-#[allow(missing_docs)] // Bogus warning (it’s not public outside the crate), ☹
-pub trait UncheckedAnyExt: Any {
- unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T;
- unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T;
- unsafe fn downcast_unchecked<T: Any>(self: Box<Self>) -> Box<T>;
+/// Methods for downcasting from an `Any`-like trait object.
+///
+/// This should only be implemented on trait objects for subtraits of `Any`, though you can
+/// implement it for other types and it’ll work fine, so long as your implementation is correct.
+pub trait Downcast {
+ /// Gets the `TypeId` of `self`.
+ fn type_id(&self) -> TypeId;
+
+ // Note the bound through these downcast methods is 'static, rather than the inexpressible
+ // concept of Self-but-as-a-trait (where Self is `dyn Trait`). This is sufficient, exceeding
+ // TypeId’s requirements. Sure, you *can* do CloneAny.downcast_unchecked::<NotClone>() and the
+ // type system won’t protect you, but that doesn’t introduce any unsafety: the method is
+ // already unsafe because you can specify the wrong type, and if this were exposing safe
+ // downcasting, CloneAny.downcast::<NotClone>() would just return an error, which is just as
+ // correct.
+ //
+ // Now in theory we could also add T: ?Sized, but that doesn’t play nicely with the common
+ // implementation, so I’m doing without it.
+
+ /// Downcast from `&Any` to `&T`, without checking the type matches.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `T` matches the trait object, on pain of *undefined behaviour*.
+ unsafe fn downcast_ref_unchecked<T: 'static>(&self) -> &T;
+
+ /// Downcast from `&mut Any` to `&mut T`, without checking the type matches.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `T` matches the trait object, on pain of *undefined behaviour*.
+ unsafe fn downcast_mut_unchecked<T: 'static>(&mut self) -> &mut T;
+
+ /// Downcast from `Box<Any>` to `Box<T>`, without checking the type matches.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `T` matches the trait object, on pain of *undefined behaviour*.
+ unsafe fn downcast_unchecked<T: 'static>(self: Box<Self>) -> Box<T>;