What? All of this stuff has been in the language since 2014 at least. If you want stack-allocated closures which can safely close over references, this is the machinery you need. Rust's implementation is the state of the art.

Really can’t see how it could be made simpler. Function pointer can’t be a closure because it means something different. You can pass pointers different functions without having multiple versions of the function you are calling. Closures have a unique type and they require you to use generics.

But the difference between a closure and a function pointer is essential complexity, not accidental complexity.

If a language were to provide only closures and not function pointers, it would not be low level enough to interface with C code and C ABI. That's definitely not what Rust needs.

This has been in Rust since before the 1.0 release.

Also, you can’t converge to a diverging number, for Rust to get close to C++’s level of complexity, the C++ WG would have to stop with their garbage.