feature: Add inlay hints for named parameters

[tgodzik]

Scala 2 version of scalameta/metals-feature-requests#184

[kasiaMarek]

I feel like it would be cool to have inlay hints for only "useful" named params. Not necessarily instead of this but maybe additionally. We could avoid showing named params:

1. for single letter param names (or xs, ys), since those are usually meaningless
2. when param name and passed argument have the same name

[tgodzik]

I feel like it would be cool to have inlay hints for only "useful" named params. Not necessarily instead of this but maybe additionally. We could avoid showing named params:

1. for single letter param names (or xs, ys), since those are usually meaningless

This really depends on a use case. Most of the times I guess yeah, but sometimes it might be useful for some symbolic names.

2. when param name and passed argument have the same name

I want to keep it consistent, if we don't show named params in some cases, users might actually think they are broken.

[tgodzik]

@harpocrates I noticed that our PR interact with each other, what do you think about the mix of named parameters and by name ones? Alternatively, we could drop '=' and have:

Suggested change

	| foo(/*x = => */1, /*y = */2, /*z = => */3)(/*w = */4, /*v = => */5)
	| foo(/*x => */1, /*y = */2, /*z => */3)(/*w = */4, /*v => */5)

[harpocrates]

I think seeing = => is probably the most reasonable thing - unfortunately in scala x => 1 starts looking like a lambda w/ arg x. I don't have a strong opinion though...

[harpocrates]

On dropping some parameter inlay-hints when they'd probably be noise: I don't really have an opinion to add, but wanted to mention this is the approach rust-analyzer took. Here's the function they used to bake in all of their heuristics:

https://github.com/rust-lang/rust-analyzer/blob/16745db3f763b6a1a315a91e8f4191b6a7a9da63/crates/ide/src/inlay_hints/param_name.rs#L111

[tgodzik]

On dropping some parameter inlay-hints when they'd probably be noise: I don't really have an opinion to add, but wanted to mention this is the approach rust-analyzer took. Here's the function they used to bake in all of their heuristics:

https://github.com/rust-lang/rust-analyzer/blob/16745db3f763b6a1a315a91e8f4191b6a7a9da63/crates/ide/src/inlay_hints/param_name.rs#L111

Thanks! Though I think we can add the heuristics later when testing the feature and user have some specific ideas. For now I will leave it as straightforward as possible.

[tgodzik]

This becomes much more complex now since all the parameters are now synthetic if we reorder the params in any way (though the flag is not set in reality). I might work on it later, but I will just do a follow up issue before mergeing. TODO