[dikaiosune]

Closures in Rust are stack-allocated and LLVM can inline them and optimize them as if they're any regular imperative code, for one thing. This means that there's less overhead from managing the iterator chains, and that they're statically dispatched which saves on runtime indirection. The borrow checker also makes sure that you don't accidentally mutate non-thread-safe data from your parallel iterators.

[pjmlp]

Regarding closures, that is also possible in Java and .NET, just you don't control when it might happen.

[Veedrac]

In Rust, each closure has a unique type, and derived expressions are templated on that type. This is key to making them statically dispatched, which is important for making the base case (sequential) fast.