Turbopack: add experimental.turbopackClient/ServerSideNestedAsyncChunking (#85827)

### What?

Allow to configure nested vs flat async chunking with a new config option.

```
experimental: {
  turbopackClientSideNestedAsyncChunking: false,
  turbopackServerSideNestedAsyncChunking: true,
}
```

Author: sokra

crates/next-api/src/project.rs

@@ -1119,6 +1119,9 @@ impl Project {
             source_maps: self.next_config().client_source_maps(self.next_mode()),
             no_mangling: self.no_mangling(),
             scope_hoisting: self.next_config().turbo_scope_hoisting(self.next_mode()),
+            nested_async_chunking: self
+                .next_config()
+                .turbo_nested_async_chunking(self.next_mode(), true),
             debug_ids: self.next_config().turbopack_debug_ids(),
             should_use_absolute_url_references: self.next_config().inline_css(),
         }))
@@ -1141,6 +1144,9 @@ impl Project {
             turbo_source_maps: self.next_config().server_source_maps(),
             no_mangling: self.no_mangling(),
             scope_hoisting: self.next_config().turbo_scope_hoisting(self.next_mode()),
+            nested_async_chunking: self
+                .next_config()
+                .turbo_nested_async_chunking(self.next_mode(), false),
             debug_ids: self.next_config().turbopack_debug_ids(),
             client_root: self.client_relative_path().owned().await?,
             asset_prefix: self.next_config().computed_asset_prefix().owned().await?,
@@ -1169,6 +1175,9 @@ impl Project {
             turbo_source_maps: self.next_config().server_source_maps(),
             no_mangling: self.no_mangling(),
             scope_hoisting: self.next_config().turbo_scope_hoisting(self.next_mode()),
+            nested_async_chunking: self
+                .next_config()
+                .turbo_nested_async_chunking(self.next_mode(), false),
             client_root: self.client_relative_path().owned().await?,
             asset_prefix: self.next_config().computed_asset_prefix().owned().await?,
         };

crates/next-core/src/next_client/context.rs

@@ -426,6 +426,7 @@ pub struct ClientChunkingContextOptions {
     pub source_maps: Vc<bool>,
     pub no_mangling: Vc<bool>,
     pub scope_hoisting: Vc<bool>,
+    pub nested_async_chunking: Vc<bool>,
     pub debug_ids: Vc<bool>,
     pub should_use_absolute_url_references: Vc<bool>,
 }
@@ -448,6 +449,7 @@ pub async fn get_client_chunking_context(
         source_maps,
         no_mangling,
         scope_hoisting,
+        nested_async_chunking,
         debug_ids,
         should_use_absolute_url_references,
     } = options;
@@ -484,7 +486,8 @@ pub async fn get_client_chunking_context(
     .export_usage(*export_usage.await?)
     .module_id_strategy(module_id_strategy.to_resolved().await?)
     .debug_ids(*debug_ids.await?)
-    .should_use_absolute_url_references(*should_use_absolute_url_references.await?);
+    .should_use_absolute_url_references(*should_use_absolute_url_references.await?)
+    .nested_async_availability(*nested_async_chunking.await?);
 
     if next_mode.is_development() {
         builder = builder
@@ -510,7 +513,6 @@ pub async fn get_client_chunking_context(
                 },
             )
             .use_content_hashing(ContentHashing::Direct { length: 16 })
-            .nested_async_availability(true)
             .module_merging(*scope_hoisting.await?);
     }
 

crates/next-core/src/next_config.rs

@@ -886,6 +886,8 @@ pub struct ExperimentalConfig {
     turbopack_source_maps: Option<bool>,
     turbopack_tree_shaking: Option<bool>,
     turbopack_scope_hoisting: Option<bool>,
+    turbopack_client_side_nested_async_chunking: Option<bool>,
+    turbopack_server_side_nested_async_chunking: Option<bool>,
     turbopack_import_type_bytes: Option<bool>,
     turbopack_use_system_tls_certs: Option<bool>,
     /// Disable automatic configuration of the sass loader.
@@ -1790,6 +1792,29 @@ impl NextConfig {
         }))
     }
 
+    #[turbo_tasks::function]
+    pub async fn turbo_nested_async_chunking(
+        &self,
+        mode: Vc<NextMode>,
+        client_side: bool,
+    ) -> Result<Vc<bool>> {
+        let option = if client_side {
+            self.experimental
+                .turbopack_client_side_nested_async_chunking
+        } else {
+            self.experimental
+                .turbopack_server_side_nested_async_chunking
+        };
+        Ok(Vc::cell(if let Some(value) = option {
+            value
+        } else {
+            match *mode.await? {
+                NextMode::Development => false,
+                NextMode::Build => client_side,
+            }
+        }))
+    }
+
     #[turbo_tasks::function]
     pub async fn turbopack_import_type_bytes(&self) -> Vc<bool> {
         Vc::cell(

