[ruby-core:91129] [Ruby trunk Bug#15543] rb_str_set_len should clear code range

Issue #15543 has been reported by nirvdrum (Kevin Menard).

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Bug #15543: rb_str_set_len should clear code range
https://bugs.ruby-lang.org/issues/15543

* Author: nirvdrum (Kevin Menard)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
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Calling `rb_str_set_len` on a `String` could alter the code range. I think this hasn't been much of an issue because of pure luck rather than anything that was deliberately designed. If called on a string that already has a `CR_UNKNOWN` code range, there's no problem because the correct values will lazily calculated.

An example invocation that could be problematic, using helper methods for writing C API specs from the Ruby Spec Suite, looks like:

```
@str = "abcdefghij"[0..-1]
@s.rb_str_set_len(@str, 1)
@str.should == "a"

@str.force_encoding(Encoding::UTF_8)
@str.valid_encoding?.should == true
@s.RSTRING_PTR_set(@str, 1, 'B'.ord)
@s.RSTRING_PTR_set(@str, 2, 0x80)
@s.rb_str_set_len(@str, 3)

p @str
@str.valid_encoding?.should == false # This line fails because the cached code range hasn't been updated.

```

The first call to `valid_encoding?` forces the code range to be calculated for the string and it's determined to be `CR_7BIT`. Then `rb_str_set_len` is called, simulating the splitting the bytes of a valid UTF-8 multi-byte character. Here, the code range is still cached as `CR_7BIT`, but the byte sequence is invalid for UTF-8.

I think the fix is a simple matter of clearing the code range in `rb_str_set_len`, but I'd appreciate a review of my analysis.




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