Issue #9278 has been updated by colindkelley (Colin Kelley).


> Perhaps if you show concrete gains in a large code-base like rails the case will be more compelling.

Working on it!  First to see how many tests actually fail from trying to mutate string literals, then to get some general performance numbers.
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Feature #9278: Magic comment "immutable: string" makes "literal".freeze the default for that file
https://bugs.ruby-lang.org/issues/9278#change-43974

Author: colindkelley (Colin Kelley)
Status: Open
Priority: Normal
Assignee: 
Category: core
Target version: 2.1.0


Building on https://bugs.ruby-lang.org/issues/9042, this pull request adds the magic comment # -*- immutable: string -*- that implies .freeze on every string literal in the file. To get a mutable string in a file that starts with the magic comment, use String.new or ''.dup.

Here is a corresponding github pull request:

    https://github.com/ruby/ruby/pull/487

For more details, background, and rationale, please see this blog post:

    http://development.invoca.com/magic-comment-immutable-string-makes-ruby-2-1s-literal-freeze-optimization-the-default/


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