在Rust中，如何检查泛型参数是否属于特定类型并强制转换为它

Question

I have several types that implement a trait (Relation). I need to pass data between them, like INSERT INTO FROM SELECT from sql.

However, some times I will move data that is coming from the same type, meaning I could use a more direct way:

impl Relation for BTree {
    fn new_from<R: Relation>(names: Schema, of: R) -> Self {
       if of is Btree { //How do this
          //Fast path
          cast(of as Btree).clone()  //And this
       } else {
          //Generic path
       }
    }
}

Answer 1

What you are trying to do should be possible using std::any . I imagine it would look something like this:

use std::any::Any;

trait Trait {
    fn foo<T: Trait + Any>(of: T) -> Self;
}

#[derive(Clone)]
struct Special;

impl Trait for Special {
    fn foo<T: Trait + Any>(of: T) -> Self {
        let of_any = &of as &dyn Any;
        if let Some(special) = of_any.downcast_ref::<Special>() {
            // Fast path
            special.clone()
        } else {
            // Generic path, pretend this is expensive
            Special
        }
    }
}