[naming-convention] Allow underscore prefix for unused parameters

[glen-84]

Repro

{
  "rules": {
    "@typescript-eslint/naming-convention": [
      "error",
      {
        "selector": "default",
        "format": ["strictCamelCase"]
      }
    ]
  }
}

onComplete: (_id, name, responseJson) => {
    // Using name and responseJson, but not id.
}

Expected Result

No error on _id.

Actual Result

Error on _id.

Additional Info

As mentioned here with Brad's response here.

When using the noUnusedParameters compiler option, you have to use an underscore to suppress TypeScript's error. However, this shouldn't be done unless the parameter is actually unused.

Versions

package	version
@typescript-eslint/eslint-plugin	2.16.0
@typescript-eslint/parser	2.16.0
TypeScript	3.7.5
ESLint	6.8.0
node	12.14.1
npm	6.13.4

[bradzacher]

I need to figure out how to do this in a performant way. Using typescript's diagnostics would be awesome, but they're super slow - ~50ms to get the diagnostics for a file, which is too slow for linting (related: #1335).

I should test tsutils's unused variables function. If that's quick, then that's a decent compromise.

The only problem with both of these routes is that without #716, this would require type information. Though we're planning on starting the 3.0.0 release soon, so that isn't that far off.

[aggmoulik]

Hy @bradzacher I think there is nothing to work in this issue so we can close it.

[bradzacher]

Sorry, the last few comments were unrelated to the root issue.

For a bit of background; the naming-convention rule was derived from a 3rd party rule made for tslint.

When I wrote it, I cut some features from that rule for the purpose of keeping complexity down and cutting features until we saw how people used them.
One feature I cut was the unused modifier. This modifier would cause a selector to only match unused variables.

Right now you can only say "all variables can have leading underscores".
With this modifier, it would allow you to instead say "only unused variables can have leading underscores".

[aggmoulik]

So, I can work on it.

[bradzacher]

This needs some exploration to find an efficient way to check if a variable is unused. You're certainly welcome to explore this if you'd like.

I don't have a clear idea of how to do this in a performant manner yet.

[bradzacher]

Once #2039 lands this should be pretty simple to do.

[tcodes0]

@bradzacher nice, #2039 got merged 💯

[evanjmg]

How do we use it? I can't get this to work

[glen-84]

@evanjmg Use what? This hasn't been implemented yet.