Have you ever being placing closure into Box<dyn Fn(...)> and wondered: "Is there a crate to avoid heap allocations for small closures?"

Wonder no more, this is the crate.

This crate provides declarative macro tiny_fn! to generates closure wrappers able store closure erasing its type.

Generated closure wrappers avoid heap allocations when wrapped closure fits inline storage.

The macro is designed to be easy to write with simple syntax that mostly reuse constructs already existing in Rust.
Behavior of generated wrappers should be obvious from the first glance.

tiny_fn! {
    struct Foo = Fn(a: i32, b: i32) -> i32;
}

let foo: Foo = Foo::new(|a, b| a + b);
assert_eq!(foo.call(1, 2), 3);

Macro expands to struct Foo definition with two public methods.

• Foo::new accepts any value that implements Fn(i32, i32) -> i32 and returns new instance of Foo.
• Foo::call follows signature specified to the macro. e.g. Foo::call accepts a: i32 and b: i32 and returns i32.
  Plainly Foo::call calls closure from which this instance of Foo was crated using a and b arguments at the same positions.

tiny_fn! macro supports defining multiple items at once.

tiny_fn! {
    struct Foo = Fn(a: i32, b: i32) -> i32;
    struct Bar = Fn() -> String;
}

let foo: Foo = Foo::new(|a, b| a + b);
let bar: Bar = Bar::new(|| "Hello, World!".to_string());

assert_eq!(foo.call(1, 2), 3);
assert_eq!(bar.call(), "Hello, World!");

tiny_fn! macro supports visibility qualifiers.

tiny_fn! {
    pub struct Foo = Fn(a: i32, b: i32) -> i32;
    struct Bar = Fn() -> String;
    pub(crate) struct Baz = Fn();
}

tiny_fn! macro supports item attributes, including documentation.

tiny_fn! {
    /// This is `Foo` wrapper for that takes two `i32`s and return `i32`.
    pub struct Foo = Fn(a: i32, b: i32) -> i32;
}

tiny_fn! macro can generate closure wrappers for any of the Fn* traits family.

tiny_fn! {
    struct A = Fn();
    struct B = FnMut();
    struct C = FnOnce();
}

let a = 42;
let a: A = A::new(|| println!("{}", a));
a.call();
a.call();

let mut b = 42;
let mut b: B = B::new(|| b += 1);
b.call();
b.call();

let c = String::from("Hello, World!");
let c: C = C::new(move || println!("{}", c));
c.call();
// c.call(); // This will not compile, because `C` can be called only once.

• A can wrap only closures that are callable when immutably borrowed. And so A::call takes &self.
• B can wrap only closures that are callable when borrowed. And so B::call takes &mut self.
• C can wrap any closures, even ones that are callable once. And so C::call takes self.

Closure wrappers can be declared generic over number of types and those types should be used in function signature.

tiny_fn! {
    struct BinOp<T> = Fn(a: T, b: T) -> T;
}

let add: BinOp<i32> = BinOp::new(|a, b| a + b);
let mul: BinOp<i32> = BinOp::new(|a, b| a * b);

assert_eq!(mul.call(add.call(1, 2), 3), 9);

Here BinOp is generic over T.
BiOp::<T>::new accepts closures bounds by Fn(T, T) -> T.

Notably T is not constrained by traits in BinOp.
Closure wrappers only move arguments and return values, so they don't need to know anything else about the type.

Closure wrappers can be declared with marker traits. Simply add | and list of + prefixed marker traits after function signature. In Fn traits family | symbol is not used, but here it is required due to declarative macro limitations.

They will be added to bounds on contained types. And if autotraits, they will be implemented for the wrapper type as well.

tiny_fn! {
    struct Foo = Fn(a: i32, b: i32) -> i32 | + Send;
}

let foo: Foo = Foo::new(|a, b| a + b);

std::thread::spawn(move || {
  foo.call(1, 2);
});

Closure wrapper generated by tiny_fn! macro always have two generic parameters besides generic types specified by macro caller:

• Lifetime 'closure.
  Wrapper contains closures bound by 'closure lifetime.
• Constant INLINE_SIZE: usize.
  Closures with size up to INLINE_SIZE and alignment requirement not exceeding tiny_fn::ALIGN will be inlined into wrapper structure directly.
  Otherwise heap allocation will occur.
  INLINE_SIZE parameter is defaulted to tiny_fn::DEFAULT_INLINE_SIZE.

Licensed under either of

• Apache License, Version 2.0, (license/APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (license/MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.