Add, update compiler error and warning messages for ref struct interfaces (#41948)

* Add, update messages in dotnet/roslyn#73567

The PR for `ref struct` interfaces added new compiler error and warning messages. It also updates a few messages. This commit creates the stubs for the new messages, and updates the text for updated messages.

* incorporate additional ref struct error conditions

* Edit pass

Author: BillWagner

.openpublishing.redirection.csharp.json

@@ -467,6 +467,10 @@
             "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs8964.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/parameter-argument-mismatch"
         },
+        {
+            "source_path_from_root": "/docs/csharp/language-reference/compiler-messages/cs9050.md",
+            "redirect_url": "/dotnet/csharp/language-reference/compiler-messages/ref-struct-errors"
+        },
         {
             "source_path_from_root": "/docs/csharp/language-reference/compiler-options/addmodule-compiler-option.md",
             "redirect_url": "/dotnet/csharp/language-reference/compiler-options/inputs"

docs/csharp/language-reference/compiler-messages/cs9050.md

@@ -53,6 +53,7 @@ f1_keywords:
   - "CS9194"
   - "CS9202"
   - "CS9211"
+  - "CS9240"
 helpviewer_keywords:
   - "CS0171"
   - "CS0188"
@@ -105,6 +106,7 @@ helpviewer_keywords:
   - "CS9194"
   - "CS9202"
   - "CS9211"
+  - "CS9240"
 ms.date: 11/02/2023
 ---
 # Resolve warnings related to language features and versions
@@ -146,6 +148,7 @@ That's be design. The text closely matches the text of the compiler error / warn
 - **CS9194**: *Argument may not be passed with the `ref` keyword. To pass `ref` arguments to `in` parameters, upgrade to language version 12 or greater.*
 - **CS9202**: *Feature is not available in C# 12.0. Please use newer language version*
 - **CS9211**: *The diagnosticId argument to the 'Experimental' attribute must be a valid identifier.*
+- **CS9240**: *Target runtime doesn't support by-ref-like generics.*
 
 In addition, the following errors and warnings relate to struct initialization changes in recent versions:
 

docs/csharp/language-reference/compiler-messages/feature-version-errors.md

@@ -32,7 +32,6 @@ f1_keywords:
   - "CS8332"
   - "CS8337"
   - "CS8338"
-  - "CS8345"
   - "CS8351"
   - "CS8373"
   - "CS8374"
@@ -98,7 +97,6 @@ helpviewer_keywords:
   - "CS8332"
   - "CS8337"
   - "CS8338"
-  - "CS8345"
   - "CS8351"
   - "CS8373"
   - "CS8374"
@@ -172,7 +170,6 @@ That's by design. The text closely matches the text of the compiler error / warn
 - [**CS8332**](#writable-reference-variables-require-a-writable-referent): *Cannot assign to a member of variable or use it as the right hand side of a `ref` assignment because it is a readonly variable*
 - [**CS8337**](#reference-variable-restrictions): *The first parameter of a '`ref`' extension method must be a value type or a generic type constrained to struct.*
 - [**CS8338**](#reference-variable-restrictions): *The first '`in`' or '`ref readonly`' parameter of the extension method must be a concrete (non-generic) value type.*
-- [**CS8345**](#ref-safety-violations): *Field or auto-implemented property cannot be of type unless it is an instance member of a `ref struct`.*
 - [**CS8351**](#ref-safety-violations): *Branches of a `ref` conditional operator cannot refer to variables with incompatible declaration scopes*
 - [**CS8373**](#incorrect-syntax): *The left-hand side of a `ref` assignment must be a ref variable.*
 - [**CS8374**](#ref-safety-violations): *Cannot ref-assign source has a narrower escape scope than destination.*
@@ -183,8 +180,8 @@ That's by design. The text closely matches the text of the compiler error / warn
 - [**CS9078**](#ref-safety-violations): *Cannot return by reference a member of parameter through a `ref` parameter; it can only be returned in a return statement*
 - [**CS9079**](#ref-safety-violations): *Cannot ref-assign because source can only escape the current method through a return statement.*
 - [**CS9096**](#ref-safety-violations): *Cannot ref-assign because source has a wider value escape scope than destination allowing assignment through source of values with narrower escapes scopes than destination.*
-- [**CS9101**](#unscoped-ref-restrictions): *UnscopedRefAttribute can only be applied to struct instance methods and properties, and cannot be applied to constructors or init-only members.*
-- [**CS9102**](#unscoped-ref-restrictions): *UnscopedRefAttribute cannot be applied to an interface implementation.*
+- [**CS9101**](#unscoped-ref-restrictions): *UnscopedRefAttribute can only be applied to struct or virtual interface instance methods and properties, and cannot be applied to constructors or init-only members.*
+- [**CS9102**](#unscoped-ref-restrictions): *UnscopedRefAttribute cannot be applied to an interface implementation  because implemented member doesn't have this attribute.*
 - [**CS9104**](#reference-variable-restrictions): *A `using` statement resource of type cannot be used in async methods or async lambda expressions.*
 - [**CS9190**](#incorrect-syntax): *`readonly` modifier must be specified after `ref`.*
 - [**CS9199**](#reference-variable-restrictions): *A `ref readonly` parameter cannot have the Out attribute.*
@@ -285,8 +282,8 @@ To fix the error, remove the reference variable where it isn't allowed:
 
