feat: make no-misleading-character-class report more granular errors

[fasttime]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[X] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What rule do you want to change?

no-misleading-character-class

What change do you want to make (place an "X" next to just one item)?

[X] Generate more warnings
[ ] Generate fewer warnings
[ ] Implement autofix
[ ] Implement suggestions

How will the change be implemented (place an "X" next to just one item)?

[ ] A new option
[X] A new default behavior
[ ] Other

Please provide some example code that this change will affect:

/* eslint no-misleading-character-class: "error" */

RegExp("[ \\ufe0f][ \\ufe0f]");

RegExp(`"[👍]"`);

What does the rule currently do for this code?

Reports one problem per RegExp expression (Playground link).

What will the rule do after it's changed?

A granular problem will be reported for each problematic character class. For the first RegExp, two problems will be reported.

What changes did you make? (Give an overview)

This is an attempt to extend granular error reporting for the no-misleading-character-class rule to string and template literal patterns whose source code doesn't match the string value. This includes literals with escape sequences, line continuations, and unescaped CRLF line breaks in templates.

This change was suggested during the implementation of granular errors, but it was postponed to keep the work manageable: #17515 (comment).

Is there anything you'd like reviewers to focus on?

• The new implementation only reports more problems on code that already has one problem with the current implementation, so I think it doesn't qualify as a breaking change, but I'd like others to confirm.
• I'm using a self-written dependency to track characters back to their positions in source code. While I'm happy to continue maintaining that project on my own, I think it would be better to extract the core of the code to ESLint for licensing reasons. Alternatively, the repo and npm package could be transferred to the eslint org or to eslint-community.
• The new implementation could be further extended to templates with interpolated expression like new RegExp(`[ \\ufe0f]${0}`), but I haven't explored that yet.

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[nzakas]

I think it's fine to produce more errors here and not mark it as breaking. It seems more like a bug fix to me.

[nzakas]

Finding this a bit difficult to understand, maybe this?

Suggested change

	if (!(node.type === "TemplateLiteral" && !node.expressions.length || node.type === "Literal")) {
	const isTemplateWithoutExpressions = node.type === "TemplateLiteral" && node.expressions.length === 0;
	if (!isTemplateWithoutExpressions && node.type !== "Literal")) {

[JoshuaKGoldberg]

Funny, I was going to suggest if (node.type !== "TemplateLiteral" || (node.type === "Literal" && node.expressions.length)) {. No preference between the two from me. 🙂

[fasttime]

Thanks! I realized that that check was always falsy, so I just removed it. It's because strings and templates without expressions always produce granular reports now, and so the default case isn't hit any more. Somehow I missed that while testing.

[nzakas]

Yeah, since you're the author of this package, is there any reason it's just not included in this PR as a utility?

[fasttime]

Note that that package contains logic that isn't needed in ESLint. The whole validation part for one, including 50% of the unit tests, is not necessary because the syntax of the arguments can be assumed to be valid since it was checked by the parser. There's also additional information about used features (for example, to tell if a string literal is valid in strict mode or not) which seems superfluous unless we expect to use it in ESLint somewhere later.

So if we are going to extract a utility from that package we should probably do some refactoring to remove the unnecessary logic.

[nzakas]

Yes, let's remove the unnecessary code, of course. As it seems like the package was created just for this purpose (?), I'm assuming that's not an issue.

[fasttime]

Done in 53e262a, thanks!

[JoshuaKGoldberg]

Looks like a great start! 👏 on going through the effort of making the parser. Looks from the files on GitHub like it was a lot of tricky work.

Since you mentioned bringing the code inline, requesting changes on that before doing a deeper review. Also I think I might have missed something with the charInfos caching?

[JoshuaKGoldberg]

[Question] Why not put the comment outside the code? It's not necessary for the test, so I'd think that preferable even ignoring the lines-around-comment shenanigan.

[fasttime]

Well, that's not for the comment inside the code, it's this comment that is producing errors now. I though that the comments in the code would make it easier to locate a test in the source code given its title, because the code as it's calculated here doesn't match what is printed in the console.

[JoshuaKGoldberg]

Funny, I was going to suggest if (node.type !== "TemplateLiteral" || (node.type === "Literal" && node.expressions.length)) {. No preference between the two from me. 🙂

[nzakas]

A little unclear here...are you intentionally moving the reader position by one? Or did you intend for const start = reader.pos + 1?

Either way, I think this line needs reworking to make its intention clear.

[nzakas]

I might rename this as TextSource. SourceReader implies that there's a read() method or something that this object is supposed to be doing, but it's just a data structure.

[fasttime]

Thanks for the feedback. I've added a read() method in 448d9a9 to read characters relative to the current position. So it's no longer necessary to store the position in a variable before accessing the source like it was done before. And I'm using += for relative position changes. I think this makes the code a little clearer but if that's not the case we can revert to how it was done previously and just address the suggestions.

[JoshuaKGoldberg]

I like the read()!

SourceReader is still a little ambiguous IMO. Maybe, CharsReader? CharactersReader? TextReader? Not a blocker from my end.

[fasttime]

Thanks! I don't have a preference for the name, so pick one that sounds better to you.

[JoshuaKGoldberg]

I'd vote for TextReader then, to align with #18082 (comment).

[fasttime]

Renamed in 072f256.

[nzakas]

Can you add some JSDoc here?

[nzakas]

Because you refactored to include a read() method in SourceReader, it might make things clearer if you also created a advance() method that moves pos rather than augmenting the value like this. So this code would become:

Suggested change

	reader.pos += octalStr.length - 1;
	reader.advance(octalStr.length - 1);

[fasttime]

good idea 👍🏻

[fasttime]

All done in b1cf05f.

[nzakas]

LGTM. Would like @JoshuaKGoldberg to review his changes before merging.

[mdjermanovic]

else can be anything, for example:

[fasttime]

Fixed in 7bb7e55.

[mdjermanovic]

We should check if the template literal has expressions. For example, this crashes:

/* eslint no-misleading-character-class: "error" */

new RegExp(`${"[👍]"}`);

TypeError: Cannot read properties of undefined (reading 'start')
Occurred while linting C:\projects\eslint\foo.js:3
Rule: "no-misleading-character-class"
    at C:\projects\eslint\lib\rules\no-misleading-character-class.js:294:64

[fasttime]

Correct! Those cases should be covered by unit tests.

[fasttime]

Fixed in 7bb7e55.

[fasttime]

This could be a bug in the current implementation:

RegExp(/[👍]/u);

Unexpected surrogate pair in character class. Use 'u' flag. (no-misleading-character-class)

The rule complains about a missing 'u' flag which is actually there:

console.log(RegExp(/[👍]/u).flags);
// "u"