I read this as "there is a public struct named Stream that takes one lifetime parameter and two type parameters. There are two members, a seed, and a next closure that takes a seed, and returns some iterator which gives a new seed and a value."

That doesn't fully describe all of it, and there's some inferences there, but that's how I hear it in my head. Some of it is experience, and some of it is context, too. Especially that "an iterator" part. I'd expect to see Iterator when talking about iterators, but the 'shape' for a lack of a better word looks like one to me.

Rust's syntax is pretty regular and modular. After some practice, the bits start to fit together, and the way to tackle more complex signatures is the same as how you tackle complex code: break it up into smaller bits, and then assemble your understanding of the bits.