等号在特征绑定中意味着什么？

Question

For example in raw_vec.rs:

pub struct RawVec<T, A: AllocRef = Global> {
    ptr: Unique<T>,
    cap: usize,
    alloc: A,
}

I can see Global is a struct that implements AllocRef trait.

I'm surprised I can't find any explanation in rust books. I appreciate any links to docs on the topic.

If I had to guess it's a way to further constrain the impls that 'A' can take, but then why need 'A' generic type at all? It seems simpler to define the field alloc as Global . Again this is just a guess. Clarification appreciated.

Thanks!

Answer 1

It's a default generic type. It will be used unless you explicitly specify another in the concrete implementation. It's mentioned in the rust book :

When we use generic type parameters, we can specify a default concrete type for the generic type. This eliminates the need for implementors of the trait to specify a concrete type if the default type works. The syntax for specifying a default type for a generic type is <PlaceholderType=ConcreteType> when declaring the generic type.