Create declaring types when creating a CodeBuilder for ITypeSymbol.

src/CodeGenHelpers/ClassBuilder.cs

@@ -343,7 +343,7 @@ internal override void Write(in CodeWriter writer)
                 IsSealed ? "sealed" : null,
                 IsAbstract ? "abstract" : null,
                 _isPartial ? "partial" : null,
-                Kind.ToString().ToLowerInvariant(),
+                KindKeyword(Kind),
                 $"{Name}{_generics}",
                 extra
             };
@@ -393,6 +393,9 @@ internal override void Write(in CodeWriter writer)
             }
         }
 
+        private static string KindKeyword(TypeKind kind) =>
+            kind == TypeKind.Struct ? "struct" : kind.ToString().ToLowerInvariant();
+
         private static void WriteClassAttributes(IEnumerable<string> classAttributes, in CodeWriter writer)
         {
             foreach (var attr in classAttributes.Distinct().OrderBy(x => x))

src/CodeGenHelpers/CodeBuilder.cs

@@ -16,6 +16,10 @@ public sealed class CodeBuilder
 
 Changes to this file may cause incorrect behavior and will be lost if
 the code is regenerated.";
+        private static readonly SymbolDisplayFormat ConstraintDisplayFormat = new SymbolDisplayFormat(
+            genericsOptions: SymbolDisplayGenericsOptions.IncludeTypeParameters,
+            typeQualificationStyle: SymbolDisplayTypeQualificationStyle.NameAndContainingTypesAndNamespaces,
+            miscellaneousOptions: SymbolDisplayMiscellaneousOptions.UseSpecialTypes);
         private string? _autoGeneratedMessage;
         private readonly List<string> _namespaceImports = new List<string>();
         private readonly List<string> _assemblyAttributes = new List<string>();
@@ -69,10 +73,97 @@ public static CodeBuilder Create(INamespaceSymbol namespaceSymbol, IndentStyle i
 
         public static ClassBuilder Create(ITypeSymbol typeSymbol, IndentStyle indentStyle = IndentStyle.Spaces)
         {
+            var containingTypes = new List<ITypeSymbol>();
+
+            if (typeSymbol.ContainingType != null)
+            {
+                var currentType = typeSymbol;
+
+                while (currentType.ContainingType is { } containingType)
+                {
+                    containingTypes.Add(containingType);
+                    currentType = containingType;
+                }
+
+                containingTypes.Reverse();
+            }
+
             var builder = Create(typeSymbol.ContainingNamespace, indentStyle);
-            return builder.AddClass(typeSymbol.Name)
-                .WithAccessModifier(typeSymbol.DeclaredAccessibility)
-                .OfType(typeSymbol.TypeKind);
+            ClassBuilder? classBuilder = null;
+            // Top-level symbols keep the historical name, accessibility, and kind.
+            // Nested symbols also repeat generics and modifiers on each partial.
+            var copyDeclarationDetails = containingTypes.Count > 0;
+
+            foreach (var containingType in containingTypes)
+            {
+                classBuilder = AddClass(containingType);
+            }
+
+            return AddClass(typeSymbol);
+
+            ClassBuilder AddClass(ITypeSymbol symbol)
+            {
+                if (classBuilder != null)
+                {
+                    classBuilder = classBuilder.AddNestedClass(symbol.Name, true);
+                }
+                else
+                {
+                    classBuilder = builder.AddClass(symbol.Name);
+                }
+
+                classBuilder
+                    .WithAccessModifier(symbol.DeclaredAccessibility)
+                    .OfType(symbol.TypeKind);
+
+                if (copyDeclarationDetails)
+                    CopyDeclarationDetails(classBuilder, symbol);
+
+                return classBuilder;
+            }
+        }
+
+        private static void CopyDeclarationDetails(ClassBuilder classBuilder, ITypeSymbol symbol)
+        {
+            if (symbol.IsStatic)
+            {
+                classBuilder.MakeStaticClass();
+            }
+            else if (symbol.TypeKind == TypeKind.Class)
+            {
+                // Structs report IsSealed, but `sealed struct` does not compile.
+                // Static classes are handled above and must not also be abstract or sealed.
+                if (symbol.IsAbstract)
+                    classBuilder.Abstract();
+
+                if (symbol.IsSealed)
+                    classBuilder.Sealed();
+            }
+
+            if (symbol is not INamedTypeSymbol namedType)
+                return;
+
+            foreach (var typeParameter in namedType.TypeParameters)
+            {
+                classBuilder.AddGeneric(typeParameter.Name, generic =>
+                {
+                    if (typeParameter.HasReferenceTypeConstraint)
+                        generic.Class();
+                    else if (typeParameter.HasUnmanagedTypeConstraint)
+                        generic.AddConstraint("unmanaged");
+                    else if (typeParameter.HasValueTypeConstraint)
+                        generic.AddConstraint("struct");
+
+                    if (typeParameter.HasNotNullConstraint)
+                        generic.AddConstraint("notnull");
+
+                    foreach (var constraintType in typeParameter.ConstraintTypes)
+                        generic.AddConstraint(constraintType.ToDisplayString(ConstraintDisplayFormat));
+
+                    if (typeParameter.HasConstructorConstraint)
+                        generic.New();
+                });
+            }
         }
 
         public CodeBuilder AddNamespaceImport(string importedNamespace)

tests/CodeGenHelpers.Tests/SampleCode/ClassNestedInStructFromTypeSymbol.cs

@@ -0,0 +1,18 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace Demo.Containers
+{
+    internal partial struct OuterStruct
+    {
+        public partial class NestedValue
+        {
+        }
+    }
+}

tests/CodeGenHelpers.Tests/SampleCode/GenericNestedClassFromTypeSymbol.cs

@@ -0,0 +1,24 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace Demo.Containers
+{
+    internal static partial class Outer<T>
+        where T : class
+    {
+        protected internal abstract partial class Middle<TItem>
+            where TItem : struct
+        {
+            private protected sealed partial class Inner<TResult>
+                where TResult : class, System.IComparable<TResult>
+            {
+            }
+        }
+    }
+}

tests/CodeGenHelpers.Tests/SampleCode/MultiLevelNestedClassFromTypeSymbol.cs

@@ -0,0 +1,21 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace Demo.Containers
+{
+    internal partial class Outer
+    {
+        protected internal partial class Middle
+        {
+            private protected partial class Inner
+            {
+            }
+        }
+    }
+}

tests/CodeGenHelpers.Tests/SampleCode/NonNestedClassFromTypeSymbol.cs

@@ -0,0 +1,15 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace Demo
+{
+    public partial class Widget
+    {
+    }
+}

tests/CodeGenHelpers.Tests/SampleCode/SingleNestedClassFromTypeSymbol.cs

@@ -0,0 +1,18 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace Demo.Containers
+{
+    public partial class Outer
+    {
+        internal partial class Inner
+        {
+        }
+    }
+}