Distribute COM interface friendly overloads across many classes

In full generation, the `FriendlyOverloadExtensions.g.cs` file was by far the largest (over 10MB in size). The C# compiler compiles types in parallel, so a very large type represents a huge unit of work that cannot be broken down into parallel compilations. It also means very large allocations for the string that renders the entire string. Splitting this up into one class per COM interface should help compilation performance, particularly in our full generation test.

src/Microsoft.Windows.CsWin32/Generator.cs

@@ -159,7 +159,6 @@ public class Generator : IDisposable
 
     private static readonly SyntaxToken SemicolonWithLineFeed = TokenWithLineFeed(SyntaxKind.SemicolonToken);
     private static readonly IdentifierNameSyntax InlineArrayIndexerExtensionsClassName = IdentifierName("InlineArrayIndexerExtensions");
-    private static readonly IdentifierNameSyntax ComInterfaceFriendlyExtensionsClassName = IdentifierName("FriendlyOverloadExtensions");
     private static readonly TypeSyntax SafeHandleTypeSyntax = IdentifierName("SafeHandle");
     private static readonly IdentifierNameSyntax IntPtrTypeSyntax = IdentifierName(nameof(IntPtr));
     private static readonly IdentifierNameSyntax UIntPtrTypeSyntax = IdentifierName(nameof(UIntPtr));
@@ -607,11 +606,7 @@ ClassDeclarationSyntax DeclarePInvokeClass(string fileNameKey) => ClassDeclarati
                 result = result.Concat(new MemberDeclarationSyntax[] { inlineArrayIndexerExtensionsClass });
             }
 
-            ClassDeclarationSyntax comInterfaceFriendlyExtensionsClass = this.DeclareComInterfaceFriendlyExtensionsClass();
-            if (comInterfaceFriendlyExtensionsClass.Members.Count > 0)
-            {
-                result = result.Concat(new MemberDeclarationSyntax[] { comInterfaceFriendlyExtensionsClass });
-            }
+            result = result.Concat(this.committedCode.ComInterfaceExtensions);
 
             if (this.committedCode.TopLevelFields.Any())
             {
@@ -3429,14 +3424,6 @@ private ClassDeclarationSyntax DeclareInlineArrayIndexerExtensionsClass()
             .AddAttributeLists(AttributeList().AddAttributes(GeneratedCodeAttribute));
     }
 
-    private ClassDeclarationSyntax DeclareComInterfaceFriendlyExtensionsClass()
-    {
-        return ClassDeclaration(ComInterfaceFriendlyExtensionsClassName.Identifier)
-            .AddMembers(this.committedCode.ComInterfaceExtensions.ToArray())
-            .WithModifiers(TokenList(TokenWithSpace(this.Visibility), TokenWithSpace(SyntaxKind.StaticKeyword), TokenWithSpace(SyntaxKind.PartialKeyword)))
-            .AddAttributeLists(AttributeList().AddAttributes(GeneratedCodeAttribute));
-    }
-
     /// <summary>
     /// Generates a type to represent a COM interface.
     /// </summary>
@@ -3724,9 +3711,8 @@ private TypeDeclarationSyntax DeclareInterfaceAsStruct(TypeDefinitionHandle type
             return null;
         }
 
-        IdentifierNameSyntax ifaceName = interfaceAsSubtype
-            ? NestedCOMInterfaceName
-            : IdentifierName(this.Reader.GetString(typeDef.Name));
+        string actualIfaceName = this.Reader.GetString(typeDef.Name);
+        IdentifierNameSyntax ifaceName = interfaceAsSubtype ? NestedCOMInterfaceName : IdentifierName(actualIfaceName);
         TypeSyntaxSettings typeSettings = this.comSignatureTypeSettings;
 
         // It is imperative that we generate methods for all base interfaces as well, ahead of any implemented by *this* interface.
@@ -3932,7 +3918,20 @@ private TypeDeclarationSyntax DeclareInterfaceAsStruct(TypeDefinitionHandle type
 
         // Only add overloads to instance collections after everything else is done,
         // so we don't leave extension methods behind if we fail to generate the target interface.
-        this.volatileCode.AddComInterfaceExtension(friendlyOverloads);
+        if (friendlyOverloads.Count > 0)
+        {
+            string ns = this.Reader.GetString(typeDef.Namespace);
+            if (this.TryStripCommonNamespace(ns, out string? strippedNamespace))
+            {
+                ns = strippedNamespace;
+            }
+
+            ClassDeclarationSyntax friendlyOverloadClass = ClassDeclaration(Identifier($"{ns.Replace('.', '_')}_{actualIfaceName}_Extensions"))
+                .WithMembers(List<MemberDeclarationSyntax>(friendlyOverloads))
+                .WithModifiers(TokenList(TokenWithSpace(this.Visibility), TokenWithSpace(SyntaxKind.StaticKeyword), TokenWithSpace(SyntaxKind.PartialKeyword)))
+                .AddAttributeLists(AttributeList().AddAttributes(GeneratedCodeAttribute));
+            this.volatileCode.AddComInterfaceExtension(friendlyOverloadClass);
+        }
 
         return ifaceDeclaration;
     }
@@ -6761,7 +6760,7 @@ private class GeneratedCode
 
         private readonly List<MethodDeclarationSyntax> inlineArrayIndexerExtensionsMembers = new();
 
-        private readonly List<MethodDeclarationSyntax> comInterfaceFriendlyExtensionsMembers = new();
+        private readonly List<ClassDeclarationSyntax> comInterfaceFriendlyExtensionsMembers = new();
 
         private bool generating;
 
@@ -6784,7 +6783,7 @@ internal GeneratedCode(GeneratedCode parent)
 
         internal IEnumerable<MemberDeclarationSyntax> GeneratedTopLevelTypes => this.specialTypes.Values.Where(st => st.TopLevel).Select(st => st.Type);
 
-        internal IEnumerable<MethodDeclarationSyntax> ComInterfaceExtensions => this.comInterfaceFriendlyExtensionsMembers;
+        internal IReadOnlyCollection<ClassDeclarationSyntax> ComInterfaceExtensions => this.comInterfaceFriendlyExtensionsMembers;
 
         internal IEnumerable<MethodDeclarationSyntax> InlineArrayIndexerExtensions => this.inlineArrayIndexerExtensionsMembers;
 
@@ -6866,13 +6865,13 @@ internal void AddInlineArrayIndexerExtension(MethodDeclarationSyntax inlineIndex
             }
         }
 
-        internal void AddComInterfaceExtension(MethodDeclarationSyntax extension)
+        internal void AddComInterfaceExtension(ClassDeclarationSyntax extension)
         {
             this.ThrowIfNotGenerating();
             this.comInterfaceFriendlyExtensionsMembers.Add(extension);
         }
 
-        internal void AddComInterfaceExtension(IEnumerable<MethodDeclarationSyntax> extension)
+        internal void AddComInterfaceExtension(IEnumerable<ClassDeclarationSyntax> extension)
         {
             this.ThrowIfNotGenerating();
             this.comInterfaceFriendlyExtensionsMembers.AddRange(extension);