 The `unscoped` qualifier on `ref` parameters isn't allowed in some locations:
 
-- **CS9101**:  *UnscopedRefAttribute can only be applied to struct instance methods and properties, and cannot be applied to constructors or  or init-only members.*
-- **CS9102**:  *UnscopedRefAttribute cannot be applied to an interface implementation.*
+- **CS9101**:  *UnscopedRefAttribute can only be applied to struct instance or virtual interface methods and properties, and cannot be applied to constructors or  or init-only members.*
+- **CS9102**:  *UnscopedRefAttribute cannot be applied to an interface implementation because implemented member doesn't have this attribute..*
 
 You must remove the `unscoped` modifier on the parameter declaration that caused the error.
 

docs/csharp/language-reference/compiler-messages/ref-modifiers-errors.md

@@ -0,0 +1,86 @@
+---
+title: Errors and warnings associated with `ref struct` types
+description: Learn about errors and warnings related to `ref struct` types. The compiler enforces several restrictions on `ref struct` types to enforce ref safety rules
+f1_keywords:
+  - "CS8343"
+  - "CS8344"
+  - "CS8345"
+  - "CS9048"
+  - "CS9050"
+  - "CS9059"
+  - "CS9241"
+  - "CS9242"
+  - "CS9243"
+  - "CS9244"
+  - "CS9245"
+  - "CS9246"
+  - "CS9247"
+helpviewer_keywords:
+  - "CS8343"
+  - "CS8344"
+  - "CS8345"
+  - "CS9048"
+  - "CS9050"
+  - "CS9059"
+  - "CS9241"
+  - "CS9242"
+  - "CS9243"
+  - "CS9244"
+  - "CS9245"
+  - "CS9246"
+  - "CS9247"
+ms.date: 07/30/2024
+---
+# Errors and warnings associated with `ref struct` types
+
+- [**CS8343**](#ref-struct-interface-implementations): *`ref structs` cannot implement interfaces*
+- [**CS8344**](#ref-struct-interface-implementations): *`foreach` statement cannot operate on enumerators in async or iterator methods because type is a `ref struct` or a type parameter that allows `ref struct`.*
+- [**CS8345**](#ref-safety-violations): *Field or auto-implemented property cannot be of type unless it is an instance member of a `ref struct`.*
+- [**CS9048**](#ref-safety-violations): *The `scoped` modifier can be used for refs and `ref struct` values only.*
+- [**CS9050**](#ref-safety-violations): *A `ref` field cannot refer to a `ref struct`.*
+- [**CS9059**](#ref-safety-violations): *A ref field can only be declared in a ref struct.*
+- [**CS9241**](#ref-struct-interface-implementations): *'ref struct' is already specified.*
+- [**CS9242**](#ref-struct-interface-implementations): *The 'allows' constraint clause must be the last constraint specified.*
+- [**CS9243**](#ref-struct-interface-implementations): *Cannot allow ref structs for a type parameter known from other constraints to be a class.*
+- [**CS9244**](#ref-struct-interface-implementations): *The type may not be a `ref struct` or a type parameter allowing ref structs in order to use it as parameter in the generic type or method.*
+- [**CS9245**](#ref-struct-interface-implementations): *Type cannot implement interface member for `ref struct` type.*
+- [**CS9246**](#ref-struct-interface-implementations): *A non-virtual instance interface member cannot be accessed on a type parameter that allows ref struct.*
