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().
We'll need to propagate this rewrite failure later, too.

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]

Can we double check that all of these enums trigger the bug. Probably best to remove any that don't.

[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.