add support for named unions in protobuf emitter

docs/emitters/protobuf/reference/decorators.md

@@ -25,7 +25,7 @@ The field index of a Protobuf message must:
 
 #### Target
 
-`ModelProperty`
+`ModelProperty | UnionVariant`
 
 #### Parameters
 

packages/compiler/src/core/type-utils.ts

@@ -18,6 +18,7 @@ import {
   TemplatedType,
   Type,
   TypeMapper,
+  Union,
   UnknownType,
   Value,
   VoidType,
@@ -164,7 +165,7 @@ export function isGlobalNamespace(
  * @returns {boolean}
  */
 export function isDeclaredInNamespace(
-  type: Model | Operation | Interface | Namespace | Enum,
+  type: Model | Operation | Interface | Namespace | Enum | Union,
   namespace: Namespace,
   options: { recursive?: boolean } = { recursive: true }
 ) {

packages/protobuf/README.md

@@ -77,7 +77,7 @@ The field index of a Protobuf message must:
 
 ##### Target
 
-`ModelProperty`
+`ModelProperty | UnionVariant`
 
 ##### Parameters
 

packages/protobuf/generated-defs/TypeSpec.Protobuf.ts

@@ -5,6 +5,7 @@ import type {
   Namespace,
   Operation,
   Type,
+  UnionVariant,
 } from "@typespec/compiler";
 
 /**
@@ -50,7 +51,7 @@ export type MessageDecorator = (context: DecoratorContext, target: Type) => void
  */
 export type FieldDecorator = (
   context: DecoratorContext,
-  target: ModelProperty,
+  target: ModelProperty | UnionVariant,
   index: number
 ) => void;
 

packages/protobuf/lib/proto.tsp

@@ -180,7 +180,7 @@ extern dec message(target: {});
  * }
  * ```
  */
-extern dec field(target: TypeSpec.Reflection.ModelProperty, index: valueof uint32);
+extern dec field(target: TypeSpec.Reflection.ModelProperty | TypeSpec.Reflection.UnionVariant, index: valueof uint32);
 
 /**
  * Reserve a field index, range, or name. If a field definition collides with a reservation, the emitter will produce

packages/protobuf/src/lib.ts

@@ -102,11 +102,12 @@ export const TypeSpecProtobufLibrary = createTypeSpecLibrary({
     "unsupported-field-type": {
       severity: "error",
       messages: {
-        unconvertible: paramMessage`cannot convert a ${"type"} to a protobuf type (only intrinsic types and models are supported)`,
+        unconvertible: paramMessage`cannot convert a ${"type"} to a protobuf type (only intrinsic types, models and unions are supported)`,
         "unknown-intrinsic": paramMessage`no known protobuf scalar for intrinsic type ${"name"}`,
         "unknown-scalar": paramMessage`no known protobuf scalar for TypeSpec scalar type ${"name"}`,
         "recursive-map": "a protobuf map's 'value' type may not refer to another map",
-        union: "a message field's type may not be a union",
+        "union-variant-map": "a protobuf oneof cannot directly include a map",
+        "union-variant-array": "a protobuf oneof cannot directly include a repeated field"
       },
     },
     "namespace-collision": {
@@ -124,6 +125,14 @@ export const TypeSpecProtobufLibrary = createTypeSpecLibrary({
           "the first variant of an enum must be set to zero to be used in a Protobuf message",
       },
     },
+    "unconvertible-union": {
+      severity: "error",
+      messages: {
+        "anonymous": "anonymous unions cannot be used in Protobuf messages",
+        "anonymous-variant": "union variants without identifiers cannot be used in Protobuf messages",
+        "no-variants": "a union must contain at least one variant to be used in a Protobuf message"
+      }
+    },
     "nested-array": {
       severity: "error",
       messages: {

packages/protobuf/src/proto.ts

@@ -14,6 +14,7 @@ import {
   resolvePath,
   StringLiteral,
   Type,
+  UnionVariant,
 } from "@typespec/compiler";
 
 import {
@@ -148,7 +149,7 @@ export const $message: MessageDecorator = (ctx: DecoratorContext, target: Type)
  */
 export const $field: FieldDecorator = (
   ctx: DecoratorContext,
-  target: ModelProperty,
+  target: ModelProperty | UnionVariant,
   fieldIndex: number
 ) => {
   if (!Number.isInteger(fieldIndex) || fieldIndex <= 0) {