Optimize when there are multiple variants of the same type

[taiki-e]

https://github.com/taiki-e/auto_enums/issues/14#issuecomment-483672675

Another problem is that the current ? operator support generates the same number of variants as the ? operators are used. This may generate code that is less efficient than boxing based error handling for functions that use many ? operators. Since proc_macro doesn't understand types (i.e., it is difficult to optimize that), proc-macro crate, which provides only this feature, it will not be very useful.

I don't know if there is any way to fix this issue at this time.

cc https://github.com/rust-lang/rfcs/issues/2414

[taiki-e]

This is actually a discussion "is there a way to optimize when there are multiple variants of the same type?"