[ruby-core:114279] [Ruby master Bug#19784] String#delete_prefix! problem

Issue #19784 has been updated by byroot (Jean Boussier).


Ok, so as suspected, in the code strings with a broken encoding are very explicitly rejected: https://github.com/ruby/ruby/commit/10082360b9124c3eaabfccf4fe10a3640c40a05e#diff-430d86fdb6c4a558ab0f1b6648bbfae1720e8bde84f026e95a52740014752040R9201

```c
if (is_broken_string(prefix)) return 0;
```

This is from the initial implementation decided in [Feature #12694], and the PR implementing it was https://github.com/ruby/ruby/pull/1632

I don't see much discussion of that behavior in either the ticket or PR, but there are tests for it so it was intentional from the PR author, not necessarily from committers.

I'd be in favor of changing it so that it matches `start_with?` behavior, but I think that needs to be discussed at the developer meeting. It's also unclear whether if accepted it would be as a feature or a bug fix.




----------------------------------------
Bug #19784: String#delete_prefix! problem
https://bugs.ruby-lang.org/issues/19784#change-103974

* Author: inversion (Yura Babak)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
Here is the snipped and the question is in the comments:
``` ruby
fp = 'with_BOM_16.txt'
body = File.read(fp).force_encoding('UTF-8')

p body                            # "\xFF\xFE1\u00001\u0000"
p body.start_with?("\xFF\xFE")    # true

body.delete_prefix!("\xFF\xFE")   # !!! why doesn't work?
p body                            # "\xFF\xFE1\u00001\u0000"
p body.start_with?("\xFF\xFE")    # true

body[0, 2] = ''
p body                            # "1\u00001\u0000"
p body.start_with?("\xFF\xFE")    # false

```
Works same 
on Linux (ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux])
and Windows (ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x64-mingw-ucrt])



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