Add emitting optional fields

packages/protobuf/README.md

@@ -54,6 +54,39 @@ If set to `true`, this emitter will not write any files. It will still validate
 
 By default, the emitter will create `message` declarations for any models in a namespace decorated with `@package` that have an `@field` decorator on every property. If this option is set to true, this behavior will be disabled, and only messages that are explicitly decorated with `@message` or that are reachable from a service operation will be emitted.
 
+### `emit-optional`
+
+**Type:** `boolean`
+
+**Default:** `false`
+
+When enabled, fields marked as optional in TypeSpec (using `?` syntax or `@optional` decorator) will be emitted with the `optional` keyword in proto3. By default, optional fields are not marked as such in the output.
+
+Example:
+
+```typespec
+model Message {
+  @field(1) requiredField: string;
+  @field(2) optionalField?: int32;
+}
+```
+
+With `emit-optional: false` (default):
+```protobuf
+message Message {
+  string requiredField = 1;
+  int32 optionalField = 2;
+}
+```
+
+With `emit-optional: true`:
+```protobuf
+message Message {
+  string requiredField = 1;
+  optional int32 optionalField = 2;
+}
+```
+
 ## Decorators
 
 ### TypeSpec.Protobuf

packages/protobuf/src/ast.ts

@@ -262,6 +262,10 @@ export interface ProtoFieldDeclaration extends ProtoDeclarationCommon {
    * Whether or not the field is repeated (i.e. an array).
    */
   repeated?: boolean;
+  /**
+   * Whether or not the field is optional.
+   */
+  optional?: boolean;
   options?: Partial<DefaultFieldOptions>;
   type: ProtoType;
   index: number;

packages/protobuf/src/lib.ts

@@ -26,6 +26,14 @@ export interface ProtobufEmitterOptions {
    * in an interface decoarated with `@service` will be emitted.
    */
   "omit-unreachable-types"?: boolean;
+
+  /**
+   * Emit optional fields.
+   *
+   * When enabled, fields marked as optional in TypeSpec (using `?` or `@optional`) will be emitted with the
+   * `optional` keyword in proto3. By default, optional fields are not marked as such in the output.
+   */
+  "emit-optional"?: boolean;
 }
 
 const EmitterOptionsSchema: JSONSchemaType<ProtobufEmitterOptions> = {
@@ -44,6 +52,12 @@ const EmitterOptionsSchema: JSONSchemaType<ProtobufEmitterOptions> = {
       description:
         "By default, the emitter will create `message` declarations for any models in a namespace decorated with `@package` that have an `@field` decorator on every property. If this option is set to true, this behavior will be disabled, and only messages that are explicitly decorated with `@message` or that are reachable from a service operation will be emitted.",
     },
+    "emit-optional": {
+      type: "boolean",
+      nullable: true,
+      description:
+        "When enabled, fields marked as optional in TypeSpec (using `?` or `@optional`) will be emitted with the `optional` keyword in proto3. By default, optional fields are not marked as such in the output.",
+    },
   },
   required: [],
 };

packages/protobuf/src/transform/index.ts

@@ -833,6 +833,11 @@ function tspToProto(program: Program, emitterOptions: ProtobufEmitterOptions): P
     // Determine if the property type is an array
     if (isArray(property.type)) field.repeated = true;
 
+    // Determine if the property is optional (when emit-optional option is enabled)
+    if (emitterOptions["emit-optional"] && property.optional) {
+      field.optional = true;
+    }
+
     return field;
   }
 

packages/protobuf/src/write.ts

@@ -178,7 +178,7 @@ function writeVariant(decl: ProtoEnumVariantDeclaration, indentLevel: number): I
 }
 
 function writeField(decl: ProtoFieldDeclaration, indentLevel: number): Iterable<string> {
-  const prefix = decl.repeated ? "repeated " : "";
+  const prefix = decl.repeated ? "repeated " : decl.optional ? "optional " : "";
   const output = prefix + `${writeType(decl.type)} ${decl.name} = ${decl.index};`;
 
   return writeDocumentationCommentFlexible(decl, output, indentLevel);

packages/protobuf/test/scenarios/optional-disabled/input/main.tsp

@@ -0,0 +1,20 @@
+import "@typespec/protobuf";
+
+using Protobuf;
+
+@package()
+namespace Test;
+
+@Protobuf.service
+interface Service {
+  foo(...Input): Output;
+}
+
+model Input {
+  @field(1) testInputField: string;
+}
+
+model Output {
+  @field(1) testOutputField?: int32;
+  @field(2) secondField?: string;
+}

packages/protobuf/test/scenarios/optional-disabled/options.json

@@ -0,0 +1,3 @@
+{
+  "emit-optional": false
+}

packages/protobuf/test/scenarios/optional-disabled/output/@typespec/protobuf/main.proto

@@ -0,0 +1,16 @@
+// Generated by Microsoft TypeSpec
+
+syntax = "proto3";
+
+message Input {
+  string testInputField = 1;
+}
+
+message Output {
+  int32 testOutputField = 1;
+  string secondField = 2;
+}
+
+service Service {
+  rpc Foo(Input) returns (Output);
+}

packages/protobuf/test/scenarios/optional/input/main.tsp

@@ -0,0 +1,20 @@
+import "@typespec/protobuf";
+
+using Protobuf;
+
+@package()
+namespace Test;
+
+@Protobuf.service
+interface Service {
+  foo(...Input): Output;
+}
+
+model Input {
+  @field(1) testInputField: string;
+}
+
+model Output {
+  @field(1) testOutputField?: int32;
+  @field(2) secondField?: string;
+}

packages/protobuf/test/scenarios/optional/options.json

@@ -0,0 +1,3 @@
+{
+  "emit-optional": true
+}

packages/protobuf/test/scenarios/optional/output/@typespec/protobuf/main.proto

@@ -0,0 +1,16 @@
+// Generated by Microsoft TypeSpec
+
+syntax = "proto3";
+
+message Input {
+  string testInputField = 1;
+}
+
+message Output {
+  optional int32 testOutputField = 1;
+  optional string secondField = 2;
+}
+
+service Service {
+  rpc Foo(Input) returns (Output);
+}