Parse option: allowLegacySDLEmptyFields (#1171)

This offers a slightly smoother adoption path for existing code that uses the legacy empty field sets.

src/language/__tests__/schema-parser-test.js

@@ -708,4 +708,17 @@ input Hello {
       { line: 2, column: 33 },
     );
   });
+
+  it('Option: allowLegacySDLEmptyFields supports type with empty fields', () => {
+    const body = 'type Hello { }';
+    expect(() => parse(body)).to.throw('Syntax Error: Expected Name, found }');
+    const doc = parse(body, { allowLegacySDLEmptyFields: true });
+    expect(doc).to.containSubset({
+      definitions: [
+        {
+          fields: [],
+        },
+      ],
+    });
+  });
 });

src/language/parser.js

@@ -119,6 +119,16 @@ export type ParseOptions = {
    */
   noLocation?: boolean,
 
+  /**
+   * If enabled, the parser will parse empty fields sets in the Schema
+   * Definition Language. Otherwise, the parser will follow the current
+   * specification.
+   *
+   * This option is provided to ease adoption of the final SDL specification
+   * and will be removed in a future major release.
+   */
+  allowLegacySDLEmptyFields?: boolean,
+
   /**
    * EXPERIMENTAL:
    *
@@ -929,6 +939,16 @@ function parseImplementsInterfaces(lexer: Lexer<*>): Array<NamedTypeNode> {
  * FieldsDefinition : { FieldDefinition+ }
  */
 function parseFieldsDefinition(lexer: Lexer<*>): Array<FieldDefinitionNode> {
+  // Legacy support for the SDL?
+  if (
+    lexer.options.allowLegacySDLEmptyFields &&
+    peek(lexer, TokenKind.BRACE_L) &&
+    lexer.lookahead().kind === TokenKind.BRACE_R
+  ) {
+    lexer.advance();
+    lexer.advance();
+    return [];
+  }
   return peek(lexer, TokenKind.BRACE_L)
     ? many(lexer, TokenKind.BRACE_L, parseFieldDefinition, TokenKind.BRACE_R)
     : [];