From: "Eregon (Benoit Daloze) via ruby-core" <ruby-core@...>
Date: 2023-03-09T20:57:43+00:00
Subject: [ruby-core:112805] [Ruby master Feature#19452] `Thread::Backtrace::Location` should have column information if possible.

Issue #19452 has been updated by Eregon (Benoit Daloze).


mame (Yusuke Endoh) wrote in #note-6:
> I think there is a confusion of what the word "reparse" means here.

I meant it as `1.` as well. I think copying/keeping the column information is the most reliable way to have this information.
Indeed it should be encoded efficiently to avoid too much of a memory increase.
I think we should measure, e.g. by how much % does it increase the memory of a Ruby program and a Rails app?
I suspect it's not much higher than keeping the node id.
In fact if we assumed e.g. no call expression is longer than 2^16 bytes long, we could fit 2 column offsets (from the start of the call) in 32-bit. Or even 4 column offsets if we need more, and then have some fallback encoding if longer than 256 (likely rare) or so.
I don't know what CRuby uses currently to keep line information though.

> I think it is possible to use four values of first_lineno, etc. as information to connect Thread::Backtrace::Location and AST node.

I think this makes sense, although it's even better if we don't need to reparse to find out column information.
It's probably more efficient to internally have the first_byteoffset and last_byteoffset, that's just two 32-bit integers and we can derive {first,last}_{lineno,column} from it.
It's also how YARP records locations.

----------------------------------------
Feature #19452: `Thread::Backtrace::Location` should have column information if possible.
https://bugs.ruby-lang.org/issues/19452#change-102307

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
I discussed this with @mame and it would be pretty useful if we could also get the column information from exception backtrace location, even if it was slow.

A POC:

```ruby
class Thread::Backtrace::Location
  if defined?(RubyVM::AbstractSyntaxTree)
    def first_column
      RubyVM::AbstractSyntaxTree.of(self, keep_script_lines: true).first_column
    end
  else
    def first_column
      raise NotImplementedError
    end
  end
end
```

It would be good to have a standard interface, so we follow the same interface as https://bugs.ruby-lang.org/issues/19451 and vice versa where it makes sense. I'll investigate it.



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