feat(noUnusedFunctionParameters): add lint for unused function parameters, instead of using noUnusedVariables

[printfn]

Summary

This adds a new lint noUnusedFunctionParameters that detects unused parameters. Previously these were detected by noUnusedVariables.

Test Plan

I added tests for the new lint.

[codspeed-hq]

CodSpeed Performance Report

Merging #2899 will not alter performance

_{Comparing printfn:ignore-unused-args (34b0b56) with main (9e4feb6)}

Summary

✅ 92 untouched benchmarks

[printfn]

Rebased and fixed merge conflicts. Would someone be able to review this?

[ematipico]

What's the reasoning that drove you to implement the option? This reasoning will be very useful in explaining to the users why this option exists and how they can use it in their code.

[printfn]

I'm using biome on a large legacy codebase, slowly migrating from ESLint. Having the option to enable noUnusedVariables would be super helpful in improving code quality, but there are thousands of unused arguments that shouldn't have to meaninglessly be renamed to have an underscore in order to still benefit from linting for unused variables.

I think that having unused arguments is much less of a code smell than entirely unused variables. In fact, especially when creating and passing around lambda functions, having unused arguments present can be really useful to readers so they immediately know that another argument is available if the code needs to be modified in the future.

While prefixing arguments with an underscore is still an option, in my case this would require renaming thousands of parameters and introducing a non-trivial amount of line noise.

One pattern that frequently comes up is:

foo((a, b, c) => b);

In code like this, having the option to provide useful argument names without having to prefix everything is very useful.

Common examples:

return new Promise((accept, reject) => { ... }); // not using `reject`

let arr = loadArr(); // `arr` has type `[string, string][]`
arr.filter(([k, v]) => v.length > 0); // `k` is unused

componentDidUpdate(prevProps: Props, prevState: State) { ... } // not using `prevState`

While in many of the examples it might make sense to remove the extra argument or add an underscore, doing this thousands of times across a legacy codebase is unreasonable and shouldn't be necessary to still benefit from noUnusedVariables.

[printfn]

It's also worth pointing out that there are many repos on GitHub that set ESLint's args to none:

https://github.com/search?q=%2F%22no-unused-vars%22%3A%5Cs*%5C%5B%5Cs*%22%28warn%7Cerror%29%22%2C%5Cs*%5C%7B.*%22args%22%3A%5Cs*%22none%22.*%5C%7D%5C%5D%2F&type=code

[Conaclos]

I think we could go beyond and create a new rule noUnusedFunctionParameters.
TSC has two "rule" for unused variables: noUnusedLocals and noUnusedParameters.
In the past we created noUnusedImports.

[printfn]

I'll give making a new noUnusedFunctionParameters rule a go.

[printfn]

Alright, pretty sure I got it all working now. I've added a new nursery lint noUnusedFunctionParameters and changed noUnusedVariables to no longer include function parameters.

[ematipico]

This changes the behaviour of the rule, and it's a breaking change. We should change this only when the new rule goes out of nursery.

[printfn]

I've reverted the changes to noUnusedVariables now

[ematipico]

We can't change the documentation like this. This change will go straight to production and users will be misled. I think we should add a new paragraph like this:

From `v1.9.0`, the rule won't check unused function parameters anymore, users should switch to [noUnusedFunctionParameters](https://biomejs.dev/linter/rules/no-unused-function-parameters/)

I wrote 1.9.0 because in v1.8.0 the new rule is nursery, and we can't suggest a rule that is still in their incubation period.

[printfn]

Updated docs as suggested

[printfn]

this is ready for another review @ematipico