@@ -6,6 +6,12 @@ module Performance
       # Identifies places where a case-insensitive string comparison
       # can better be implemented using `casecmp`.
       #
+      # This cop is disabled by default because `String#casecmp` only works with
+      # ASCII characters. See https://github.com/rubocop/rubocop/issues/9753.
+      #
+      # If you are working only with ASCII characters, then this cop can be
+      # safely enabled.
+      #
       # @safety
       #   This cop is unsafe because `String#casecmp` and `String#casecmp?` behave
       #   differently when using Non-ASCII characters.