It borrowed some FP concepts, it's influenced by FP but it's definitely not an FP language. It's very much a language about imperative, explicit control. And that's a good thing for the use-cases that Rust targets.
It doesn't have in-built immutable data structures, most basic data structures and types are mutable and have mutate in place methods as their primary way of interaction. Struct fields being private by default is a dead giveaway (unnecessary in an FP language). Closures and functions are not the same thing and you cannot do much with them except calling them and passing them around.
I would rather describe it as an imperative language with lightweight OO (via traits) and some FP sprinkled on top. Note I'm not saying you cannot write FP code in it or that Rust doesn't have great unique benefits.
> It doesn't have in-built immutable data structures
Immutable data structures by default would involve avoidable overhead, so that's why they aren't a built in. There are well-known crates that do provide persistent data structures in Rust.
> Struct fields being private by default is a dead giveaway (unnecessary in an FP language).
Data hiding is just as useful in functional programming; in particular, it enables you to expose an interface to data that stays valid under any isomorphism, while preserving desired invariants. Structs with generally accessible and updatable fields are a special case.
It doesn't have in-built immutable data structures, most basic data structures and types are mutable and have mutate in place methods as their primary way of interaction. Struct fields being private by default is a dead giveaway (unnecessary in an FP language). Closures and functions are not the same thing and you cannot do much with them except calling them and passing them around.
I would rather describe it as an imperative language with lightweight OO (via traits) and some FP sprinkled on top. Note I'm not saying you cannot write FP code in it or that Rust doesn't have great unique benefits.