Rust has two kinds of types commonly referred to as
- the first is
std::future::Futurefrom Rust’s standard library.
- the second is
futures::future::Futurefrom the futures-rs crate.
The future defined in the futures-rs crate was the original implementation of the type. To enable the
async/await syntax, the core Future trait was moved into Rust’s standard library and became
std::future::Future. In some sense, the
std::future::Future can be seen as a minimal subset of
It is critical to understand the difference between
futures::future::Future, and the approach that
async-std takes towards them. In itself,
std::future::Future is not something you want to interact with as a user—except by calling
.await on it. The inner workings of
std::future::Future are mostly of interest to people implementing
Future. Make no mistake—this is very useful! Most of the functionality that used to be defined on
Future itself has been moved to an extension trait called
FuturesExt. From this information, you might be able to infer that the
futures library serves as an extension to the core Rust async features.
In the same tradition as
async-std re-exports the core
std::future::Future type. You can actively opt into the extensions provided by the
futures-preview crate by adding it to your
Cargo.toml and importing
async-std aims to be a stable and reliable library, at the level of the Rust standard library. This also means that we don't rely on the
futures library for our interface. Yet, we appreciate that many users have come to like the conveniences that
futures-rs brings. For that reason,
async-std implements all
futures traits for its types.
Luckily, the approach from above gives you full flexibility. If you care about stability a lot, you can just use
async-std as is. If you prefer the
futures library interfaces, you link those in. Both uses are first class.
There’s some support functions that we see as important for working with futures of any kind. These can be found in the
async_std::future module and are covered by our stability guarantees.
Due to limitations of the Rust compiler, those are currently implemented in
async_std, but cannot be implemented by users themselves.