added support for $literal when a constant string begins with a dollar sign.

Author: craiggwilson

src/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectionTranslatorTests_Project.cs

@@ -288,6 +288,16 @@ public async Task Should_translate_less_than_or_equal()
             result.Value.Result.Should().BeFalse();
         }
 
+        [Test]
+        public async Task Should_translate_literal_when_a_constant_strings_begins_with_a_dollar()
+        {
+            var result = await Project(x => new { Result = x.A == "$1" });
+
+            result.Projection.Should().Be("{ Result: { \"$eq\": [\"$A\", { \"$literal\": \"$1\" }] }, _id: 0 }");
+
+            result.Value.Result.Should().BeFalse();
+        }
+
         [Test]
         public async Task Should_translate_map_with_value()
         {

src/MongoDB.Driver/Linq/Translators/AggregateProjectionTranslator.cs

@@ -109,7 +109,15 @@ private BsonValue BuildValue(Expression node)
                     case ExpressionType.Conditional:
                         return BuildConditional((ConditionalExpression)node);
                     case ExpressionType.Constant:
-                        return BsonValue.Create(((ConstantExpression)node).Value);
+                        var value = BsonValue.Create(((ConstantExpression)node).Value);
+                        var stringValue = value as BsonString;
+                        if (stringValue != null && stringValue.Value.StartsWith("$"))
+                        {
+                            value = new BsonDocument("$literal", value);
+                        }
+                        // TODO: there may be other instances where we should use a literal...
+                        // but I can't think of any yet.
+                        return value;
                     case ExpressionType.Convert:
                     case ExpressionType.ConvertChecked:
                         return BuildValue(((UnaryExpression)node).Operand);