CSHARP-4126: Replace parameter names that aren't valid server variable names with generated variable names.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Misc/NameGenerator.cs

@@ -40,7 +40,7 @@ public string GetVarName(string symbolName)
                 return symbolName;
             }
 
-            return $"_v{_varCounter++}";
+            return $"v__{_varCounter++}";
 
             static bool IsValidVarName(string name)
             {
@@ -52,7 +52,7 @@ static bool IsValidVarName(string name)
 
                 static bool IsValidFirstChar(char c)
                 {
-                    return c == '_' || IsBetween(c, 'a', 'z') || IsBetween(c, 'A', 'Z');
+                    return IsBetween(c, 'a', 'z');
                 }
 
                 static bool IsValidSubsequentChar(char c)

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4126Tests.cs

@@ -0,0 +1,51 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Linq;
+using FluentAssertions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4126Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Test()
+        {
+            var collection = GetCollection<C>();
+            CreateCollection(
+                collection,
+                new C { Id = 1, A = new[] { 1, -2, -3 } });
+
+            var queryable = collection.AsQueryable()
+                .Select(_p0 => new { Id = _p0.Id, A = _p0.A.Select(_p1 => Math.Abs(_p1)) });
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, "{ $project : { Id : '$_id', A : { $map : { input : '$A', as : 'v__0', in : { $abs : '$$v__0' } } }, _id : 0 } }");
+
+            var results = queryable.ToList();
+            results.Should().HaveCount(1);
+            results[0].Id.Should().Be(1);
+            results[0].A.Should().Equal(1, 2, 3);
+        }
+
+        public class C
+        {
+            public int Id { get; set; }
+            public int[] A { get; set; }
+        }
+    }
+}

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Linq3IntegrationTest.cs

@@ -51,6 +51,11 @@ protected void CreateCollection<TDocument>(IMongoCollection<TDocument> collectio
             }
         }
 
+        protected void CreateCollection<TDocument>(IMongoCollection<TDocument> collection, params TDocument[] documents)
+        {
+            CreateCollection(collection, (IEnumerable<TDocument>)documents); ;
+        }
+
         protected IMongoCollection<TDocument> GetCollection<TDocument>(string collectionName = null)
         {
             var databaseName = DriverTestConfiguration.DatabaseNamespace.DatabaseName;