It will automatically implement the most general, relaxed version (FnMut I think?) and only restrict itself further to FnOnce and Fn based on what you do inside the closure.
So, it can be tricky to know what's going on, and making a code change can change the contract of the closure and therefore where and how it can be used.
(I invite rust experts to correct me if any of the above is mistaken - I always forget the order of precedence for FnOnce/Fn/FnMut and which implies which)
The least restrictive for the function itself is the opposite order: FnOnce (it can do anything to its environment, including possibly consuming things without putting them back into a consistent state), followed by FnMut (it has exclusive access to its environment, and so is allowed to mutate it, but not destroy it), followed by Fn (it has only shared access to its environment and therefore is not allowed to mutate it).
Since these orders are inverses of each other, functions that are easier to write are harder to call and vice versa. That’s why they implement the trait with the minimum amount of power possible, so that they can be called in more places.
The way I remember the ordering is by thinking about the restrictions the various Fn traits provide from a caller's perspective:
1. FnOnce can only ever be called once and cannot be called concurrently. This is the most restrictive.
2. FnMut can be called multiple times but cannot be called concurrently. This is less restrictive than FnOnce.
3. Fn can be called multiple times and can even be called concurrently. This is the least restrictive.
It’s more precise to say that Fn can be called even when you only have shared access to it, which is a necessary, but not sufficient, condition for being able to be called concurrently.
FnOnce
FnMut
Fn
I've been using this on my vibewasm project to provide host function conversion to keep a C callable function in the front surface but doing my own custom wasm calling convention while capturing a persistent context pointer to the wasm store.
There is a side effect though: it is essentially unsound as you have to leak the object to the libffi closure which is a form of JIT -- dynamic code generation, it is, meaning Rust will have no way of knowing the lifetime of the pointer, or you have to always keep the libffi closure alive, meaning it is a permanent leak. I tried mitigate this by storing the closure in the wasm store, and we use that as the ultimate closure lifetime by designation -- any libffi function callback post store destruction is undefined behavior though.
So then I'm forced to define a trait for the function, define a struct (the closure) to store the references I want to close over, choose the mutability and lifetimes, instantiate it manually and pass that. Then the implementation of the method (that may only be a few lines) is not located inline so readability may suffer.
fn foo<F, T>(f: F)
where
F: Fn(T),
T: ToString,
{
f("Hello World")
}
fn foo(f: impl for<T: ToString> Fn(T)) {
f(“Hello World”);
f(3.14);
}
fn main() {
f(|x| println!("{}", x.to_string());
}
trait FnToString {
fn call(&self, x: impl ToString);
}
fn foo(f: impl FnToString) {
f.call("Hello World");
f.call(3.14);
}
struct AnFnToString;
impl FnToString for AnFnToString {
fn call(&self, x: impl ToString) {
println!("{}", x.to_string());
}
}
fn main() {
foo(AnFnToString);
}the move keywords captures everything. Sometimes I want a little bit more flexibility, like C++ lambdas.
I agree it would be nice, in particular to make it easier to understand when learning the concept.
Also worthy of note is that there is talk to add a syntax for explicit captures.
In fact I can’t remember the last time I had to fight with them.
[1]: https://github.com/rust-lang/rust/issues?q=is%3Aopen%20is%3A...
Their closures are essentially the equivalent of Rust's `Rc<dyn Fn(...) -> ...>` with some sugar for expressing the function type and hiding all the `.clone()`s needed.
It's easy to get simplier results if you support less features and use cases.