Cover empty field-selections (#4228)

Closes graphql/graphql-js#3790

This follows the spec closer to the letter as we consider empty
selection-sets invalid as well. This however does not cover empty
operation-definitions, which should also be considered invalid.

The parser itself considers empty-selections invalid as it requires you
to specify a name before a closing curly, hence why testing happens a
bit more manual here. Despite our parser not considering this valid, I
think this is good to validate when folks use a separate parser or
construct documents manually as this is valid in our type-system.

src/validation/__tests__/ScalarLeafsRule-test.ts

@@ -1,8 +1,15 @@
 import { describe, it } from 'mocha';
 
+import { expectJSON } from '../../__testUtils__/expectJSON.js';
+
+import type { DocumentNode } from '../../language/ast.js';
+import { OperationTypeNode } from '../../language/ast.js';
+import { Kind } from '../../language/kinds.js';
+
 import { ScalarLeafsRule } from '../rules/ScalarLeafsRule.js';
+import { validate } from '../validate.js';
 
-import { expectValidationErrors } from './harness.js';
+import { expectValidationErrors, testSchema } from './harness.js';
 
 function expectErrors(queryStr: string) {
   return expectValidationErrors(ScalarLeafsRule, queryStr);
@@ -35,6 +42,39 @@ describe('Validate: Scalar leafs', () => {
     ]);
   });
 
+  it('object type having only one selection', () => {
+    const doc: DocumentNode = {
+      kind: Kind.DOCUMENT,
+      definitions: [
+        {
+          kind: Kind.OPERATION_DEFINITION,
+          operation: OperationTypeNode.QUERY,
+          selectionSet: {
+            kind: Kind.SELECTION_SET,
+            selections: [
+              {
+                kind: Kind.FIELD,
+                name: { kind: Kind.NAME, value: 'human' },
+                selectionSet: { kind: Kind.SELECTION_SET, selections: [] },
+              },
+            ],
+          },
+        },
+      ],
+    };
+
+    // We can't leverage expectErrors since it doesn't support passing in the
+    // documentNode directly. We have to do this because this is technically
+    // an invalid document.
+    const errors = validate(testSchema, doc, [ScalarLeafsRule]);
+    expectJSON(errors).toDeepEqual([
+      {
+        message:
+          'Field "human" of type "Human" must have at least one field selected.',
+      },
+    ]);
+  });
+
   it('interface type missing selection', () => {
     expectErrors(`
       {

src/validation/rules/ScalarLeafsRule.ts

@@ -41,6 +41,15 @@ export function ScalarLeafsRule(context: ValidationContext): ASTVisitor {
               { nodes: node },
             ),
           );
+        } else if (selectionSet.selections.length === 0) {
+          const fieldName = node.name.value;
+          const typeStr = inspect(type);
+          context.reportError(
+            new GraphQLError(
+              `Field "${fieldName}" of type "${typeStr}" must have at least one field selected.`,
+              { nodes: node },
+            ),
+          );
         }
       }
     },