clarify defensive copies

docs/csharp/language-reference/keywords/in-parameter-modifier.md

@@ -9,7 +9,7 @@ helpviewer_keywords:
 
 # in parameter modifier (C# Reference)
 
-The `in` keyword causes arguments to be passed by reference. It makes the formal parameter an alias for the argument, which must be a variable. In other words, any operation on the parameter is made on the argument. It is like the [ref](ref.md) or [out](out-parameter-modifier.md) keywords, except that `in` arguments cannot be modified by the called method. Whereas `ref` arguments may be modified, `out` arguments must be modified by the called method, and those modifications are observable in the calling context.
+The `in` keyword causes arguments to be passed by reference but ensures the argument is not modified. It makes the formal parameter an alias for the argument, which must be a variable. In other words, any operation on the parameter is made on the argument. It is like the [ref](ref.md) or [out](out-parameter-modifier.md) keywords, except that `in` arguments cannot be modified by the called method. Whereas `ref` arguments may be modified, `out` arguments must be modified by the called method, and those modifications are observable in the calling context.
 
 [!code-csharp-interactive[cs-in-keyword](../../../../samples/snippets/csharp/language-reference/keywords/in-ref-out-modifier/InParameterModifier.cs#1)]  
 
@@ -115,14 +115,8 @@ You can't use the `in`, `ref`, and `out` keywords for the following kinds of met
 - The first argument of an extension method cannot have the `in` modifier unless that argument is a struct.
 - The first argument of an extension method where that argument is a generic type (even when that type is constrained to be a struct.)
 
+You can learn more about the `in` modifier, how it differs from `ref` and `out` in the article on [Write safe efficient code](../../write-safe-efficient-code.md).
+
 ## C# Language Specification  
 
  [!INCLUDE[CSharplangspec](~/includes/csharplangspec-md.md)]  
-
-## See also
-
-- [C# Reference](../index.md)
-- [C# Programming Guide](../../programming-guide/index.md)
-- [C# Keywords](index.md)
-- [Method Parameters](method-parameters.md)
-- [Write safe efficient code](../../write-safe-efficient-code.md)

docs/csharp/write-safe-efficient-code.md

@@ -158,12 +158,7 @@ can't be modified. Attempts to do so result in a compile-time error.
 
 The `readonly` modifier is required on the declaration of `originReference`.
 
-The compiler enforces that the caller can't modify the reference. Attempts
-to assign the value directly generate a compile-time error. However, the compiler
-can't know if any member method modifies the state of the struct.
-To ensure that the object isn't modified, the compiler creates a copy and
-calls member references using that copy. Any modifications are to that
-defensive copy.
+The compiler enforces that the caller can't modify the reference. Attempts to assign the value directly generate a compile-time error. In other cases, the compiler allocates a defensive copy unless it can safely use the readonly reference. Static analysis rules determine if the struct could be modified. The compiler doesn't create a defensive copy when the struct is a `readonly struct` or the member is a `readonly` member of the struct. Defensive copies aren't needed to pass the struct as an `in` argument.
 
 ## Apply the `in` modifier to `readonly struct` parameters larger than `System.IntPtr.Size`