This is still a bit rough and can be optimized (because ArcanistLintResult::getMessages() returns a list<ArcanistLintMessage> roughly sorted by (line, char). I wanted to get some initial feedback before investing too much time in this.

Are "code" and "name" meaningfully different often enough to matter? The linters which don't define different codes are usually external linters, I think, and we don't have nearly as much of a burden to test them (e.g., our test suite does not need to cover all of the behavior of pylint or whatever, just that the binding operates correctly). The first-party linters where we do care about behavior (like XHPAST) have codes per message.

Specifically, could we simplify this by allowing the first field to either be a severity or a code, like error:3 vs TXT17:3? (Although this is a little fuzzy, it seems reasonable to expect that no linter will ever define "error" or "warning" as a code.)

Or could we just replace "severity" with "code" wholesale? That would be a bit of a pain to migrate, but could maybe just be automated.

I think this is generally headed to a good place, but that we might be able to end up with a simpler patch.

Are "code" and "name" meaningfully different often enough to matter?

Probably not, I just wasn't sure how far to take this.

Specifically, could we simplify this by allowing the first field to either be a severity or a code, like error:3 vs TXT17:3? (Although this is a little fuzzy, it seems reasonable to expect that no linter will ever define "error" or "warning" as a code.)

I don't see what that achieves, besides a slightly less verbose syntax? Longer term, I'd imagine that we'd have something more flexible such as severity=error|line=3|char=3|code=TXT1.

I should mention that one of the reasons that I want to pursue this is because I noticed that we aren't parsing the lint message correctly from lessc for ArcanistLesscLinter (although my current diff doesn't check the lint message text).

arc lint -- test.less 
>>> Lint for test.less:


   Error  (LESSCSyntaxError) Unknown lint message!
    Expected ')' got '('

               1 .a {
    >>>        2   something: (12 (13 + 5 -23) + 5);
               3 }

Possibly another plan of attack here would be to throw an exception if we can't parse the linter message.