refac: replace MultiPeek by Peekable

[marcospb19]

MultiPeek is only useful in cases when it's necessary
to peek multiple items ahead.

[ytmimi]

Thanks for your first contribution to rustfmt 🎉

It looks like MultiPeek was added in this PR #2527, so I'd want to double check that we don't regress into the issue that it was trying to solve for.

I also want to note that I've been working on a fix for #4879 on my fork of rustfmt here, and I use MultiPeek to look ahead several characters.

[marcospb19]

Thanks for your first contribution to rustfmt tada

Thanks!

Sorry, I re-checked MultiPeek implementation and I can confirm this DOES break the function is_raw_string_suffix.

I will turn this PR into a draft and I'll see if I can add a test to check this behavior, and compare the two codes with it.

[ytmimi]

Great! Keep me posted on how the investigation goes!

[marcospb19]

@ytmimi my investigation was inconclusive, a test was added to check for this:

https://github.com/rust-lang/rustfmt/blob/master/tests/target/issue-2526.rs

However, the test has not failed after I replaced MultiPeek by Peekable, which I do not fully understand, maybe it does not break things, but anyways, I think I prefer not to touch on this piece of code.

Let me know if you have any thoughts on this tho.

[ytmimi]

Unfortunately I'd need to dive into this myself to understand what's going on here. I'm happy to closing this PR as there isn't an immediate need to change this part of the codebase.

If you've still got some interest in contributing to rustfmt I'd encourage you to take a look at the backlog and see if any open issue catches your attention. If you need any help don't hesitate to reach out 😁