Yes, for Option the default value is quite obvious. I'm talking about Result, because it's the second best-known sum type, and it doesn't have an obvious default value.
Since it's implemented for Option<T> you can do it the same way [3] for that, but that's not available for Result<T,U> since the semantics are different from Option which is basically just Rust's alternative for 'null'/'nil' (but verifiable by the compiler).
Thus you'd have to choose what variant of the enum you want to have at initialization, after which you can use the Default implementation of the enclosed type, if available (and so on, if there are more nested types).
Though honestly I can't recall using the kind of pattern in Rust that would "need" something like that. Through the handling of code blocks as expressions, early returns etc. you for example don't need to initialize a variable before having an if-/match-/for-/while-/etc. block, you can simply use that block as an expression on the right side of the assignment [4].
[0]: https://play.rust-lang.org/?version=stable&mode=debug&editio...
[1]: https://doc.rust-lang.org/stable/error-index.html#E0381
[2]: https://play.rust-lang.org/?version=stable&mode=debug&editio...
[3]: https://play.rust-lang.org/?version=stable&mode=debug&editio...
[4]: https://play.rust-lang.org/?version=stable&mode=debug&editio...