docs(turbopack): Move mdbook "tasks" docs over to turbo_tasks::function rustdoc (#91132)

Moving content from: https://turbopack-rust-docs.vercel.sh/turbo-engine/tasks.html

Very few changes, mostly updates to the introduction and links.

Rendered:

![Screenshot 2026-03-09 at 19-07-57 function in turbo_tasks - Rust.png](https://app.graphite.com/user-attachments/assets/ad7eb1d5-aee4-40ea-840d-7f17b59278dc.png)

Author: bgw

turbopack/crates/turbo-tasks/README.md

@@ -11,13 +11,11 @@ Turbo Tasks defines 4 primitives:
 - **[Collectibles][crate::TurboTasks::emit_collectible]:** Values emitted in functions that bubble up the call graph and can be collected in parent functions. Collectibles are deduplicated by [cell id equality].
 
 It defines some derived elements from that:
-- **[Tasks][book-tasks]:** An instance of a function together with its arguments.
+- **Tasks:** An instance of a function together with its arguments.
 - **[`Vc`s ("Value Cells")][`Vc`]:** References to locations associated with tasks where values are stored. The contents of a cell can change after the reexecution of a function due to invalidation. A [`Vc`] can be read to get [a read-only reference][crate::ReadRef] to the stored data, representing a snapshot of that cell at that point in time.
 
 [blog-post]: https://nextjs.org/blog/turbopack-incremental-computation
 [cell id equality]: crate::ResolvedVc#equality--hashing
-[book-cells]: https://turbopack-rust-docs.vercel.sh/turbo-engine/cells.html
-[book-tasks]: https://turbopack-rust-docs.vercel.sh/turbo-engine/tasks.html
 [`Vc`]: crate::Vc
 
 ## Functions and Tasks

turbopack/crates/turbo-tasks/function.md

@@ -0,0 +1,140 @@
+Tasks are created by defining a Rust function annotated with the `#[turbo_tasks::function]` macro and calling it with arguments. Each unique combination of function and arguments create a new task at runtime. Tasks are the fundamental units of work within the build system.
+
+```rust
+#[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>`].
+- The **external signature** of a task always **returns a [`Vc<T>`]** or an [`OperationVc<T>`].
+- **Generics** (type or lifetime parameters) are **not supported** in task functions.
+
+[`Vc<T>`]: crate::Vc
+[`TaskInput`]: crate::TaskInput
+
+## 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>`**.
+
+[`ResolvedVc<T>`]: crate::ResolvedVc
+[`FromTaskInput`]: crate::FromTaskInput
+
+### 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>>`][Future]** type implicitly returned by an async function is **flattened into the `Vc<T>` type**, which implements [`IntoFuture`] and can be `.await`ed.
+
+Some of this logic is represented by the [`TaskOutput`] trait and its associated [`Return`] type.
+
+[`TaskOutput`]: crate::task::TaskOutput
+[`Return`]: crate::task::TaskOutput::Return
+
+### External Signature Example
+
+As an example, the method
+
+```rust
+#[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
+
+```rust
+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 and Self
+
+Tasks can be methods associated with a value or a trait implementation using the [`arbitrary_self_types` nightly compiler feature][self-types].
+
+[self-types]: https://github.com/rust-lang/rfcs/blob/master/text/3519-arbitrary-self-types-v2.md
+
+### Inherent Implementations
+
+```rust
+#[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
+
+```rust
+#[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.

turbopack/crates/turbo-tasks/src/lib.rs

@@ -71,7 +71,7 @@ pub use anyhow::{Error, Result};
 use auto_hash_map::AutoSet;
 use rustc_hash::FxHasher;
 pub use shrink_to_fit::ShrinkToFit;
-pub use turbo_tasks_macros::{TaskInput, function, turbobail, turbofmt, value_impl};
+pub use turbo_tasks_macros::{TaskInput, turbobail, turbofmt, value_impl};
 
 pub use crate::{
     capture_future::TurboTasksPanic,
@@ -159,6 +159,10 @@ macro_rules! fxindexset {
     };
 }
 
+#[doc = include_str!("../function.md")]
+#[rustfmt::skip]
+pub use turbo_tasks_macros::function;
+
 /// Implements [`VcValueType`] for the given `struct` or `enum`. These value types can be used
 /// inside of a "value cell" as [`Vc<...>`][Vc].
 ///