Pub trait Thin = Pointee<Metadata = ()>?

What I don't understand in the code shown below is line 77. Line 53 only defines a trait containing an association type, but line 77 does have this operation (Metadata=()), how is it associated, can you explain it in detail?

If you read the comment above it at line 76, you'll see it says the unstable trait aliases.

And you'll find the RFC 1733 with this line:

It’s also possible to partially bind associated types of the right hand side:

trait IntoIntIterator = IntoIterator<Item=i32>;

I know what the trait alias means, but I still don't understand how it (Metadata=()) is passed through