Syntax for the unit constraint?

[Centril]

In the following example:

type FooBox<trait Extra = ()> = Box<dyn Foo + Extra>;

We have used () as the "unit trait", that is, a "trait" that adds no constraints.
This takes the approach in Haskell where we have the unit type (), the unit value () (of type ()), and the unit constraint ().

In Rust like Haskell, we do use () for the unit type and () for the unit value, but we understandably have no unit constraint.

Is this syntax legible in the sense that we understand that Extra = () means that
by default, and if nothing else is passed, the type alias becomes Box<dyn Foo>?

If not, what is a better syntax?

[comex]

c.f. rust-lang/rust#41517 (comment)

[Centril]

@comex

Oh wow; Tho that is on the RHS instead of LHS.
Then you'd write: type FooBox<trait Extra = > = Box<..>.
That becomes weirdly unbalanced :/

[kennytm]

Use Sized or ?Sized if you really need something not empty.

[Centril]

@kennytm hmm... should it not be "if you need something empty" ?

[kennytm]

@Centril I mean if FooBox<trait Extra = > is unacceptable (i.e. you don't want an empty spot there), you may use Sized or ?Sized depending on the situation.

[Centril]

@kennytm Right; Nicely spotted 👍