How we can support other parsers better (babel-eslint, typescript)

There’s a good amount of previous discussion in https://github.com/eslint/eslint/issues/4640 and part of the wish list issue, but wanted to bring up an issue for this in general.

Just wanted to brainstorm what we can do to make it easier to support experimental syntax or syntax/nodes that core rules might not know about.

• There’s currently monkeypatching being done on VisitorKeys for estraverse to account for unknown node types as well as for escope to add variable references and visit them so not sure what we can do there except figure out how if we have add more formal hooks?
• And there’s the issue with eslint-plugin-babel which everyone eventually stumbles upon (eslint issue -> babel-eslint -> eslint-plugin-babel). Because of the use of new syntax, users will still want to use old rules. However those rules may error out when it sees new syntax. This leads to https://github.com/babel/babel-eslint/issues/131 where either babel-eslint needs to convert the experimental syntax to something eslint rules understand (and thus the AST is different and unexpected for the rules that try to operate on the new syntax) or we copy paste a specific version of the rule and modify a line or two.

Example: I think the main difference for arrow-parens is the line https://github.com/babel/eslint-plugin-babel/blob/master/rules/arrow-parens.js#L24 to account for node.async: if (node.async) token = context.getTokenAfter(token);. However the rule can get updated upstream and will be out of sync (like in https://github.com/babel/eslint-plugin-babel/issues/63).

How can we extend built-in rules, extend on top?

• And other parsers need to figure out a good way to run all eslint tests or eslint add integration tests (since we got https://github.com/babel/babel-eslint/issues/267 before) - there were some updates to the tester so just need to further investigate the issues there.

[not-an-aardvark]

I just published eslint-rule-composer, which should hopefully make it easier to create variants of rules in eslint-plugin-babel without copy-pasting and changing a line.

[mysticatea]

A small rough idea, it’s to add an event for parsers. My image is:

function addressARule(context) {
    var sourceCode = context.getSourceCode();
    var token = sourceCode.getFirstToken(context.reportedItem.loc);
    if (token.type === "Keyword" && token.value === "async") {
        // It can cancel this report.
        context.cancel();

        // Or it can modify this report.
        context.reportedItem.loc.column += 5;
    }
}

module.exports = {
    parse: function() {},

    // This is called in RuleContext#report(), then this modifies the report.
    onReported: function(context) {
        switch (context.ruleId) {
            case "a-rule": addressARule(context); break;
            case "another-rule": addressAnotherRule(context); break;
        }
    }
}

I think such event has smaller risk than monkey patching.