fix(schematype): deprecate isAsync option for custom validators

Fix #6700

lib/schematype.js

@@ -10,6 +10,7 @@ const $type = require('./schema/operators/type');
 const get = require('./helpers/get');
 const immediate = require('./helpers/immediate');
 const schemaTypeSymbol = require('./helpers/symbols').schemaTypeSymbol;
+const util = require('util');
 const utils = require('./utils');
 const validatorErrorSymbol = require('./helpers/symbols').validatorErrorSymbol;
 
@@ -597,6 +598,11 @@ SchemaType.prototype.validate = function(obj, message, type) {
       }
       properties = {message: message, type: type, validator: obj};
     }
+
+    if (properties.isAsync) {
+      handleIsAsync();
+    }
+
     this.validators.push(properties);
     return this;
   }
@@ -620,6 +626,15 @@ SchemaType.prototype.validate = function(obj, message, type) {
   return this;
 };
 
+/*!
+ * ignore
+ */
+
+const handleIsAsync = util.deprecate(function handleIsAsync() {},
+  'Mongoose: the `isAsync` option for custom validators is deprecated. Make ' +
+  'your async validators return a promise instead: ' +
+  'https://mongoosejs.com/docs/validation.html#async-custom-validators');
+
 /**
  * Adds a required validator to this SchemaType. The validator gets added
  * to the front of this SchemaType's validators array using `unshift()`.