fix: throw when unsupported fields are detected

`required` is a field in proto2 but not in proto3 so throw if it
is encountered while generating `.ts` from `.proto`.

It would be better to detect `proto2` from the `syntax` directive
but it's not in the output of the `pbjs -t json` command.

Fixes #34

packages/protons/src/index.ts

@@ -595,6 +595,10 @@ function defineModule (def: ClassDef): ModuleDef {
           fieldDef.repeated = fieldDef.rule === 'repeated'
           fieldDef.optional = !fieldDef.repeated && fieldDef.options?.proto3_optional === true
           fieldDef.map = fieldDef.keyType != null
+
+          if (fieldDef.rule === 'required') {
+            throw new Error('"required" fields are not allowed in proto3 - please convert your proto2 definitions to proto3')
+          }
         }
       }
 

packages/protons/test/fixtures/proto2.proto

@@ -0,0 +1,5 @@
+syntax = "proto2";
+
+message Message {
+  required string requiredField = 1;
+}

packages/protons/test/unsupported.spec.ts

@@ -0,0 +1,12 @@
+/* eslint-env mocha */
+/* eslint-disable @typescript-eslint/restrict-template-expressions */
+
+import { expect } from 'aegir/chai'
+import { execa } from 'execa'
+
+describe('unsupported', () => {
+  it('should refuse to generate source from proto2 definition', async () => {
+    await expect(execa('dist/bin/protons.js', ['./test/fixtures/proto2.proto'])).to.eventually.be.rejected()
+      .with.property('stderr').that.contain('"required" fields are not allowed in proto3')
+  })
+})