Ideas around anonymous enum types

language design
163

Yep, you must not “override” any blanket implementations for Any, otherwise this would be a problem:

use std::any::Any;

fn main() {
    let x: enum(i32, bool) = 42;
    // ^^^ implements Any based on dispatch
    let y: &dyn Any = &x;
    // ^^^ unsize coercion: enum(i32, bool) is sized and implements Any
    let z: &i32 = y.downcast_ref::<i32>().unwrap()
    // ^^^ created internally by casting y, so we get an invalid reference (UB)
    // ^^^ succeeds since y.type_id() gives is TypeId of `i32` by dispatch
    // see: https://doc.rust-lang.org/src/core/any.rs.html#195-204
}