Fix: allow fallthrough comment inside block (fixes #14701)

[bakkot]

Prerequisites checklist

• I have read the contributing guidelines.

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

[x] Bug fix
[x] Changes an existing rule

What rule do you want to change?

no-fallthrough

Does this change cause the rule to produce more or fewer warnings?

fewer

How will the change be implemented? (New option, new default behavior, etc.)?

new default

Please provide some example code that this change will affect:

switch (foo) {
  case 1: {
    let x = value;
    console.log(value);
    // falls through
  }
  case 2: {
    doSomething();
  }
}

What does the rule currently do for this code?

warn

What will the rule do after it's changed?

not warn

What changes did you make? (Give an overview)

Now when the case consists of a single BlockStatement the code will look for the fallthrough comment to be the final comment in the case, as well where it looked previously.

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

In the situation in question - i.e., when the case consists of exactly one statement, which is a BlockStatement - I've chosen to check for the fallthrough comment in both places. This means that it becomes possible to have a comment after the fallthrough comment, as long as it's after the block, as in

switch (foo) {
  case 1: {
    let x = value;
    console.log(value);
    // falls through
  }

  // some other comment
  case 2: {
    doSomething();
  }
}

I think that's correct, personally. However, I'm happy to be more restrictive and say that the fallthrough comment must also be the final comment: that is, to only look in the new place when there are no comments after the block.

[mdjermanovic]

@bakkot thanks for the PR!

In the situation in question - i.e., when the case consists of exactly one statement, which is a BlockStatement - I've chosen to check for the fallthrough comment in both places. This means that it becomes possible to have a comment after the fallthrough comment, as long as it's after the block, as in

switch (foo) {
  case 1: {
    let x = value;
    console.log(value);
    // falls through
  }

  // some other comment
  case 2: {
    doSomething();
  }
}

I agree, I think it's fine to allow any comments after } when the fallsthrough comment is last in { ... }.

[mdjermanovic]

All changes look good, I only have some small requests about the documentation and a few additional tests.

[mdjermanovic]

LGTM, thanks!

[snitin315]

LGTM

[btmills]

Thanks, and congrats on your first ESLint pull request, @bakkot! This will be released today.

[evelynhathaway]

Oh yay! I submitted a PR for this a long time ago but it got closed. I think this also fixes #4652, #7889, #9080, and #9559