fix panic on failure to format generics in enum

[ding-young]

fixes #5738

Description

Above issue reports panic when formatting complex enum with generic parameters.
This is because rustfmt simply calls unwrap() on Rewrite (RewriteResult in future), the return value from format_generics.
It is better not to panic on this sort of rewrite failure.
Therefore, this pr restores original snippet when we fail to format generics in enum instead of calling unwrap().

• This pr returns None early and then leave entire enum unformatted instead of calling unwrap().
• visit_enum is refactored with format_enum that returns Rewrite. Later pr will replace this rewrite with RewriteResult

TODO

• Revisit test case
• Any other solutions ? Calling existing functions provided in visitor.rs?
• check whether version gate is required (originally it would've panic, so I'm not sure if this is a formatting change)
• check other related issues like Crash during rustfmt #6378

[ytmimi]

@ding-young thanks for picking this one up! I wonder if now would be a good time to refactor visit_enum, and introduce a format_enum function, which we can return a RewriteResult from, similar to how visit_static and visit_struct are implemented. Then if we fail to format the generics we can just return early with an error and leave the enum unformatted.

Let me know what you think?

[ytmimi]

Can we also add this smaller reproducible case from the original issue:

enum Node where P::<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S<S>>>>>>>>>>>>>>>>>>>>>>>>:  {
    Cons,
}

[ytmimi]

@ding-young there were also some linked issues like #6137, #6318, #6378. Would be good to check these issues out as well. #6378 doesn't seem to have a minimal test case so feel free to ignore that one.

[ding-young]

Thank you for listing all the related issues! I included only the cases that originally lead to a rustfmt crash.