fix: don't emit unnecessary null-handling if source and target are nullable

src/Riok.Mapperly/Descriptors/Mappings/NullDelegateMapping.cs

@@ -45,6 +45,11 @@ public override ExpressionSyntax Build(TypeMappingBuildContext ctx)
         if (_delegateMapping.SourceType.IsNullable())
             return _delegateMapping.Build(ctx);
 
+        if (SourceType.IsNullable() && TargetType.IsNullable() && SymbolEqualityComparer.Default.Equals(SourceType, TargetType))
+        {
+            return _delegateMapping.Build(ctx);
+        }
+
         if (!SourceType.IsNullable())
         {
             // if the target type is a nullable value type, there needs to be an additional cast in some cases

test/Riok.Mapperly.IntegrationTests/_snapshots/StaticMapperTest.SnapshotGeneratedSource_NET6_0.verified.cs

@@ -13,7 +13,7 @@ public static partial int DirectInt(int value)
         [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "0.0.1.0")]
         public static partial int? DirectIntNullable(int? value)
         {
-            return value == null ? default(int?) : value.Value;
+            return value;
         }
 
         [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "0.0.1.0")]

test/Riok.Mapperly.Tests/Mapping/DictionaryTest.cs

@@ -44,7 +44,7 @@ public void DictionaryToSameDictionaryDeepCloningInDisabledNullableContext()
                 var target = new global::System.Collections.Generic.Dictionary<string, string?>(source.Count);
                 foreach (var item in source)
                 {
-                    target[item.Key] = item.Value == null ? default : item.Value;
+                    target[item.Key] = item.Value;
                 }
                 return target;
                 """

test/Riok.Mapperly.Tests/Mapping/NullableTest.cs

@@ -164,6 +164,13 @@ TestSourceBuilderOptions.Default with
 
     [Fact]
     public void NonNullableToNullableValueType()
+    {
+        var source = TestSourceBuilder.Mapping("int[]?", "int[]?");
+        TestHelper.GenerateMapper(source).Should().HaveSingleMethodBody("return source;");
+    }
+
+    [Fact]
+    public void NonNullableToNullableValueType2()
     {
         var source = TestSourceBuilder.Mapping("DateTime", "DateTime?");
         TestHelper.GenerateMapper(source).Should().HaveSingleMethodBody("return (global::System.DateTime?)source;");

test/Riok.Mapperly.Tests/Mapping/ToObjectDeepCloningTest.cs

@@ -30,7 +30,7 @@ public void NullableObjectToNullableObjectDeepCloning()
         TestHelper
             .GenerateMapper(source, TestHelperOptions.AllowInfoDiagnostics)
             .Should()
-            .HaveMapMethodBody("return source == null ? default : source;")
+            .HaveMapMethodBody("return source;")
             .HaveDiagnostic(DiagnosticDescriptors.MappedObjectToObjectWithoutDeepClone)
             .HaveAssertedAllDiagnostics();
     }

test/Riok.Mapperly.Tests/_snapshots/DictionaryTest.DictionaryToDictionaryWithNonNullableKeyAndValueDeepCloningInDisabledNullableContext#Mapper.g.verified.cs

@@ -27,7 +27,7 @@ public partial class Mapper
         var target = new global::System.Collections.Generic.Dictionary<string, string?>(source.Count);
         foreach (var item in source)
         {
-            target[item.Key] = item.Value == null ? default : item.Value;
+            target[item.Key] = item.Value;
         }
         return target;
     }

test/Riok.Mapperly.Tests/_snapshots/QueryableProjectionNullableTest.NestedPropertyWithDeepCloneable#Mapper.g.verified.cs

@@ -46,7 +46,7 @@ public partial class Mapper
         var target = new string?[source.Length];
         for (var i = 0; i < source.Length; i++)
         {
-            target[i] = source[i] == null ? default : source[i]!;
+            target[i] = source[i];
         }
         return target;
     }