Turbo Tasks: Tasks and Functions

A Task is an instance of a function along with its arguments. These tasks are the fundamental units of work within the build system.

Defining Tasks

Tasks are defined using Rust functions annotated with the #[turbo_tasks::function] macro.

#![allow(unused)]
fn main() {
#[turbo_tasks::function]
fn add(a: i32, b: i32) -> Vc<Something> {
    // Task implementation goes here...
}
}

Tasks can be implemented as either a synchronous or asynchronous function.
Arguments must implement the TaskInput trait. Usually these are primitives or types wrapped in Vc<T>. For details, see the task inputs documentation.
The external signature of a task always returns Vc<T>. This can be thought of as a lazily-executed promise to a value of type T.
Generics (type or lifetime parameters) are not supported in task functions.

External Signature Rewriting

The #[turbo_tasks::function] macro rewrites the arguments and return values of functions. The rewritten function signature is referred to as the “external signature”.

Argument Rewrite Rule

Function arguments with the ResolvedVc<T> type are rewritten to Vc<T>.
- The value cell is automatically resolved when the function is called. This reduces the work needed to convert between Vc<T> and ResolvedVc<T> types.
- This rewrite applies for ResolvedVc<T> types nested inside of Option<ResolvedVc<T>> and Vec<ResolvedVc<T>>. For more details, refer to the FromTaskInput trait.
Method arguments of &self are rewritten to self: Vc<Self>.

Return Type Rewrite Rules

A return type of Result<Vc<T>> is rewritten into Vc<T>.
- The Result<Vc<T>> return type allows for idiomatic use of the ? operator inside of task functions.
A function with no return type is rewritten to return Vc<()> instead of ().
The impl Future<Output = Vc<T>> type implicitly returned by an async function is flattened into the Vc<T> type, which represents a lazy and re-executable version of the Future.

Some of this logic is represented by the TaskOutput trait and its associated Return type.

External Signature Example

As an example, the method

#![allow(unused)]
fn main() {
#[turbo_tasks::function]
async fn foo(
    &self,
    a: i32,
    b: Vc<i32>,
    c: ResolvedVc<i32>,
    d: Option<Vec<ResolvedVc<i32>>>,
) -> Result<Vc<i32>> {
    // ...
}
}

will have an external signature of

#![allow(unused)]
fn main() {
fn foo(
    self: Vc<Self>,           // was: &self
    a: i32,
    b: Vc<i32>,
    c: Vc<i32>,               // was: ResolvedVc<i32>
    d: Option<Vec<Vc<i32>>>,  // was: Option<Vec<ResolvedVc<i32>>>
) -> Vc<i32>;                 // was: impl Future<Output = Result<Vc<i32>>>
}

Methods with a Self Argument

Tasks can be methods associated with a value or a trait implementation using the arbitrary_self_types nightly compiler feature.

Inherent Implementations

#![allow(unused)]
fn main() {
#[turbo_tasks::value_impl]
impl Something {
    #[turbo_tasks::function]
    fn method(self: Vc<Self>, a: i32) -> Vc<SomethingElse> {
        // Receives the full `Vc<Self>` type, which we must `.await` to get a
        // `ReadRef<Self>`.
        vdbg!(self.await?.some_field);

        // The `Vc` type is useful for calling other methods declared on
        // `Vc<Self>`, e.g.:
        self.method_resolved(a)
    }

    #[turbo_tasks::function]
    fn method_resolved(self: ResolvedVc<Self>, a: i32) -> Vc<SomethingElse> {
        // Same as above, but receives a `ResolvedVc`, which can be `.await`ed
        // to a `ReadRef` or dereferenced (implicitly or with `*`) to `Vc`.
        vdbg!(self.await?.some_field);

        // The `ResolvedVc<Self>` type can be used to call other methods
        // declared on `Vc<Self>`, e.g.:
        self.method_ref(a)
    }

    #[turbo_tasks::function]
    fn method_ref(&self, a: i32) -> Vc<SomethingElse> {
        // As a convenience, receives the fully resolved version of `self`. This
        // does not require `.await`ing to read.
        //
        // It can access fields on the struct/enum and call methods declared on
        // `Self`, but it cannot call other methods declared on `Vc<Self>`
        // (without cloning the value and re-wrapping it in a `Vc`).
        Vc::cell(SomethingElse::new(self.some_field, a))
    }
}
}

Declaration Location: The methods are defined on Vc<T> (i.e. Vc<Something>::method and Vc<Something>::method2), not on the inner type.
&self Syntactic Sugar: The &self argument of a #[turbo_tasks::function] implicitly reads the value from self: Vc<Self>.
External Signature Rewriting: All of the signature rewrite rules apply here. self can be ResolvedVc<T>. async and Result<Vc<T>> return types are supported.

Trait Implementations

#![allow(unused)]
fn main() {
#[turbo_tasks::value_impl]
impl Trait for Something {
    #[turbo_tasks::function]
    fn method(self: Vc<Self>, a: i32) -> Vc<SomethingElse> {
        // Trait method implementation...
        //
        // `self: ResolvedVc<Self>` and `&self` are also valid argument types!
    }
}
}

For traits, only the external signature (after rewriting) must align with the trait definition.

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