json byte position doesn't start at top of file

[sophiajt]

From the IRC conversation:

1:17 AM jntrnr: re json compiler output, i totally forgot to ask about byte_{start,end}! it's either not mapping to the byte indices of the source file (is it maybe counting them after after macro expansion or something?) or i'm stupid ;) do you know what's up this that? it'd make rustfix (and editor plugin i imagine) a bit easier to implement if we could use byte indices
4:38 AM killercup: you have to look at how they're created
4:38 AM src/libsyntax/json.rs: byte_start: span.lo.0,
4:39 AM see, that's wrong
4:39 AM that's the byte start within the crate
4:39 AM without the CodeMap you can't know where the file starts
4:39 AM it should subtract the file start
4:40 AM assuming the start and end are in the same file

The fix should be to alter the json output so that lo/hi of the span's byte position start at the top of the file rather than at a codemap offset.

[Mark-Simulacrum]

As far as I can tell, this hasn't happened. @jonathandturner This feels like it may be an E-easy bug. Could you provide some more instructions, perhaps?

[sophiajt]

@Mark-Simulacrum - it may be, but tbh I totally forgot what I was thinking when I wrote this 😅 It's been a while since I worked on any error message code

[Mark-Simulacrum]

Based on the IRC log, I think the problem is that these lines need to be something more than they are, going through some sort of CodeMap or maybe CodeMapper? I'm not familiar with this code, though, so I don't know what exactly needs to happen here.

Would you perhaps know who we could cc for some instructions here?

[sophiajt]

Looks like the compiler team and @killercup sitting down together and figuring it out (if they haven't already)

[killercup]

Not sure if anything changed since last year, but I just wrote this small example program that still crashes when the message is in another file: https://github.com/killercup/rustfix/blob/do-byte-indices-work/examples/do-byte-indices-work.rs

You can play around with it by adding unused warnings in various places.