Explain this syntax

help
1

Does this mean any type appointed to the alias MaxLocks must implement the trait Get<T> ?


pub trait Get<T> {
	/// Return the current value.
	fn get() -> T;
}

type MaxLocks: Get<u32>;
2

The syntax is invalid.

error: free type alias without body
 --> src/lib.rs:6:1
  |
6 | type MaxLocks: Get<u32>;
  | ^^^^^^^^^^^^^^^^^^^^^^^-
  |                        |
  |                        help: provide a definition for the type: `= <type>;`
1 Like
3

It is defined here.

4

And used here

5

Aha, so it's part of a trait. That changes things quite a lot. It's an associated type. Whenever you implement the trait, you have to pick some type for the MaxLocks type, and you can only pick types that implement the specified trait.

2 Likes
6

"The specific trait" being Get<T>?

7

Yes.

8

A typical example for this in the standard library is IntoIterator.

You can do something like

fn foo<I: IntoIterator<Item = u32>>(x: I) {
    let y = x.into_iter();
    let mut r: u32 = 0;
    y.for_each(|item| r += item);
}

The fact that we can call for_each on y relies on the fact that y is an iterator (of u32s). The type of y is the return type of I::into_iter, which is an associated type in the IntoIterator trait.

pub trait IntoIterator {
    type Item;
    type IntoIter: Iterator<Item = Self::Item>;
    fn into_iter(self) -> Self::IntoIter;
}

the : Iterator<Item = Self::Item> part if that associated type declaration is what makes sure that we do in fact get an iterator.

If the trait was something like

pub trait IntoIterator {
    type IntoIter;
    fn into_iter(self) -> Self::IntoIter;
}

instead, you’d need to put the bound for every user of the trait, so foo would need to be written

fn foo<I: IntoIterator>(x: I)
where
    I::IntoIter: Iterator<Item = u32>,
{
    let y = x.into_iter();
    let mut r: u32 = 0;
    y.for_each(|item| r += item);
}

The Item type in IntoIterator is a convenience feature allowing us to specify the item type of the resulting iterator in an IntoIterator<Item = ...> bound.

9

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