- Feature Name: closure_to_fn_coercion
- Start Date: 2016-03-25
- RFC PR: [rust-lang/rfcs#1558](https://github.com/rust-lang/rfcs/pull/1558)
- Rust Issue: [rust-lang/rust#39817](https://github.com/rust-lang/rust/issues/39817)

## Summary
[summary]: #summary

A closure that does not move, borrow, or otherwise access (capture) local
variables should be coercible to a function pointer (`fn`).

## Motivation
[motivation]: #motivation

Currently in Rust, it is impossible to bind anything but a pre-defined function
as a function pointer. When dealing with closures, one must either rely upon
Rust's type-inference capabilities, or use the `Fn` trait to abstract for any
closure with a certain type signature.

It is not possible to define a function while at the same time binding it to a
function pointer.

This is, admittedly, a convenience-motivated feature, but in certain situations
the inability to bind code this way creates a significant amount of boilerplate.
For example, when attempting to create an array of small, simple, but unique functions,
it would be necessary to pre-define each and every function beforehand:

```rust
fn inc_0(var: &mut u32) {}
fn inc_1(var: &mut u32) { *var += 1; }
fn inc_2(var: &mut u32) { *var += 2; }
fn inc_3(var: &mut u32) { *var += 3; }

const foo: [fn(&mut u32); 4] = [
  inc_0,
  inc_1,
  inc_2,
  inc_3,
];
```

This is a trivial example, and one that might not seem too consequential, but the
code doubles with every new item added to the array. With a large amount of elements,
the duplication begins to seem unwarranted.

A solution, of course, is to use an array of `Fn` instead of `fn`:

```rust
const foo: [&'static Fn(&mut u32); 4] = [
  &|var: &mut u32| {},
  &|var: &mut u32| *var += 1,
  &|var: &mut u32| *var += 2,
  &|var: &mut u32| *var += 3,
];
```

And this seems to fix the problem. Unfortunately, however, because we use
a reference to the `Fn` trait, an extra layer of indirection is added when
attempting to run `foo[n](&mut bar)`.

Rust must use dynamic dispatch in this situation; a closure with captures is nothing
but a struct containing references to captured variables. The code associated with a
closure must be able to access those references stored in the struct.

In situations where this function pointer array is particularly hot code,
any optimizations would be appreciated. More generally, it is always preferable
to avoid unnecessary indirection. And, of course, it is impossible to use this syntax
when dealing with FFI.

Aside from code-size nits, anonymous functions are legitimately useful for programmers.
In the case of callback-heavy code, for example, it can be impractical to define functions
out-of-line, with the requirement of producing confusing (and unnecessary) names for each.
In the very first example given, `inc_X` names were used for the out-of-line functions, but
more complicated behavior might not be so easily representable.

Finally, this sort of automatic coercion is simply intuitive to the programmer.
In the `&Fn` example, no variables are captured by the closures, so the theory is
that nothing stops the compiler from treating them as anonymous functions.

## Detailed design
[design]: #detailed-design

In C++, non-capturing lambdas (the C++ equivalent of closures) "decay" into function pointers
when they do not need to capture any variables. This is used, for example, to pass a lambda
into a C function:

```cpp
void foo(void (*foobar)(void)) {
    // impl
}
void bar() {
    foo([]() { /* do something */ });
}
```

With this proposal, rust users would be able to do the same:

```rust
fn foo(foobar: fn()) {
    // impl
}
fn bar() {
    foo(|| { /* do something */ });
}
```

Using the examples within ["Motivation"](#motivation), the code array would
be simplified to no performance detriment:

```rust
const foo: [fn(&mut u32); 4] = [
  |var: &mut u32| {},
  |var: &mut u32| *var += 1,
  |var: &mut u32| *var += 2,
  |var: &mut u32| *var += 3,
];
```

Because there does not exist any item in the language that directly produces
a `fn` type, even `fn` items must go through the process of reification. To
perform the coercion, then, rustc must additionally allow the reification of
unsized closures to `fn` types. The implementation of this is simplified by the
fact that closures' capture information is recorded on the type-level.

*Note:* once explicitly assigned to an `Fn` trait, the closure can no longer be
coerced into `fn`, even if it has no captures.

```rust
let a: &Fn(u32) -> u32 = |foo: u32| { foo + 1 };
let b: fn(u32) -> u32 = *a; // Can't re-coerce
```

## Drawbacks
[drawbacks]: #drawbacks

This proposal could potentially allow Rust users to accidentally constrain their APIs.
In the case of a crate, a user returning `fn` instead of `Fn` may find
that their code compiles at first, but breaks when the user later needs to capture variables:

```rust
// The specific syntax is more convenient to use
fn func_specific(&self) -> (fn() -> u32) {
  || return 0
}

fn func_general<'a>(&'a self) -> impl Fn() -> u32 {
  move || return self.field
}
```

In the above example, the API author could start off with the specific version of the function,
and by circumstance later need to capture a variable. The required change from `fn` to `Fn` could
be a breaking change.

We do expect crate authors to measure their API's flexibility in other areas, however, as when
determining whether to take `&self` or `&mut self`. Taking a similar situation to the above:

```rust
fn func_specific<'a>(&'a self) -> impl Fn() -> u32 {
  move || return self.field
}

fn func_general<'a>(&'a mut self) -> impl FnMut() -> u32 {
  move || { self.field += 1; return self.field; }
}
```

This aspect is probably outweighed by convenience, simplicity, and the potential for optimization
that comes with the proposed changes.

## Alternatives
[alternatives]: #alternatives

### Function literal syntax

With this alternative, Rust users would be able to directly bind a function
to a variable, without needing to give the function a name.

```rust
let foo = fn() { /* do something */ };
foo();
```

```rust
const foo: [fn(&mut u32); 4] = [
  fn(var: &mut u32) {},
  fn(var: &mut u32) { *var += 1 },
  fn(var: &mut u32) { *var += 2 },
  fn(var: &mut u32) { *var += 3 },
];
```

This isn't ideal, however, because it would require giving new semantics
to `fn` syntax. Additionally, such syntax would either require explicit return types,
or additional reasoning about the literal's return type.

```rust
fn(x: bool) { !x }
```

The above function literal, at first glance, appears to return `()`. This could be
potentially misleading, especially in situations where the literal is bound to a
variable with `let`.

As with all new syntax, this alternative would carry with it a discovery barrier.
Closure coercion may be preferred due to its intuitiveness.

### Aggressive optimization

This is possibly unrealistic, but an alternative would be to continue encouraging
the use of closures with the `Fn` trait, but use static analysis to determine
when the used closure is "trivial" and does not need indirection.

Of course, this would probably significantly complicate the optimization process, and
would have the detriment of not being easily verifiable by the programmer without
checking the disassembly of their program.

## Unresolved questions
[unresolved]: #unresolved-questions

Should we generalize this behavior in the future, so that any zero-sized type that
implements `Fn` can be converted into a `fn` pointer?
