`impl Fn/FnOnce/FnMut` don't stand for function pointers, but rather function items in Rust, and as such they are zero sized so Rust can provide optimizations regarding function items specifically at compile time.
They can decay to function pointers (`$(unsafe)? $(extern)? fn($(inp),*) $(-> $(output))?`, example: unsafe extern fn() -> i32), which you can freely cast between function pointers, `*const/mut T` pointers or `usize`s. https://doc.rust-lang.org/reference/types/function-pointer.h...
They can decay to function pointers (`$(unsafe)? $(extern)? fn($(inp),*) $(-> $(output))?`, example: unsafe extern fn() -> i32), which you can freely cast between function pointers, `*const/mut T` pointers or `usize`s. https://doc.rust-lang.org/reference/types/function-pointer.h...