[ruby-core:103153] [Ruby master Bug#17771] String#start_with? should not construct MatchData or set $~

Issue #17771 has been reported by headius (Charles Nutter).

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Bug #17771: String#start_with? should not construct MatchData or set $~
https://bugs.ruby-lang.org/issues/17771

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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I am working on making $~ more thread-safe in JRuby and came across this unexpected behavior:

```ruby
$ rvm ruby-3.0 do ruby -e '"foo".start_with?(/foo/); p $~'
#<MatchData "foo">
```

The `start_with?` method was added 11 years ago in https://bugs.ruby-lang.org/issues/3388 but I do not think the set of $~ was an intended feature. The `start_with?` method could be much faster and more thread-safe if it did not use the frame-local backref slot and did not allocate a MatchData.

Compare with `match?` which was added specifically (without MatchData or backref setting) to provide a fast way to check if a Regexp matches.

I propose that `start_with?` stop constructing MatchData, stop setting backref, and provide only its boolean result in the same way as `match?`.



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