fix: add modifying nested struct diagnostic

src/Riok.Mapperly/AnalyzerReleases.Shipped.md

@@ -105,3 +105,4 @@ RMG043  | Mapper   | Warning  | Enum fallback values are only supported for the
 RMG044  | Mapper   | Warning  | An ignored enum member can not be found on the source enum
 RMG045  | Mapper   | Warning  | An ignored enum member can not be found on the target enum
 RMG046  | Mapper   | Error    | The used C# language version is not supported by Mapperly, Mapperly requires at least C# 9.0
+RMG047  | Mapper   | Error    | Cannot map to member path due to modifying a temporary value

src/Riok.Mapperly/Descriptors/MappingBodyBuilders/ObjectMemberMappingBodyBuilder.cs

@@ -135,6 +135,28 @@ public static bool ValidateMappingSpecification(
             return false;
         }
 
+        // cannot assign to intermediate value type
+        // invalid mapping when second to last member is a property that returns a value type
+        if (targetMemberPath.Path.Count > 1)
+        {
+            var member = targetMemberPath.Path[targetMemberPath.Path.Count - 2];
+            if (member is PropertyMember { Type: { IsValueType: true, IsRefLikeType: false } })
+            {
+                ctx.BuilderContext.ReportDiagnostic(
+                    DiagnosticDescriptors.CannotMapToTemporarySourceMember,
+                    ctx.Mapping.SourceType,
+                    sourceMemberPath.FullName,
+                    sourceMemberPath.Member.Type,
+                    ctx.Mapping.TargetType,
+                    targetMemberPath.FullName,
+                    targetMemberPath.Member.Type,
+                    member.Name,
+                    member.Type
+                );
+                return false;
+            }
+        }
+
         // a target member path part is init only
         var noInitOnlyPath = allowInitOnlyMember ? targetMemberPath.ObjectPath : targetMemberPath.Path;
         if (noInitOnlyPath.Any(p => p.IsInitOnly))

src/Riok.Mapperly/Diagnostics/DiagnosticDescriptors.cs

@@ -410,4 +410,13 @@ internal static class DiagnosticDescriptors
         DiagnosticSeverity.Error,
         true
     );
+
+    public static readonly DiagnosticDescriptor CannotMapToTemporarySourceMember = new DiagnosticDescriptor(
+        "RMG047",
+        "Cannot map to member path due to modifying a temporary value",
+        "Cannot map from member {0}.{1} of type {2} to member path {3}.{4} of type {5} because {6}.{7} is a value type, returning a temporary value",
+        DiagnosticCategories.Mapper,
+        DiagnosticSeverity.Error,
+        true
+    );
 }

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

@@ -443,4 +443,34 @@ public void ShouldIgnoreStaticConstructorAndDiagnostic()
             .HaveDiagnostic(DiagnosticDescriptors.CouldNotCreateMapping)
             .HaveAssertedAllDiagnostics();
     }
+
+    [Fact]
+    public void ModifyingTemporaryStructShouldDiagnostic()
+    {
+        var source = TestSourceBuilder.MapperWithBodyAndTypes(
+            """
+            [MapProperty("StringValue", "NestedValue.StringValue")]
+            partial B Map(A src);
+            """,
+            "class A { public string StringValue { get; set; } }",
+            "class B { public C NestedValue { get; set; } }",
+            "struct C { public string StringValue { get; set; } }"
+        );
+
+        TestHelper
+            .GenerateMapper(source, TestHelperOptions.AllowAllDiagnostics)
+            .Should()
+            .HaveDiagnostic(DiagnosticDescriptors.SourceMemberNotMapped)
+            .HaveDiagnostic(
+                DiagnosticDescriptors.CannotMapToTemporarySourceMember,
+                "Cannot map from member A.StringValue of type string to member path B.NestedValue.StringValue of type string because NestedValue.C is a value type, returning a temporary value"
+            )
+            .HaveAssertedAllDiagnostics()
+            .HaveSingleMethodBody(
+                """
+                var target = new global::B();
+                return target;"
+                """
+            );
+    }
 }