Linguist: Syntax highlighting splits unicode characters

[eric-wieser]

Preliminary Steps

Please confirm you have...

• [x] reviewed How Linguist Works,
• [x] reviewed the Troubleshooting docs,
• [x] considered implementing an override,
• [x] verified an issue has not already been logged for your issue (linguist issues).

Problem Description

Syntax highlighting is splitting a unicode codepoint into two garbled halves:

https://github.com/leanprover-community/mathlib/blob/d4477fa7f79beea1058f72fc3741c88a1832d9a1/src/group_theory/ore_localization.lean#L29

To eliminate browser interference, you can reproduce the issue with

import requests
w = requests.get("https://github.com/leanprover-community/mathlib/blob/d4477fa7f79beea1058f72fc3741c88a1832d9a1/src/group_theory/ore_localization.lean#L29")
c = w.content
i = c.find(b'has_coe_to_sort')
print(c[i:][:65])
print('⟨'.encode('utf8'))

which prints

b'has_coe_to_sort (ore_set M) := \xe2\x9f<span class="pl-k">\xa8Type</span>*'
b'\xe2\x9f\xa8'

\xe2\x9f\xa8 is the utf8 encoding of ⟨, which has somehow ended up with a span tag right in the middle of it. The hypothesis over at https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/github.20syntax.20highlighting is that some kind of chunking is going on in the file.

An even simpler reproduction is https://gist.github.com/eric-wieser/caef77bc87edc0feae06bd91b0d241f2/756f85e2f06618ef2b7261e7ec3fca0aa0d73e2f.

While I know that the "How Linguist Works" page says that highlighting issues belong in upstream repos, this looks like a highlighting issue with how the grammar files themselves are processed.

URL of the affected repository:

As above, https://github.com/leanprover-community/mathlib/blob/d4477fa7f79beea1058f72fc3741c88a1832d9a1/src/group_theory/ore_localization.lean#L29

Last modified on:

[lildude]

🤔 I think this really _is_ a problem with the grammar and not a problem with the way the grammars are being processed as using the old Lean Textmate grammar we used to use, using the exact same parsing/processing, works as expected:

• Old Textmate grammar: lightshow results

... versus ...

• Current VSCode grammar: lightshow results

We switched grammars in https://github.com/github/linguist/pull/4546.

[eric-wieser]

Thanks for doing that test.

What does the conversion between the various types of grammar file? Could it be a bug that applies only to json grammars (https://github.com/leanprover/vscode-lean/blob/master/syntaxes/lean.json) and not to tmLanguage grammars (https://github.com/leanprover/Lean.tmbundle/blob/master/Syntaxes/Lean.tmLanguage)?

[lildude]

What does the conversion between the various types of grammar file?

There are two but in this case it's an internal library called PrettyLights heavily based off the Textmate grammar processing. It's not open source because of various licensing requirements.

The other is the grammar processor in this repo which produces the JSON files used by PrettyLights in production. These files are attached to each release.

Could it be a bug that applies only to json grammars and not to tmLanguage grammars

It's possible, but I don't think so as ultimately we convert all grammars to JSON.

I've taken the source.lean.json file from v7.5.0 which still uses the TextMate grammar and the same from v7.11.1 which uses the new VSCode grammar, both produced by the grammar compiler in this repo, and placed in this gist and you can see the problem stays with the newer grammar:

• Old grammar lightshow results
• New grammar lightshow results

I've taken a look at the history of the new grammar and I can see it switched from TextMate to JSON in this commit. Using that JSON file produces the same syntax highlighting at the old TextMate grammar as can be see here so I think this confirms this is definitely an issue with the grammar itself.

I'm not very good with writing grammars, but I know @Alhadis is quite the dab hand; he may be able to spot where things are going wrong in the current version of the grammar.

[eric-wieser]

Thanks for the further investigation. I think you're right that the tmLanguage vs json format is irrelevant, but I still think this is a bug in the grammar application, not the grammar itself. This extremely minimal example fails:

{
  "name": "Lean",
  "scopeName": "source.lean",
  "patterns": [
    {
      "name": "storage.type.lean",
      "match": "\\b(Prop|Type|Sort)\\b"
    }
  ]
}

https://gist.github.com/eric-wieser/c5a9efea2581d65fda99ec2816177fde

https://github-lightshow.herokuapp.com/?utf8=%E2%9C%93&scope=from-url&grammar_format=auto&grammar_url=https%3A%2F%2Fgist.github.com%2Feric-wieser%2Fc5a9efea2581d65fda99ec2816177fde%2Fraw%2F58eb1e47a0086a3d5ab405ebeb4cd909ba7dc59e%2Flean-tiny.json&grammar_text=&code_source=from-text&code_url=&code=x+%E2%9F%A8Type%E2%9F%A9

[Alhadis]

Okay, this is weird. leanprover/vscode-lean@deb64b0 appears to be the commit that broke the syntax highlighting, according to the Lightshow results:

• leanprover/vscode-lean@8690a57 (Commit prior to deb64b0)
• leanprover/vscode-lean@deb64b0

However, that doesn't make sense, because the commit in question only added a single keyword to an unrelated pattern:

~diff
- "match": "\b(Prop|Type)\b",
+ "match": "\b(Prop|Type|Sort)\b",
~

I agree with @eric-wieser; I don't believe the grammar file is to blame.

[eric-wieser]

@Alhadis is spot on - I found that if I removed either Prop| or |Sort from the example in https://github.com/github/linguist/issues/5069#issuecomment-716606285, everything works.

[lildude]

🤔 Interesting. Good work peeps. I've reached out to the internal maintainers of the syntax highlighter to get some 👀 on this.