`no-extra-semi` mistakenly removes necessary semicolon when `semi` is `never`

Tell us about your environment

• ESLint Version: 3.13.1
• Node Version: 6.9.3
• npm Version: 4.1.1 What parser (default, Babel-ESLint, etc.) are you using? default Please show your full configuration:

rules:
  semi:
    - error
    - never
  no-extra-semi:
    - error

What did you do? Please include the actual source code causing the issue.

var foo = function(){};
;[1].map(foo)

What did you expect to happen?

var foo = function(){}
;[1].map(foo)

What actually happened? Please include the actual, raw output from ESLint.

var foo = function(){}
[1].map(foo)

When semi is never, no-extra-semi is true, and there are whitespaces between two semicolons, those rules will delete both of semicolons. semi deletes the first one, and no-extra-semi deletes the second one.

[not-an-aardvark]

@alberto I disagree. The rules are both working correctly, it’s just that their fixes happen to interfere with each other. semi removes the first semicolon (which is correct behavior, because the first semicolon is unnecessary in the original source text). no-extra-semi removes the second semicolon (which is also correct behavior, because the second semicolon is also unnecessary in the original source text). Neither rule knows anything about what the other rule is doing, so core is the only good place to handle this.

If we start adding exceptions to specific fixers to prevent them from conflicting with fixers from other rules, then we’re going to end up with a lot of duplicated logic. For example, the semi rule will have to detect no-extra-semi errors in order to determine whether it should handle the fix. With hundreds of rules, this will quickly become very difficult to reason about, and it also won’t work for custom autofixable rules.

Core is designed to prevent this by isolating fixes. If a rule makes a fix that is safe on its own, core is supposed to guarantee that the fix will be safe when applied with other fixes. This bug violates the guarantee, so it’s an issue with core, not with the rules.

[not-an-aardvark]

The bug that was initially reported in this issue was fixed by https://github.com/eslint/eslint/commit/4d35a81c9ddae2e77409bb6d82adeb62e1e1c33c, but I’m going to reopen this since I think we do need to eventually figure out a better general solution.