Why was the dyn syntax introduced? [duplicate]

Question

In Rust 1.27.0 a new syntax is introduced - the dyn keyword was added.

  // old => new   Box<Foo> => Box<dyn Foo>   &Foo => &dyn Foo   &mut Foo => &mut dyn Foo 

What does it actually do and why was it added?

Answer 1

This helps differentiate between traits/trait objects and structs; &Foo, Box<Foo> and impl Bar for Foo were ambiguous, because in all of them Foo could have been a trait or a struct.

With the addition of dyn this is no longer ambiguous, as traits are distinguished by the dyn keyword:

// trait objects (new dyn syntax) &Foo     => &dyn Foo &mut Foo => &mut dyn Foo Box<Foo> => Box<dyn Foo>  // structs (no change) &Bar &mut Bar Box<Bar>