[ruby-core:100987] [Ruby master Feature#17259] Kernel#warn should ignore <internal: entries

Issue #17259 has been updated by Eregon (Benoit Daloze).


PR to RubyGems so RubyGems won't need to override Kernel#warn anymore: https://github.com/rubygems/rubygems/pull/4075

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Feature #17259: Kernel#warn should ignore <internal: entries
https://bugs.ruby-lang.org/issues/17259#change-88653

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Target version: 3.0
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`Kernel#warn` currently does not skip `<internal:` entries from core library methods defined in Ruby.
This can cause rather unhelpful locations to be used for warnings.

For instance:
```
$ ruby -v --disable=gems -e 'def deprecated; warn "use X instead", uplevel: 1; end; tap(&:deprecated)'
ruby 3.0.0preview1 (2020-09-25 master 0096d2b895) [x86_64-linux]
<internal:kernel>:90: warning: use X instead
# expected: "-e:1: warning: use X instead"
```

Note that RubyGems overrides Kernel#warn since https://github.com/rubygems/rubygems/pull/2442 and https://github.com/rubygems/rubygems/blob/c1bafab1d84e0aad06e377e9db4b74cccab4b43a/lib/rubygems/core_ext/kernel_warn.rb#L42,
so `--disable-gems` is needed to observe this behavior.
I think it is very suboptimal that RubyGems needs to monkey-patch Kernel#warn to remove RubyGems' `require` from `Kernel#warn` location.
That is both fragile (as we've seen from various incompatible behavior and bugs in that monkey-patch) and inefficient (walking the stack multiple times).

So I would suggest to actually skip all backtraces entries starting with `<internal:` for `Kernel#warn(message, uplevel:)`.
BTW this is already what [TruffleRuby does](https://github.com/oracle/truffleruby/blob/c3bcccfd8db7d460e23f0371e7ceaf5fdb71275c/src/main/ruby/truffleruby/core/kernel.rb#L656).

As a bonus, by filtering out `<internal:`, RubyGems could define its `require` in an `eval(code, nil, '<internal:rubygems-require>', line)` and it would automatically be skipped, without needing to monkey-patch Kernel#warn at all!



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