What makes something a "trait object"?

Solution 1:

You have trait objects when you have a pointer to a trait. Box, Arc, Rc and the reference & are all, at their core, pointers. In terms of defining a "trait object" they work in the same way.

"Trait objects" are Rust's take on dynamic dispatch. Here's an example that I hope helps show what trait objects are:

// define an example struct, make it printable
#[derive(Debug)]
struct Foo;

// an example trait
trait Bar {
    fn baz(&self);
}

// implement the trait for Foo
impl Bar for Foo {
    fn baz(&self) {
        println!("{:?}", self)
    }
}

// This is a generic function that takes any T that implements trait Bar.
// It must resolve to a specific concrete T at compile time.
// The compiler creates a different version of this function
// for each concrete type used to call it so &T here is NOT
// a trait object (as T will represent a known, sized type
// after compilation)
fn static_dispatch<T>(t: &T)
where
    T: Bar,
{
    t.baz(); // we can do this because t implements Bar
}

// This function takes a pointer to a something that implements trait Bar
// (it'll know what it is only at runtime). &dyn Bar is a trait object.
// There's only one version of this function at runtime, so this
// reduces the size of the compiled program if the function
// is called with several different types vs using static_dispatch.
// However performance is slightly lower, as the &dyn Bar that
// dynamic_dispatch receives is a pointer to the object +
// a vtable with all the Bar methods that the object implements.
// Calling baz() on t means having to look it up in this vtable.
fn dynamic_dispatch(t: &dyn Bar) {
    // ----------------^
    // this is the trait object! It would also work with Box<dyn Bar> or
    // Rc<dyn Bar> or Arc<dyn Bar>
    //
    t.baz(); // we can do this because t implements Bar
}

fn main() {
    let foo = Foo;
    static_dispatch(&foo);
    dynamic_dispatch(&foo);
}

For further reference, there is a good Trait Objects chapter of the Rust book

Solution 2:

Short Answer: You can only make object-safe traits into trait objects.

Object-Safe Traits: Traits that do not resolve to concrete type of implementation. In practice two rules govern if a trait is object-safe.

1. The return type isn’t Self.
2. There are no generic type parameters.

Any trait satisfying these two rules can be used as trait objects.

Example of trait that is object-safe can be used as trait object:

trait Draw {
    fn draw(&self);
}

Example of trait that cannot be used as trait object:

trait Draw {
    fn draw(&self) -> Self;
}

For detailed explanation: https://doc.rust-lang.org/book/second-edition/ch17-02-trait-objects.html