It does compile.
> because `x` does not have the type `fn (i32, i32) -> i32` (that's reserved for "normal" functions).
Closures can cast to function pointers just fine as long as they don't capture anything.
The difference is meaningful here. You have to allocate a closure (and deal with its lifetime and the lifetimes of the variables it references) but the anonymous function is just a pointer to static code in the binary. That's the entire difficulty with closures in non-GC languages.
This distinction matters less in GC languages where you're not thinking about lifetimes either way.
Also, your arguments only partly apply in Rust. Rust doesn't heap-allocate closures. And you also often don't have to deal with lifetimes - a closure that captures variables by move or copy is perfectly self-contained
The difference between a closure that captures and a closure that doesn't is like the difference between `(T)` and `()` - same kind of thing, so it adheres to the same terms and behaviors
That's incorrect, a closure in Rust compiles down to a static function that takes its environment as an argument. None of that requires a heap allocation in the above code.
According to the Rust reference:
> Closure types > > A closure expression produces a closure value with a unique, anonymous type that cannot be written out.
https://doc.rust-lang.org/reference/types/closure.html
Even a non-capturing closure is written using a closure expression and produces a unique, anonymous type.
> Unlike a plain function, a closure allows the function to access those captured variables through the closure's copies of their values or references, even when the function is invoked outside their scope.
> The term closure is often used as a synonym for anonymous function, though strictly, an anonymous function is a function literal without a name, while a closure is an instance of a function, a value, whose non-local variables have been bound either to values or to storage locations (depending on the language; see the lexical environment section below).
My understanding is that your use of 'capture' matches up with the the use of 'bound' in the description above.
https://en.wikipedia.org/wiki/Closure_(computer_programming)
