Span<T> is a "ref struct" type, and thus has a lot of restrictions on its use. For instance, you can't have one as a field or property in a class, so you can't assign it to outside of the scope that it was declared in.
You can assign the span to an out of scope variable as long as it does not violate the scoping of the memory referenced by that said span. The closer primitive to C#'s Span<T> is Rust's &mut [T] since both are subject to lifetime analysis (of course the one in C#[0] is quite rudimentary in comparison to Rust).
