Interpolated string handlers should avoid ref struct (#44123)

* Interpolated string handlers should avoid ref struct

* Drop that they can't implement interfaces

Author: Youssef1313

docs/csharp/advanced-topics/performance/interpolated-string-handler.md

@@ -93,7 +93,7 @@ Finally, notice that the last warning doesn't invoke the interpolated string han
 
 > [!IMPORTANT]
 >
-> The version of the `Logger` for this section is a `ref struct`. A `ref struct` minimizes allocations because it must be stored on the stack. However, `ref struct` types, in general, can't implement interfaces. This can cause compatibility issues for unit test frameworks and mocking types that use `ref struct` types for implementation.
+> Use `ref struct` for interpolated string handlers only if absolutely necessary. Using `ref struct` will have limitations as they must be stored on the stack. For example, they will not work if an interpolated string hole contains an `await` expression because the compiler will need to store the handler in the compiler-generated `IAsyncStateMachine` implementation.
 
 ## Add more capabilities to the handler
 

docs/csharp/advanced-topics/performance/snippets/interpolated-string-handler/Logger-v2.cs

@@ -5,7 +5,7 @@ namespace interpolated_string_handler.Version2
 {
     // <CoreInterpolatedStringHandler>
     [InterpolatedStringHandler]
-    public ref struct LogInterpolatedStringHandler
+    public struct LogInterpolatedStringHandler
     {
         // Storage for the built-up string
         StringBuilder builder;

docs/csharp/advanced-topics/performance/snippets/interpolated-string-handler/logger-v3.cs

@@ -4,7 +4,7 @@
 namespace interpolated_string_handler.Version3
 {
     [InterpolatedStringHandler]
-    public ref struct LogInterpolatedStringHandler
+    public struct LogInterpolatedStringHandler
     {
         // Storage for the built-up string
         StringBuilder builder;

docs/csharp/advanced-topics/performance/snippets/interpolated-string-handler/logger-v4.cs

@@ -4,7 +4,7 @@
 namespace interpolated_string_handler.Version4
 {
     [InterpolatedStringHandler]
-    public ref struct LogInterpolatedStringHandler
+    public struct LogInterpolatedStringHandler
     {
         // Storage for the built-up string
         StringBuilder builder;