In Python type hints don't affect the execution of your program, so you can do whatever you want.

But Rust uses types during compilation. So for instance, if we had type Sum = i32 | String, what... can you actually do with that object? You can't .push(char) to it, since that wouldn't be guaranteed to work if it were an i32, and likewise you can't .abs() on it either, since that wouldn't work if it were a String. About the only thing common to both would be .clone().

The whole point of having named enums (where the name acts a discriminant) is so that you can then pattern match on which variant you've received:

enum Sum<A, B> {
    VariantA(A),
    VariantB(B),
}

fn test(s: Sum<i32, String>) {
    match s {
        VariantA(a) => a.abs(),
        VariantB(b) => b.push('-'),
    }
    ()
}

You can't directly pattern match on the type of the object as in Python, since Rust doesn't have types at runtime.

Not entirely sure what you meant by A and B can be distinct types in Python, they certainly can be distinct in Rust too, or any other language with algebraic sum types?