Bindings in Rust have to initialized with an explicit value if the first use comes before the first modification/assignment otherwise [0] you get an error message with error E0381 [1]. Even if that explicit assignment is just through the use of the type's default method [2], if available.

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...