cm, wit/bindgen: add JSON marshaling for enum types

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 `list`, `enum`, and `record` types.
 - [`wasm-tools`](https://crates.io/crates/wasm-tools) is now vendored as a WebAssembly module, executed using [Wazero](https://wazero.io/). This allows package `wit` and `wit-bindgen-go` to run on any supported platform without needing to separately install `wasm-tools`.
 
 ### Changed

cm/CHANGELOG.md

@@ -7,7 +7,12 @@ 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`.
+- Initial support for JSON serialization of WIT `list`, `enum`, and `record` types.
+- Added `cm.CaseUnmarshaler` helper for text and JSON unmarshaling of `enum` and `variant` types.
+
+### Changed
+
+- Breaking: package `cm`: removed `bool` from `Discriminant` type constraint. It was not used by code generation.
 
 ## [v0.1.0] — 2024-12-14
 

cm/case.go

@@ -0,0 +1,51 @@
+package cm
+
+import "errors"
+
+// CaseUnmarshaler returns an function that can unmarshal text into
+// [variant] or [enum] case T.
+//
+// [enum]: https://component-model.bytecodealliance.org/design/wit.html#enums
+// [variant]: https://component-model.bytecodealliance.org/design/wit.html#variants
+func CaseUnmarshaler[T ~uint8 | ~uint16 | ~uint32](cases []string) func(v *T, text []byte) error {
+	if len(cases) <= linearScanThreshold {
+		return func(v *T, text []byte) error {
+			if len(text) == 0 {
+				return errEmpty
+			}
+			s := string(text)
+			for i := 0; i < len(cases); i++ {
+				if cases[i] == s {
+					*v = T(i)
+					return nil
+				}
+			}
+			return errNoMatchingCase
+		}
+	}
+
+	m := make(map[string]T, len(cases))
+	for i, v := range cases {
+		m[v] = T(i)
+	}
+
+	return func(v *T, text []byte) error {
+		if len(text) == 0 {
+			return errEmpty
+		}
+		s := string(text)
+		c, ok := m[s]
+		if !ok {
+			return errNoMatchingCase
+		}
+		*v = c
+		return nil
+	}
+}
+
+const linearScanThreshold = 16
+
+var (
+	errEmpty          = errors.New("empty text")
+	errNoMatchingCase = errors.New("no matching case")
+)

cm/case_test.go

@@ -0,0 +1,34 @@
+package cm
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestCaseUnmarshaler(t *testing.T) {
+	tests := []struct {
+		name  string
+		cases []string
+	}{
+		{"nil", nil},
+		{"empty slice", []string{}},
+		{"a b c", strings.SplitAfter("abc", "")},
+		{"a b c d e f g", strings.SplitAfter("abcdefg", "")},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			f := CaseUnmarshaler[uint8](tt.cases)
+			for want, c := range tt.cases {
+				var got uint8
+				err := f(&got, []byte(c))
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				if got != uint8(want) {
+					t.Errorf("f(%q): got %d, expected %d", c, got, want)
+				}
+			}
+		})
+	}
+}

cm/variant.go

@@ -3,10 +3,9 @@ package cm
 import "unsafe"
 
 // Discriminant is the set of types that can represent the tag or discriminator of a variant.
-// Use bool for 2-case variant types, result<T>, or option<T> types, uint8 where there are 256 or
-// fewer cases, uint16 for up to 65,536 cases, or uint32 for anything greater.
+// Use uint8 where there are 256 or fewer cases, uint16 for up to 65,536 cases, or uint32 for anything greater.
 type Discriminant interface {
-	bool | uint8 | uint16 | uint32
+	uint8 | uint16 | uint32
 }
 
 // Variant represents a loosely-typed Component Model variant.

cm/variant_test.go

@@ -17,18 +17,18 @@ func TestVariantLayout(t *testing.T) {
 		size   uintptr
 		offset uintptr
 	}{
-		{"variant { string; string }", Variant[bool, string, string]{}, sizePlusAlignOf[string](), ptrSize},
-		{"variant { bool; string }", Variant[bool, string, bool]{}, sizePlusAlignOf[string](), ptrSize},
-		{"variant { string; _ }", Variant[bool, string, string]{}, sizePlusAlignOf[string](), ptrSize},
-		{"variant { _; _ }", Variant[bool, string, struct{}]{}, sizePlusAlignOf[string](), ptrSize},
-		{"variant { u64; u64 }", Variant[bool, uint64, uint64]{}, 16, alignOf[uint64]()},
-		{"variant { u32; u64 }", Variant[bool, uint64, uint32]{}, 16, alignOf[uint64]()},
-		{"variant { u64; u32 }", Variant[bool, uint64, uint32]{}, 16, alignOf[uint64]()},
-		{"variant { u8; u64 }", Variant[bool, uint64, uint8]{}, 16, alignOf[uint64]()},
-		{"variant { u64; u8 }", Variant[bool, uint64, uint8]{}, 16, alignOf[uint64]()},
-		{"variant { u8; u32 }", Variant[bool, uint32, uint8]{}, 8, alignOf[uint32]()},
-		{"variant { u32; u8 }", Variant[bool, uint32, uint8]{}, 8, alignOf[uint32]()},
-		{"variant { [9]u8, u64 }", Variant[bool, [9]byte, uint64]{}, 24, alignOf[uint64]()},
+		{"variant { string; string }", Variant[uint8, string, string]{}, sizePlusAlignOf[string](), ptrSize},
+		{"variant { bool; string }", Variant[uint8, string, bool]{}, sizePlusAlignOf[string](), ptrSize},
+		{"variant { string; _ }", Variant[uint8, string, string]{}, sizePlusAlignOf[string](), ptrSize},
+		{"variant { _; _ }", Variant[uint8, string, struct{}]{}, sizePlusAlignOf[string](), ptrSize},
+		{"variant { u64; u64 }", Variant[uint8, uint64, uint64]{}, 16, alignOf[uint64]()},
+		{"variant { u32; u64 }", Variant[uint8, uint64, uint32]{}, 16, alignOf[uint64]()},
+		{"variant { u64; u32 }", Variant[uint8, uint64, uint32]{}, 16, alignOf[uint64]()},
+		{"variant { u8; u64 }", Variant[uint8, uint64, uint8]{}, 16, alignOf[uint64]()},
+		{"variant { u64; u8 }", Variant[uint8, uint64, uint8]{}, 16, alignOf[uint64]()},
+		{"variant { u8; u32 }", Variant[uint8, uint32, uint8]{}, 8, alignOf[uint32]()},
+		{"variant { u32; u8 }", Variant[uint8, uint32, uint8]{}, 8, alignOf[uint32]()},
+		{"variant { [9]u8, u64 }", Variant[uint8, [9]byte, uint64]{}, 24, alignOf[uint64]()},
 	}
 
 	for _, tt := range tests {