Issue #17930 has been updated by Eregon (Benoit Daloze).


> I'll try to implement it again in prelude.rb, or think of creating a private, MRI-specific default gem.

Why not exposing column information on Thread::Backtrace::Location?
That would be a proper public API.
Given that Thread::Backtrace::Location already exposes code locations (path and line currently), it feels natural that it can provide column information too.

Is the problem that does not show the proper place for the call and underlines the receiver too?

This feature should really be available on all Rubies, not just CRuby, so I think we need to expose proper APIs to make it work.

Another possibility might be for NoMethodError#message to already include the underlining (and be multiple lines then),
or to add a method on `NoMethodError` that would returns the underlying String or column information.
Of course that would be less general and be useful for fewer use cases.

----------------------------------------
Feature #17930: Add column information into error backtrace
https://bugs.ruby-lang.org/issues/17930#change-92406

* Author: mame (Yusuke Endoh)
* Status: Assigned
* Priority: Normal
* Assignee: mame (Yusuke Endoh)
----------------------------------------
Consider the following code and error.

```
data["data"].first["field"] #=> undefined method `[]` for nil:NilClass
```

There are two possibilities; the variable `data` is nil, or the return value of `first` is nil. Unfortunately, the error message is less informative to say which.

This proposal allows to help identifying which method call failed.

```
$ ruby -r ./sample/no_method_error_ext.rb err1.rb
err1.rb:2:in `<main>': undefined method `[]' for nil:NilClass (NoMethodError)

data["data"].first["field"]
                  ^^^^^^^^^
```

## Proposal

I'd like to propose a feature to get column information from each `Thread::BacktraceLocation`. Maybe it is good to provide the following four methods:

* `Thread::BacktraceLocation#first_lineno`
* `Thread::BacktraceLocation#first_column`
* `Thread::BacktraceLocation#last_lineno`
* `Thread::BacktraceLocation#last_column`

These names came from `RubyVM::AbstraceSyntaxTree::Node`'s methods.

## Implementation

Here is a proof-of-concept implementation: https://github.com/ruby/ruby/pull/4540

See https://github.com/ruby/ruby/pull/4540/commits/6ff516f4985826e9f9c5606638001c3c420f7cad for an example usage.
(Note that, currently, you need to build ruby with `./configure cflags=-DEXPERIMENTAL_ISEQ_NODE_ID` to enable the feature.)

To put it simply, this PR provides only a raw API, `Thread::BacktraceLocation#node_id`. To get actual column information, you need to manually identify `RubyVM::AbstractSyntaxTree::Node` that corresponds to `Thread::BacktraceLocation#node_id`.
But it would be arguable to expose "node_id", so I will wrap it as the above four methods if this is accepted.

Credit: the original implementation was done by @yui-knk.

## Drawback

To use this feature, we need to enable `-DEXPERIMENTAL_ISEQ_NODE_ID` to add "node_id" information (a subtree ID of the original abstract syntax tree) into each byte code instruction. If we provide this feature, the option should be enabled by default. However, the option increases memory consumption.

I performed a simple experiment: I created a scaffold app by `rails new`, and measured the memory usage after `rails s`. The result was 97 MB without `-DEXPERIMENTAL_ISEQ_NODE_ID`, and 100 MB with the option enabled.

In my opinion, it is not so large, but requiring more gems will increase the difference. I will appriciate it if anyone could provide the actual memory increase in a more practical Rails app.

Do you think this feature deserves the memory increase?

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