Support column breakpoints

[brettcannon]

... if it's possible.

https://code.visualstudio.com/updates/v1_39#_improved-ui-for-column-breakpoints

[int19h]

This might be feasible with frame eval, since it operates on bytecode level, where individual opcodes could be mapped to specific expressions on the line. I'd expect it to be a very big item, though; @fabioz, correct me if I'm wrong.

It is technically possible to do with regular tracing on 3.7+ with f_trace_opcodes, but it's likely to be unfeasible for performance reasons.

[fabioz]

@int19h sort of... right now the frame eval mode puts bytecode just to say that the context needs to be traced and then steps out of the way (the tracing mode is still the mode that'll make the break later on)... in this mode the bytecode itself would need to make the break -- the problem is that if it does that, other features may not work properly (if I remember well, the goto line feature requires the new line to be set during a frame.f_trace call). Maybe we could try to renumber the lines for the bytecode internally and have a mapping from the original lines to the new lines, but this can get quite tricky too and would involve lots of bytecode manipulation (which could change between versions)...

It's possible that to have all features working, the option for f_trace_opcodes may be required (regarding performance, I also expect it to be slower as there will be more tracing calls, but I'm not sure how much slower this would be... we could be able to mitigate that doing more in native code -- being 3.7+ only also helps here so we wouldn't need to support older versions).

-- I agree, this is definitely going to be a pretty big item (and probably needs an investigation period to discover the best way to structure this feature).

[int19h]

No traction on this to date; closing until we decide we actually need it.