make with_layer return Vc<Self> (#5563)

### Description

Using the new ability to return `Vc<Self>`

Co-authored-by: Alex Kirszenberg <alexkirsz@users.noreply.github.com>

crates/turbopack-build/src/chunking_context.rs

@@ -275,10 +275,10 @@ impl ChunkingContext for BuildChunkingContext {
     }
 
     #[turbo_tasks::function]
-    async fn with_layer(self: Vc<Self>, layer: String) -> Result<Vc<Box<dyn ChunkingContext>>> {
+    async fn with_layer(self: Vc<Self>, layer: String) -> Result<Vc<Self>> {
         let mut context = self.await?.clone_value();
         context.layer = (!layer.is_empty()).then(|| layer.to_string());
-        Ok(Vc::upcast(BuildChunkingContext::new(Value::new(context))))
+        Ok(BuildChunkingContext::new(Value::new(context)))
     }
 
     #[turbo_tasks::function]

crates/turbopack-core/src/chunk/chunking_context.rs

@@ -49,7 +49,7 @@ pub trait ChunkingContext {
         Vc::cell("".to_string())
     }
 
-    fn with_layer(self: Vc<Self>, layer: String) -> Vc<Box<dyn ChunkingContext>>;
+    fn with_layer(self: Vc<Self>, layer: String) -> Vc<Self>;
 
     fn chunk_group(self: Vc<Self>, entry: Vc<Box<dyn Chunk>>) -> Vc<OutputAssets>;
 

crates/turbopack-dev/src/chunking_context.rs

@@ -301,10 +301,10 @@ impl ChunkingContext for DevChunkingContext {
     }
 
     #[turbo_tasks::function]
-    async fn with_layer(self: Vc<Self>, layer: String) -> Result<Vc<Box<dyn ChunkingContext>>> {
+    async fn with_layer(self: Vc<Self>, layer: String) -> Result<Vc<Self>> {
         let mut context = self.await?.clone_value();
         context.layer = (!layer.is_empty()).then(|| layer.to_string());
-        Ok(Vc::upcast(DevChunkingContext::new(Value::new(context))))
+        Ok(DevChunkingContext::new(Value::new(context)))
     }
 
     #[turbo_tasks::function]

crates/turbopack/src/transition/mod.rs

@@ -4,7 +4,7 @@ use std::collections::HashMap;
 
 use anyhow::Result;
 pub use context_transition::ContextTransition;
-use turbo_tasks::{Value, Vc};
+use turbo_tasks::{Value, ValueDefault, Vc};
 use turbopack_core::{
     compile_time_info::CompileTimeInfo, module::Module, reference_type::ReferenceType,
     source::Source,
@@ -88,3 +88,11 @@ pub trait Transition {
 
 #[turbo_tasks::value(transparent)]
 pub struct TransitionsByName(HashMap<String, Vc<Box<dyn Transition>>>);
+
+#[turbo_tasks::value_impl]
+impl ValueDefault for TransitionsByName {
+    #[turbo_tasks::function]
+    fn value_default() -> Vc<Self> {
+        Vc::cell(Default::default())
+    }
+}