CSHARP-4116: Optimize { $expr : true } to { }.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Optimizers/AstSimplifier.cs

@@ -13,7 +13,10 @@
 * limitations under the License.
 */
 
+using MongoDB.Bson;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Expressions;
+using MongoDB.Driver.Linq.Linq3Implementation.Ast.Filters;
+using MongoDB.Driver.Linq.Linq3Implementation.Ast.Stages;
 using MongoDB.Driver.Linq.Linq3Implementation.Ast.Visitors;
 
 namespace MongoDB.Driver.Linq.Linq3Implementation.Ast.Optimizers
@@ -105,6 +108,21 @@ static AstExpression UltimateGetFieldInput(AstGetFieldExpression getField)
             }
         }
 
+        public override AstNode VisitMatchStage(AstMatchStage node)
+        {
+            node = (AstMatchStage)base.VisitMatchStage(node);
+
+            if (node.Filter is AstExprFilter exprFilter &&
+                exprFilter.Expression is AstConstantExpression constantExpression &&
+                constantExpression.Value is BsonBoolean booleanValue)
+            {
+                var simpleFilter = booleanValue.Value ? AstFilter.MatchesEverything() : AstFilter.MatchesNothing();
+                return AstStage.Match(simpleFilter);
+            }
+
+            return node;
+        }
+
         public override AstNode VisitUnaryExpression(AstUnaryExpression node)
         {
             // { $first : <arg> } => { $arrayElemAt : [<arg>, 0] } (or -1 for $last)

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4116Tests.cs

@@ -0,0 +1,38 @@
+/* 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.Linq;
+using MongoDB.Bson;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4116Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [InlineData(false, "{ $match : { _id : { $type : -1 } } }")]
+        [InlineData(true, "{ $match : { } }")]
+        public void Optimize_match_with_expr(bool value, string expectedStage)
+        {
+            var collection = GetCollection<BsonDocument>();
+            var queryable = collection.AsQueryable()
+                .Where(x => value);
+
+            var stages = Translate(collection, queryable);
+
+            AssertStages(stages, expectedStage);
+        }
+    }
+}

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

@@ -25,6 +25,11 @@ namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests
 {
     public abstract class Linq3IntegrationTest
     {
+        protected void AssertStages(IEnumerable<BsonDocument> stages, params string[] expectedStages)
+        {
+            AssertStages(stages, (IEnumerable<string>)expectedStages);
+        }
+
         protected void AssertStages(IEnumerable<BsonDocument> stages, IEnumerable<string> expectedStages)
         {
             stages.Should().Equal(expectedStages.Select(json => BsonDocument.Parse(json)));