CSHARP-5572: Implement new KnownSerializerFinder

src/MongoDB.Bson/Serialization/IBsonSerializerExtensions.cs

@@ -50,6 +50,38 @@ public static TValue Deserialize<TValue>(this IBsonSerializer<TValue> serializer
             return serializer.Deserialize(context, args);
         }
 
+        /// <summary>
+        /// Gets the serializer for a base type starting from a serializer for a derived type.
+        /// </summary>
+        /// <param name="serializer">The serializer for the derived type.</param>
+        /// <param name="baseType">The base type.</param>
+        /// <returns>The serializer for the base type.</returns>
+        public static IBsonSerializer GetBaseTypeSerializer(this IBsonSerializer serializer, Type baseType)
+        {
+            if (!baseType.IsAssignableFrom(serializer.ValueType))
+            {
+                throw new ArgumentException($"{baseType} is not assignable from {serializer.ValueType}.");
+            }
+
+            return BsonSerializer.LookupSerializer(baseType); // TODO: should be able to navigate from serializer
+        }
+
+        /// <summary>
+        /// Gets the serializer for a derived type starting from a serializer for a base type.
+        /// </summary>
+        /// <param name="serializer">The serializer for the base type.</param>
+        /// <param name="derivedType">The derived type.</param>
+        /// <returns>The serializer for the derived type.</returns>
+        public static IBsonSerializer GetDerivedTypeSerializer(this IBsonSerializer serializer, Type derivedType)
+        {
+            if (!serializer.ValueType.IsAssignableFrom(derivedType))
+            {
+                throw new ArgumentException($"{serializer.ValueType} is not assignable from {derivedType}.");
+            }
+
+            return BsonSerializer.LookupSerializer(derivedType); // TODO: should be able to navigate from serializer
+        }
+
         /// <summary>
         /// Gets the discriminator convention for a serializer.
         /// </summary>

src/MongoDB.Driver/Linq/Linq3Implementation/ExtensionMethods/ExpressionExtensions.cs

@@ -60,5 +60,17 @@ public static TValue GetConstantValue<TValue>(this Expression expression, Expres
             var message = $"Expression must be a constant: {expression} in {containingExpression}.";
             throw new ExpressionNotSupportedException(message);
         }
+
+        public static bool IsConvert(this Expression expression, out Expression operand)
+        {
+            if (expression is UnaryExpression unaryExpression && unaryExpression.NodeType == ExpressionType.Convert)
+            {
+                operand = unaryExpression.Operand;
+                return true;
+            }
+
+            operand = null;
+            return false;
+        }
     }
 }

src/MongoDB.Driver/Linq/Linq3Implementation/GroupingWithOutputExpressionStageDefinitions.cs

@@ -64,7 +64,8 @@ private AstStage RenderProjectStage(
             out IBsonSerializer<TOutput> outputSerializer)
         {
             var partiallyEvaluatedOutput = (Expression<Func<TGrouping, TOutput>>)PartialEvaluator.EvaluatePartially(_output);
-            var context = TranslationContext.Create(translationOptions);
+            var parameter = partiallyEvaluatedOutput.Parameters.Single();
+            var context = TranslationContext.Create(partiallyEvaluatedOutput, initialNode: parameter, initialSerializer: inputSerializer, translationOptions: translationOptions);
             var outputTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, partiallyEvaluatedOutput, inputSerializer, asRoot: true);
             var (projectStage, projectSerializer) = ProjectionHelper.CreateProjectStage(outputTranslation);
             outputSerializer = (IBsonSerializer<TOutput>)projectSerializer;
@@ -106,7 +107,8 @@ protected override AstStage RenderGroupingStage(
             out IBsonSerializer<IGrouping<TValue, TInput>> groupingOutputSerializer)
         {
             var partiallyEvaluatedGroupBy = (Expression<Func<TInput, TValue>>)PartialEvaluator.EvaluatePartially(_groupBy);
-            var context = TranslationContext.Create(translationOptions);
+            var parameter = partiallyEvaluatedGroupBy.Parameters.Single();
+            var context = TranslationContext.Create(partiallyEvaluatedGroupBy, initialNode: parameter, initialSerializer: inputSerializer, translationOptions: translationOptions);
             var groupByTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, partiallyEvaluatedGroupBy, inputSerializer, asRoot: true);
 
             var valueSerializer = (IBsonSerializer<TValue>)groupByTranslation.Serializer;
@@ -150,7 +152,8 @@ protected override AstStage RenderGroupingStage(
             out IBsonSerializer<IGrouping<AggregateBucketAutoResultId<TValue>, TInput>> groupingOutputSerializer)
         {
             var partiallyEvaluatedGroupBy = (Expression<Func<TInput, TValue>>)PartialEvaluator.EvaluatePartially(_groupBy);
-            var context = TranslationContext.Create(translationOptions);
+            var parameter = partiallyEvaluatedGroupBy.Parameters.Single();
+            var context = TranslationContext.Create(partiallyEvaluatedGroupBy, initialNode: parameter, initialSerializer: inputSerializer, translationOptions: translationOptions);
             var groupByTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, partiallyEvaluatedGroupBy, inputSerializer, asRoot: true);
 
             var valueSerializer = (IBsonSerializer<TValue>)groupByTranslation.Serializer;
@@ -188,7 +191,8 @@ protected override AstStage RenderGroupingStage(
             out IBsonSerializer<IGrouping<TValue, TInput>> groupingOutputSerializer)
         {
             var partiallyEvaluatedGroupBy = (Expression<Func<TInput, TValue>>)PartialEvaluator.EvaluatePartially(_groupBy);
-            var context = TranslationContext.Create(translationOptions);
+            var parameter = partiallyEvaluatedGroupBy.Parameters.Single();
+            var context = TranslationContext.Create(partiallyEvaluatedGroupBy, initialNode: parameter, initialSerializer: inputSerializer, translationOptions: translationOptions);
             var groupByTranslation = ExpressionToAggregationExpressionTranslator.TranslateLambdaBody(context, partiallyEvaluatedGroupBy, inputSerializer, asRoot: true);
             var pushElements = AstExpression.AccumulatorField("_elements", AstUnaryAccumulatorOperator.Push, AstExpression.RootVar);
             var groupBySerializer = (IBsonSerializer<TValue>)groupByTranslation.Serializer;