impl Trait

TicketStore::to_dos returns a Vec<&Ticket>.
That signature introduces a new heap allocation every time to_dos is called, which may be unnecessary depending on what the caller needs to do with the result. It'd be better if to_dos returned an iterator instead of a Vec, thus empowering the caller to decide whether to collect the results into a Vec or just iterate over them.

That's tricky though! What's the return type of to_dos, as implemented below?

#![allow(unused)]
fn main() {
impl TicketStore {
    pub fn to_dos(&self) -> ??? {
        self.tickets.iter().filter(|t| t.status == Status::ToDo)
    }
}
}

Unnameable types

The filter method returns an instance of std::iter::Filter, which has the following definition:

#![allow(unused)]
fn main() {
pub struct Filter<I, P> { /* fields omitted */ }
}

where I is the type of the iterator being filtered on and P is the predicate used to filter the elements.
We know that I is std::slice::Iter<'_, Ticket> in this case, but what about P?
P is a closure, an anonymous function. As the name suggests, closures don't have a name, so we can't write them down in our code.

Rust has a solution for this: impl Trait.

impl Trait

impl Trait is a feature that allows you to return a type without specifying its name. You just declare what trait(s) the type implements, and Rust figures out the rest.

In this case, we want to return an iterator of references to Tickets:

#![allow(unused)]
fn main() {
impl TicketStore {
    pub fn to_dos(&self) -> impl Iterator<Item = &Ticket> {
        self.tickets.iter().filter(|t| t.status == Status::ToDo)
    }
}
}

That's it!

Generic?

impl Trait in return position is not a generic parameter.

Generics are placeholders for types that are filled in by the caller of the function. A function with a generic parameter is polymorphic: it can be called with different types, and the compiler will generate a different implementation for each type.

That's not the case with impl Trait. The return type of a function with impl Trait is fixed at compile time, and the compiler will generate a single implementation for it. This is why impl Trait is also called opaque return type: the caller doesn't know the exact type of the return value, only that it implements the specified trait(s). But the compiler knows the exact type, there is no polymorphism involved.

RPIT

If you read RFCs or deep-dives about Rust, you might come across the acronym RPIT.
It stands for "Return Position Impl Trait" and refers to the use of impl Trait in return position.

Exercise

The exercise for this section is located in 06_ticket_management/08_impl_trait