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.
rust
tiny_fn! {
struct Foo = Fn(a: i32, b: i32) -> i32;
}
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.
rust
tiny_fn! {
struct Foo = Fn(a: i32, b: i32) -> i32;
struct Bar = Fn() -> String;
}
[tiny_fn!] macro supports visibility qualifiers.
rust
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.
rust
tiny_fn! {
/// This is `Foo` wrapper for that takes two `i32`s and return `i32`.
pub struct Foo = Fn(a: i32, b: i32) -> i32;
}
Fn*] traits family[tiny_fn!] macro can generate closure wrappers for any of the [Fn*] traits family.
rust
tiny_fn! {
struct A = Fn();
struct B = FnMut();
struct C = FnOnce();
}
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.
rust
tiny_fn! {
struct BinOp<T> = Fn(a: T, b: T) -> T;
}
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 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
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.