Essentially Box<Foo> is static dispatch. You know at compile time which exact methods you are going to call. Box<dyn Foo> is dynamic dispatch. Since Foo is a trait, at compile time you won't know which methods are called, it depends on the type of the object passed in (as long is it implements the Foo trait).
If Foo is a struct then Box<Foo> is static dispatch. If Foo is a trait Box<dyn Foo> is dynamic dispatch, but then so is Box<Foo> - but that is the regrettable point. So from now on, if Foo is a trait we should be writing Box<dyn Foo>, which is largely syntactic sugar to make the situation more clear?
I don't fully understand what the release notes are saying about using impl Trait. When would you use Box<impl Foo> vs Box<dyn Foo>?
It’s not that you would use Box<impl Foo>, but when taking about the two features, it’s easier to compare them, as they’re actually distinct syntactically.
Essentially Box<Foo> is static dispatch. You know at compile time which exact methods you are going to call. Box<dyn Foo> is dynamic dispatch. Since Foo is a trait, at compile time you won't know which methods are called, it depends on the type of the object passed in (as long is it implements the Foo trait).