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`String#+@` is 2-3 times faster than `String#dup` because it can directly go through `rb_str_dup` instead of using the generic much slower `rb_obj_dup`. This fact led to the existance of the ugly `Performance/UnfreezeString` rubocop performance rule that encourage users to rewrite the much more readable and convenient `"foo".dup` into the ugly `(+"foo")`. Let's make that rubocop rule useless. ``` compare-ruby: ruby 3.3.0dev (2023-11-20T02:02:55Z master 701b065) [arm64-darwin22] last_commit=[ruby/prism] feat: add encoding for IBM865 (ruby/prism#1884) built-ruby: ruby 3.3.0dev (2023-11-20T12:51:45Z faster-str-lit-dup 6b745bb) [arm64-darwin22] warming up.. | |compare-ruby|built-ruby| |:------|-----------:|---------:| |uplus | 16.312M| 16.332M| | | -| 1.00x| |dup | 5.912M| 16.329M| | | -| 2.76x| ```
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@byroot This is really great 💯 |
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Starting with Ruby 3.4, there is a gradual plan to freeze strings: https://bugs.ruby-lang.org/issues/20205#note-35 Using Ruby 3.4dev with code prior to this PR results in the following warning: ```console $ cd path/to/ruby/lrama $ ruby -v ruby 3.4.0dev (2024-05-27T01:45:38Z master 5853a38043) [x86_64-darwin23] $ RUBYOPT=-W:deprecated bundle exec rake (snip) /Users/koic/src/github.com/ruby/lrama/lib/lrama/output.rb:74: warning: literal string will be frozen in the future ``` This warning suggests that applying the frozen string magic comment will lead to `FrozenError`. So, this indicates that the future behavior of (default frozen) string will produce `FrozenError`. This PR applies the frozen string magic comment to detect `FrozenError` and uses `String#dup` to prevent it: https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72#ruby-34 This ensures compatibility for the future. NOTE: From Ruby 3.3+, there is no speed difference between `String#+@` and `String#dup`. For readability, `String#dup` has been chosen: ruby/ruby#8952
koic
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Starting with Ruby 3.4, there is a gradual plan to freeze strings: https://bugs.ruby-lang.org/issues/20205#note-35 Using Ruby 3.4dev with code prior to this PR results in the following warning: ```console $ cd path/to/ruby/lrama $ ruby -v ruby 3.4.0dev (2024-05-27T01:45:38Z master 5853a38043) [x86_64-darwin23] $ RUBYOPT=-W:deprecated bundle exec rake (snip) /Users/koic/src/github.com/ruby/lrama/lib/lrama/output.rb:74: warning: literal string will be frozen in the future ``` This warning suggests that applying the frozen string magic comment will lead to `FrozenError`. So, this indicates that the future behavior of (default frozen) string will produce `FrozenError`. This PR applies the frozen string magic comment to detect `FrozenError` and uses `String#dup` to prevent it: https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72#ruby-34 This ensures compatibility for the future. NOTE: From Ruby 3.3+, there is no speed difference between `String#+@` and `String#dup`. For readability, `String#dup` has been chosen: ruby/ruby#8952
koic
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Starting with Ruby 3.4, there is a gradual plan to freeze strings: https://bugs.ruby-lang.org/issues/20205#note-35 Using Ruby 3.4dev with code prior to this PR results in the following warning: ```console $ cd path/to/ruby/lrama $ ruby -v ruby 3.4.0dev (2024-05-27T01:45:38Z master 5853a38043) [x86_64-darwin23] $ RUBYOPT=-W:deprecated bundle exec rake (snip) /Users/koic/src/github.com/ruby/lrama/lib/lrama/output.rb:74: warning: literal string will be frozen in the future ``` This warning suggests that applying the frozen string magic comment will lead to `FrozenError`. So, this indicates that the future behavior of (default frozen) string will produce `FrozenError`. This PR applies the frozen string magic comment to detect `FrozenError` and uses `String#dup` to prevent it: https://gist.github.com/fxn/bf4eed2505c76f4fca03ab48c43adc72#ruby-34 This ensures compatibility for the future. NOTE: From Ruby 3.3+, there is no speed difference between `String#+@` and `String#dup`. For readability, `String#dup` has been chosen: ruby/ruby#8952
toshimaru
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Jan 8, 2026
This change removes the Performance/UnfreezeString cop from the RuboCop configuration to align with the upstream change in Rails PR #50113. After the change in ruby/ruby#8952, String#dup also having the same performance of String#+@. This makes the cop obsolete.
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String#+@is 2-3 times faster thanString#dupbecause it can directly go throughrb_str_dupinstead of using the generic much slowerrb_obj_dup.This fact led to the existence of the ugly
Performance/UnfreezeStringrubocop performance rule that encourage users to rewrite the much more readable and convenient"foo".dupinto the ugly(+"foo").Let's make that rubocop rule useless.