cm: implement JSON [un]marshaling for List types

.github/workflows/test.yaml

@@ -41,7 +41,7 @@ jobs:
   test-go:
     name: Test with Go
     runs-on: ubuntu-latest
-    timeout-minutes: 5
+    timeout-minutes: 15
     strategy:
       matrix:
         go-version: ["1.22", "1.23"]

cm/CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 ### Added
 
 - Initial support for Component Model [async](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Async.md) types `stream`, `future`, and `error-context`.
+- Initial support for JSON serialization of WIT types, starting with `list` and `record`.
 
 ## [v0.1.0] — 2024-12-14
 

cm/list.go

@@ -1,6 +1,10 @@
 package cm
 
-import "unsafe"
+import (
+	"bytes"
+	"encoding/json"
+	"unsafe"
+)
 
 // List represents a Component Model list.
 // The binary representation of list<T> is similar to a Go slice minus the cap field.
@@ -58,3 +62,57 @@ func (l list[T]) Data() *T {
 func (l list[T]) Len() uintptr {
 	return l.len
 }
+
+// MarshalJSON implements json.Marshaler.
+func (l list[T]) MarshalJSON() ([]byte, error) {
+	if l.len == 0 {
+		return []byte("[]"), nil
+	}
+
+	s := l.Slice()
+	var zero T
+	if unsafe.Sizeof(zero) == 1 {
+		// The default Go json.Encoder will marshal []byte as base64.
+		// We override that behavior so all int types have the same serialization format.
+		// []uint8{1,2,3} -> [1,2,3]
+		// []uint32{1,2,3} -> [1,2,3]
+		return json.Marshal(sliceOf(s))
+	}
+	return json.Marshal(s)
+}
+
+type slice[T any] []entry[T]
+
+func sliceOf[S ~[]E, E any](s S) slice[E] {
+	return *(*slice[E])(unsafe.Pointer(&s))
+}
+
+type entry[T any] [1]T
+
+func (v entry[T]) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v[0])
+}
+
+// UnmarshalJSON implements json.Unmarshaler.
+func (l *list[T]) UnmarshalJSON(data []byte) error {
+	if bytes.Equal(data, nullLiteral) {
+		return nil
+	}
+
+	var s []T
+	err := json.Unmarshal(data, &s)
+	if err != nil {
+		return err
+	}
+
+	l.data = unsafe.SliceData([]T(s))
+	l.len = uintptr(len(s))
+
+	return nil
+}
+
+// nullLiteral is the JSON representation of a null literal.
+// By convention, to approximate the behavior of Unmarshal itself,
+// Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.
+// See https://pkg.go.dev/encoding/json#Unmarshaler for more information.
+var nullLiteral = []byte("null")