Provides end_col_offset for mypy ast nodes (pr proposal)

[devmessias]

I can work and send a PR solving this

Feature
The mypy ast nodes sould have the end_col_offset attribute if this information is available in the original python AST nodes

Pitch

AST nodes from python version 3.8 and up providess the end_col_offset attribute. This information can be used to adress some issues in mypy:

#12288
#4868
#12513

and also in LibCST

Instagram/LibCST#451

The reason is because the end_col_offset allows to associate a section in the source code with the type information.
We can latter on implement something similar to pyre query API using this feature.

I working on in a TypeVisitor that represents a MypyFile node as json string (similar to pyre query result), which maybe in the future can be used by dmypy.
But I need end_col_offest to be exposed, currently, I have a local copy of mypy that exposes the end_col_offset. I can work in a PR to integrate this modification in mypy. I don't think that PR will change so much stuff.

[JelleZijlstra]

Seems useful! We could also use this in mypy itself for giving an output range for errors.

[devmessias]

Hi @JelleZijlstra . Can I start working in a PR to expose that?

[JelleZijlstra]

Sure! We'll have to think about how to use this new data in mypy's output and about what to do on mypy 3.6/3.7 where the feature is not yet available.

[devmessias]

Sure! We'll have to think about how to use this new data in mypy's output and about what to do on mypy 3.6/3.7 where the feature is not yet available.

For mypy 3.6/3.7 this could be an optional int as well the end_line attr
fc79289 (first commit) I need to apply the same modifications in the pyi files.

Regarding the use of this new data, I've made a comment on #12288. I can work in a separate PR for that because I think that changing the report system needs a lot of discussion and also will create much more changes in the codebase than just exposing the end_col_offset

What I doing in my project (pyastrx) using the end_col_offset is creating a report with ^ and > chars like this

The other thing that I have now using the end_col_offset is a mypy-query system that returns to me a json with the types similar to pyre query. Although this is just in my local repo using my modified mypy