I think this comes from functional programming. I'd just call it "everything is an expression" (which isn't quite true in Rust but it's a lot more true than it is in traditional imperative languages like C++ and Python).

The only things in Rust that are real statements are `let` statements, and item statements (e.g. declaring an `fn` inside a function). All other statements are in fact expressions, although some always return `()` so they're not really useful as such.