style: use Self to avoid unnecessary repetition

[theodorebje]

Motivation

This PR is part of the broader PR #3394 (Activate more lints)

Repeating type names over and over again quickly becomes a problem if the name of the type ever changes, or a refactored is required. It also makes it more difficult to copy code from one method to another.

Solution

In impls, Self can be used instead of the struct/enum being implemented, to reduce unnecessary repetition. This PR uses Self as often as possible. This fixes the use_self clippy lint.

[theodorebje]

Copying over the previous discussion from #3394

Originally posted by mladedav in #3394 (comment)

Personally, I'm not a fan of this on 3rd party structs like Option<T> here or String somewhere else. Self::from_utf8 looks just weird to me.

So I wouldn't add this lint, but I get that this is stylistic, so if other people have different opinions, I don't mind.
Originally posted by theodorebje in #3394 (comment)

jplatte What do you think about this? Should we only use Self for Axum's types?

[jplatte]

Yeah, let's be a bit more conservative and not change any of the places that David had some concerns about. You can still enable the lint, just #[allow] it in those specific places (there weren't many, right?).

[mladedav]

I think everything here is fine, if I didn't miss something, it's really just constructors and enum variants (and a few signatures).

[theodorebje]

With all checks passing, this is ready to merge 👌

[mladedav]

@jplatte if you want to change some of the allows to Self, we can, otherwise let's merge this.

[theodorebje]

Thanks everyone!