Why do I need "use rand::Rng" to call gen() on rand::thread_rng()?

I don't know the specific library you're using, but I can guess at the problem. I would guess that Rng is a trait which defines gen. Traits can be thought of as somewhat like Java's interfaces: they enable ad-hoc polymorphism by allowing you to define different behaviors for the same function on different datatypes.

However, Rust's traits fix one major problem (well, they fix several major problems, but one that's relevant here) with Java's interfaces. In Java, if you define an interface, then anyone writing a class can implement the interface, but you can't implement it for other people. In particular, the built-in types String and int and the like can never implement any new interfaces downstream. In Rust, either the trait writer or the struct/enum writer can implement the trait.

But this poses another issue. Now, if I have a value foo of type Foo and I write foo.bar(), then bar might not be a method defined on Foo; it might be something some trait writer implemented in some other file. We can't go search every Rust file on your computer for possible matching traits, so Rust makes the logical decision to restrict this search to traits that are in scope. If you want to call foo.bar() and bar is a method on trait Bar, then trait Bar has to be in scope when you call it. Otherwise, Rust won't see it.

So, in your case, thread_rng() returns a rand::prelude::ThreadRng. The method gen is not defined on rand::prelude::ThreadRng. Instead, it's defined on a trait called rand::Rng which is *implemented by ThreadRng. That trait has to be in-scope to use the method.

That's quite different from what I used to know.

It feels different because it is indeed different. You are probably used to dynamic dispatch via some kind of virtual method table (as in e.g. C++), or, in case of JS, to dynamic dispatch by looking up either the own properties of the receiver object, or its ancestors via the __proto__-chain. In any case, the object on which you are invoking a method carries around some data that tells it how to get the method that you're invoking. Given the signature of the invoked method, the receiver object itself knows how to get the method with that signature.

That's not the only way, though. For example,

• modules / functors in OCaml or SML
• Typeclasses in Haskell
• implicits / givens in Scala
• traits in Rust

work on a rather different principle: the methods are not tied to the receiver, but to the module / typeclass / given / trait instances. In each case, those are entities that are separate from the receiver of the method call. It opens some new possibilities, e.g. it allows you to do some ad-hoc polymorphism (i.e. to define instances of traits after the fact, for types that are not necessarily under your control). At the same time, the compiler typically requires a bit more information from you in order to be able to select the correct instances: it behaves somewhat like a little type-directed search engine, or even a little "theorem prover", and for this to work, you have to tell the compiler where to look for the suitable building blocks for those synthetically generated instances.

If you've never worked before with any language that has a compiler with a subsystem that is "searching for instances" based on type information, this should indeed feel quite foreign. The error messages and the solution approaches do indeed feel rather different, because instead of comparing your implementation against an interface and looking for conflicts, you have to guide this instance-searching mechanism by providing more hints (e.g. by importing more traits etc.).

In your particular case, rand::thread_rng returns a struct ThreadRng. On its own, the struct knows nothing about the gen method, because this method is not tied directly to the struct. Instead, it's defined in the Rng trait. But at the same time, it could be defined in some entirely unrelated trait, and have some completely different meaning. In order to disambiguate the intended meaning, you therefore have to explicitly specify that you want to work with the Rng trait. This is why you have to mention it in the use-clause.