Which Rust 1.2 containers support trait objects?

Question

In the Rust Edition Guide it says that in Rust 1.2, more container types support trait objects. It gave the example of Rc<T>, but it didn't give a complete list. What other containers support trait objects in Rust 1.2+?

Answer 1

Containers that support trait objects are containers that have a ?Sized bound on their containee type.

By default with generics, all types are Sized implicitly as this is what you want most of the time, and adding a Sized on almost every generic would be annoying. This behaviour is different from other traits and can be avoided by adding a ?Sized bound.

struct Foo<T>; // implicit `T: Sized` bound. T cannot be a trait object.

struct Bat<T: ?Sized>; // T can be a trait object.

You can see on the repository that Rc indeed used to be declared pub struct Rc<T> and was later changed to pub struct Rc<T: ?Sized>. GitHub lists this change as being part of Rust 1.1, but I guess we had to wait 1.2 to get it in stable.

Other containers that work on trait objects are Box, Arc, Cell and all the like of those smart pointers.

Containers that do not work on trait objects are Vec, HashMap and generally containers that might store more that one instance (collections). This is because 2 instances of the same trait objects might have different sizes (if they have different concrete type), and collections usually store elements continuously, requiring a constant size.