crates/next-core/src/next_edge/context.rs

@@ -208,6 +208,7 @@ pub struct EdgeChunkingContextOptions {
     pub turbo_source_maps: Vc<bool>,
     pub no_mangling: Vc<bool>,
     pub scope_hoisting: Vc<bool>,
+    pub nested_async_chunking: Vc<bool>,
     pub client_root: FileSystemPath,
     pub asset_prefix: RcStr,
 }
@@ -229,6 +230,7 @@ pub async fn get_edge_chunking_context_with_client_assets(
         turbo_source_maps,
         no_mangling,
         scope_hoisting,
+        nested_async_chunking,
         client_root,
         asset_prefix,
     } = options;
@@ -259,7 +261,8 @@ pub async fn get_edge_chunking_context_with_client_assets(
         SourceMapsType::None
     })
     .module_id_strategy(module_id_strategy.to_resolved().await?)
-    .export_usage(*export_usage.await?);
+    .export_usage(*export_usage.await?)
+    .nested_async_availability(*nested_async_chunking.await?);
 
     if !next_mode.is_development() {
         builder = builder
@@ -300,6 +303,7 @@ pub async fn get_edge_chunking_context(
         turbo_source_maps,
         no_mangling,
         scope_hoisting,
+        nested_async_chunking,
         client_root,
         asset_prefix,
     } = options;
@@ -336,7 +340,8 @@ pub async fn get_edge_chunking_context(
         SourceMapsType::None
     })
     .module_id_strategy(module_id_strategy.to_resolved().await?)
-    .export_usage(*export_usage.await?);
+    .export_usage(*export_usage.await?)
+    .nested_async_availability(*nested_async_chunking.await?);
 
     if !next_mode.is_development() {
         builder = builder

crates/next-core/src/next_server/context.rs

@@ -1000,6 +1000,7 @@ pub struct ServerChunkingContextOptions {
     pub turbo_source_maps: Vc<bool>,
     pub no_mangling: Vc<bool>,
     pub scope_hoisting: Vc<bool>,
+    pub nested_async_chunking: Vc<bool>,
     pub debug_ids: Vc<bool>,
     pub client_root: FileSystemPath,
     pub asset_prefix: RcStr,
@@ -1022,6 +1023,7 @@ pub async fn get_server_chunking_context_with_client_assets(
         turbo_source_maps,
         no_mangling,
         scope_hoisting,
+        nested_async_chunking,
         debug_ids,
         client_root,
         asset_prefix,
@@ -1058,7 +1060,8 @@ pub async fn get_server_chunking_context_with_client_assets(
     .module_id_strategy(module_id_strategy.to_resolved().await?)
     .export_usage(*export_usage.await?)
     .file_tracing(next_mode.is_production())
-    .debug_ids(*debug_ids.await?);
+    .debug_ids(*debug_ids.await?)
+    .nested_async_availability(*nested_async_chunking.await?);
 
     builder = builder.source_map_source_type(if next_mode.is_development() {
         SourceMapSourceType::AbsoluteFileUri
@@ -1106,6 +1109,7 @@ pub async fn get_server_chunking_context(
         turbo_source_maps,
         no_mangling,
         scope_hoisting,
+        nested_async_chunking,
         debug_ids,
         client_root,
         asset_prefix,
@@ -1142,7 +1146,8 @@ pub async fn get_server_chunking_context(
     .module_id_strategy(module_id_strategy.to_resolved().await?)
     .export_usage(*export_usage.await?)
     .file_tracing(next_mode.is_production())
-    .debug_ids(*debug_ids.await?);
+    .debug_ids(*debug_ids.await?)
+    .nested_async_availability(*nested_async_chunking.await?);
 
     if next_mode.is_development() {
         builder = builder.source_map_source_type(SourceMapSourceType::AbsoluteFileUri);

packages/next/src/server/config-schema.ts

@@ -301,6 +301,8 @@ export const experimentalSchema = {
   turbopackTreeShaking: z.boolean().optional(),
   turbopackRemoveUnusedExports: z.boolean().optional(),
   turbopackScopeHoisting: z.boolean().optional(),
+  turbopackClientSideNestedAsyncChunking: z.boolean().optional(),
+  turbopackServerSideNestedAsyncChunking: z.boolean().optional(),
   turbopackImportTypeBytes: z.boolean().optional(),
   turbopackUseSystemTlsCerts: z.boolean().optional(),
   turbopackUseBuiltinBabel: z.boolean().optional(),

packages/next/src/server/config-shared.ts

@@ -413,6 +413,18 @@ export interface ExperimentalConfig {
    */
   turbopackScopeHoisting?: boolean
 
+  /**
+   * Enable nested async chunking for client side assets. Defaults to true in build mode and false in dev mode.
+   * This optimization computes all possible paths through dynamic imports in the applications to figure out the modules needed at dynamic imports for every path.
+   */
+  turbopackClientSideNestedAsyncChunking?: boolean
+
+  /**
+   * Enable nested async chunking for server side assets. Defaults to false in dev and build mode.
+   * This optimization computes all possible paths through dynamic imports in the applications to figure out the modules needed at dynamic imports for every path.
+   */
+  turbopackServerSideNestedAsyncChunking?: boolean
+
   /**
    * Enable filesystem cache for the turbopack dev server.
    */