throw when opts is used

lib/cli/run.js

@@ -279,6 +279,13 @@ exports.builder = yargs =>
         );
       }
 
+      if (argv.opts) {
+        throw createUnsupportedError(
+          `--opts: configuring Mocha via 'mocha.opts' is DEPRECATED and no longer supported.
+          Please use a configuration file instead.`
+        );
+      }
+
       // load requires first, because it can impact "plugin" validation
       handleRequires(argv.require);
       validatePlugin(argv, 'reporter', Mocha.reporters);

test/integration/options/opts.spec.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var invokeMocha = require('../helpers').invokeMocha;
+
+describe('--opts', function() {
+  it('should report deprecation', function(done) {
+    invokeMocha(
+      ['--opts', './test/mocha.opts'],
+      function(err, res) {
+        if (err) {
+          return done(err);
+        }
+        expect(
+          res,
+          'to have failed with output',
+          /'mocha.opts' is DEPRECATED/i
+        );
+        done();
+      },
+      'pipe'
+    );
+  });
+});