perf(turbo-tasks): Use native async traits for TaskInput (not the asy…

…nc_trait macro) (vercel#70084)

## What?

Use rustc's relatively new native support for async traits: https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html
Instead of the `async_trait` macro: https://docs.rs/async-trait/latest/async_trait/

The main differences are:

- `async_trait` supports dynamic dispatch. The compiler-native version does not yet.
- The compiler-native version often requires using messier `-> impl Future<...> + Send + '_` return types to declare additional bounds (usually `Send`).
- `async_trait` forcibly boxes your returned `Future<...>` as a `Box<dyn Future<...>>`.

## Why?

- We don't need dynamic dispatch.
- Most of the `TaskInput` code is macro-generated, so the uglier `impl Future` syntax doesn't matter.
- This is a hot codepath, and there's overhead associated with boxing a `dyn Future`. Removing the dynamic dispatch avoids pointer indirection and gives the compiler more opportunities to inline code.

## Benchmarks

No measurable change.

```
cargo bench -p turbopack-bench -p turbopack-cli -- "hmr_to_eval/Turbopack CSR"
```

```
bench_hmr_to_eval/Turbopack CSR/1000 modules
                        time:   [15.439 ms 15.633 ms 15.978 ms]
                        change: [-1.3769% +1.0017% +3.3534%] (p = 0.46 > 0.05)
                        No change in performance detected.
```


```
TURBOPACK_BENCH_STRESS=yes cargo bench -p turbo-tasks-memory 500
```

```
turbo_tasks_memory_stress/fibonacci/500
                        time:   [539.45 ms 544.10 ms 548.41 ms]
                        thrpt:  [228.39 Kelem/s 230.20 Kelem/s 232.18 Kelem/s]
                 change:
                        time:   [-1.3718% -0.1154% +1.1538%] (p = 0.87 > 0.05)
                        thrpt:  [-1.1406% +0.1155% +1.3909%]
                        No change in performance detected.
Found 1 outliers among 20 measurements (5.00%)
  1 (5.00%) low mild
```

turbopack/crates/turbo-tasks-macros/src/derive/task_input_macro.rs

@@ -168,8 +168,17 @@ pub fn derive_task_input(input: TokenStream) -> TokenStream {
 
             #[allow(non_snake_case)]
             #[allow(unreachable_code)] // This can occur for enums with no variants.
-            async fn resolve(&self) -> turbo_tasks::Result<Self> {
-                #resolve_impl
+            #[allow(clippy::manual_async_fn)] // some impls need the manual return type to work :(
+            fn resolve(
+                &self,
+            ) -> impl
+                ::std::future::Future<Output = turbo_tasks::Result<Self>> +
+                ::std::marker::Send +
+                '_
+            {
+                async move {
+                    #resolve_impl
+                }
             }
         }
     }

turbopack/crates/turbo-tasks/src/task/task_input.rs

@@ -1,7 +1,6 @@
-use std::{any::Any, fmt::Debug, hash::Hash};
+use std::{any::Any, fmt::Debug, future::Future, hash::Hash};
 
 use anyhow::Result;
-use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 
 use crate::{
@@ -12,10 +11,9 @@ use crate::{
 /// [`#[turbo_tasks::function]`][crate::function] argument.
 ///
 /// See also [`ConcreteTaskInput`].
-#[async_trait]
 pub trait TaskInput: Send + Sync + Clone + Debug + PartialEq + Eq + Hash {
-    async fn resolve(&self) -> Result<Self> {
-        Ok(self.clone())
+    fn resolve(&self) -> impl Future<Output = Result<Self>> + Send + '_ {
+        async { Ok(self.clone()) }
     }
     fn is_resolved(&self) -> bool {
         true
@@ -28,7 +26,6 @@ pub trait TaskInput: Send + Sync + Clone + Debug + PartialEq + Eq + Hash {
 macro_rules! impl_task_input {
     ($($t:ty),*) => {
         $(
-            #[async_trait]
             impl TaskInput for $t {}
         )*
     };
@@ -48,7 +45,6 @@ impl_task_input! {
     ValueTypeId
 }
 
-#[async_trait]
 impl<T> TaskInput for Vec<T>
 where
     T: TaskInput,
@@ -70,7 +66,6 @@ where
     }
 }
 
-#[async_trait]
 impl<T> TaskInput for Option<T>
 where
     T: TaskInput,
@@ -97,7 +92,6 @@ where
     }
 }
 
-#[async_trait]
 impl<T> TaskInput for Vc<T>
 where
     T: Send,
@@ -220,7 +214,6 @@ impl<'de, T> Deserialize<'de> for TransientInstance<T> {
 
 macro_rules! tuple_impls {
     ( $( $name:ident )+ ) => {
-        #[async_trait]
         impl<$($name: TaskInput),+> TaskInput for ($($name,)+)
         where $($name: TaskInput),+
         {