Is there any crate can hold multiple types but only one is possible

Hi, guys. Is there any crate out there which can hold multiple types but only one type is possible, much like an union.


struct Foo {
    a: SomeType<AType, BType, CType>,

so I can do like

let mut foo = Foo {
    a: SomeType::new(AType);
// foo.a is a AType.

foo.a = SomeType::new(BType);
// foo.a is a BType by now.

Can't you just use an enum?

enum SomeType<A, B, C> {
    AType(A), BType(B), CType(C),


If you want to store the state, but not a value of the type, then a regular enum should suffice. Types in Rust aren't objects, so you can't store them directly. You could do something clever with extra traits and associated types implemented on unit structs to mark something as "a type", but the easiest way is to just use a regular enum and a match.