Callbacks: `absl::AnyInvocable` and `Fn`/`FnMut`/`FnOnce`

Crubit supports bidirectional function callbacks across C++ and Rust:

Rust bindings for C++ callbacks (absl::AnyInvocable): Calling C++ APIs that take or return absl::AnyInvocable from Rust.
C++ bindings for Rust callbacks (Fn, FnMut, FnOnce): Calling Rust APIs that accept closures from C++ via rs::FnRef and rs::Fn.

Type mappings

Direction	C++ type	Rust type	Semantics
C++ $\to$ Rust	`absl::AnyInvocable<R(...) const>`	`Box<dyn Fn(...) -> R + Send + Sync>`	Multiple invocations
C++ $\to$ Rust	`absl::AnyInvocable<R(...) &&>`	`Box<dyn FnOnce(...) -> R + Send>`	Single-use invocation
C++ $\to$ Rust	`absl::AnyInvocable<R(...)>` (non-const)	`Box<dyn Fn(...) -> R + Send + Sync>`	Promoted to `Fn`
Rust $\to$ C++	`rs::FnRef<R(...) const>`	`&dyn Fn(...) -> R` / `impl Fn(...) -> R`	Borrowed, zero allocation
Rust $\to$ C++	`rs::FnRef<R(...)>`	`&mut dyn FnMut(...) -> R` / `impl FnMut(...) -> R`	Borrowed mutable, zero allocation
Rust $\to$ C++	`rs::Fn<R(...) const>`	`Box<dyn Fn(...) -> R>`	Owned
Rust $\to$ C++	`rs::Fn<R(...)>`	`Box<dyn FnMut(...) -> R>`	Owned mutable
Rust $\to$ C++	`rs::Fn<R(...) &&>`	`Box<dyn FnOnce(...) -> R>` / `impl FnOnce(...) + 'static`	Owned, single-use (`std::move`)
Rust $\to$ C++	`rs::DynCallable<Sig>`	(Rust callback in struct)	Implicitly converts to `absl::AnyInvocable<Sig>`

Examples

Runnable, tested examples for both directions are in examples/types/callbacks/.