feat: implement tuple return type to tuple struct assist

[rmehri01]

This PR implements the convert_tuple_return_type_to_struct assist, for converting the return type of a function or method from a tuple to a tuple struct. Additionally, it moves the to_camel_case and char_has_case functions from case_conv to stdx so that they can be used similar to to_lower_snake_case.

tuple_return_type_to_tuple_struct.webm

Currently, the assist puts the struct definition above the function, or above the nearest impl or trait if applicable and only rewrites literal tuples that are returned in the body of the function. Additionally, it only attempts to rewrite simple tuple pattern usages with the corresponding tuple struct pattern but does so across files and modules.

I think that this is sufficient for the majority of use cases but I could be wrong. One thing I'm still not sure how to approach is handling Self and generics/lifetimes in the tuple type to be extracted. I was thinking of either manually figuring out what lifetimes and generics are in scope and using them (sort of similar to the generate_function assist) or maybe using ctx.sema.resolve_type and generic_params on hir::Type but this seems to not deal with lifetimes.

Closes #14293

[Veykril]

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[Veykril]

Could you add a comment about the magic 5 here?

[rmehri01]

Oh yeah sorry, this is mostly a relic, I was basically trying to handle this case where we don't want to replace the outer tuple with the struct pattern. So, I was trying to get the direct parent of the call or method call expression but I've pushed a cleaner version now.

let ((bar1, bar2), foo) = (bar(), 3);

Also, in this case it would be nice to replace the corresponding pattern but I'm not sure if/how we should do that? Like:

let (BarResult(bar1, bar2), foo) = (bar(), 3);

[Veykril]

Ye that's tricky, we can only do heuristics here I think.

[rmehri01]

Yeah maybe we can leave it for now then, I think figuring out how to handle generics is more important.

[Veykril]

@bors r+

[bors]

📌 Commit 9ba8dbc has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9ba8dbc with merge ab62c01...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing ab62c01 to master...