Currently, the implementation for From<Box<T>> for Pin<Box<T>> is defined as:

impl<T> From<Box<T>> for Pin<Box<T>>

https://doc.rust-lang.org/nightly/std/pin/struct.Pin.html#impl-From%3CBox%3CT%3E%3E

This does not allow trait objects to be pinned. It is however possible to call Box::pinned, and convert that into a trait object.

Is it intentional that T is required to be Sized on the From implementation?

Defining it as this:

impl<T: ?Sized> From<Box<T>> for Pin<Box<T>>

allows to call Pin::from(trait_object)