+- [**CS9247**](#ref-struct-interface-implementations): *foreach statement cannot operate on enumerators of type because it is a type parameter that allows ref struct and it is not known at compile time to implement `IDisposable`.*
+
+## ref safety violations
+
+- **CS8345**: *Field or auto-implemented property cannot be of type unless it is an instance member of a `ref struct`.*
+- **CS9048**: *The `scoped` modifier can be used for refs and `ref struct` values only.*
+- **CS9050**: *A `ref` field cannot refer to a `ref struct`.*
+- **CS9059**: *A `ref` field can only be declared in a `ref struct`.*
+
+A [`ref struct`](../builtin-types/ref-struct.md) type can include `ref` fields. Other types aren't allowed `ref` fields. The compiler enforces restrictions on the declarations and use of `ref struct` types to enforce ref safety rules on instances of any `ref struct` type:
+
+- Only `ref struct` types can contain auto-implemented `ref` properties.
+- Only `ref struct` types or `ref` variables can have the `scoped` modifier.
+- A `ref` field can be declared only in a `ref struct` type.
+- A `ref` field can't refer to a `ref struct` type/
+
+Violating any of these rules produces one of the listed errors. If you intended to use that language feature, convert the type to a `ref struct`. Otherwise, remove the disallowed construct.
+
+## ref struct interface implementations
+
+- **CS8343**: *`ref structs` cannot implement interfaces*
+- **CS8344**: *`foreach` statement cannot operate on enumerators in async or iterator methods because type is a `ref struct` or a type parameter that allows `ref struct`.*
+- **CS9241**: *'ref struct' is already specified.*
+- **CS9242**: *The 'allows' constraint clause must be the last constraint specified.*
+- **CS9243**: *Cannot allow ref structs for a type parameter known from other constraints to be a class.*
+- **CS9244**: *The type may not be a `ref struct` or a type parameter allowing ref structs in order to use it as parameter in the generic type or method.*
+- **CS9245**: *Type cannot implement interface member for `ref struct` type.*
+- **CS9246**: *A non-virtual instance interface member cannot be accessed on a type parameter that allows ref struct.*
+- **CS9247**: *foreach statement cannot operate on enumerators of type because it is a type parameter that allows ref struct and it is not known at compile time to implement `IDisposable`.*
+
+Prior to C# 13, [`ref struct`](../builtin-types/ref-struct.md) types can't implement interfaces; the compiler generates *CS8343*. Beginning with C# 13, `ref struct` types can implement interfaces, subject to the following rules:
+
+- A `ref struct` can't be converted to an instance of an interface it implements. This restriction includes the implicit conversion when you use a `ref struct` type as an argument when the parameter is an interface type. The conversion results in a boxing conversion, which violates ref safety.
+- A `ref struct` that implements an interface *must* implement all interface members. The `ref struct` must implement members where the interface includes a default implementation.
+
+Beginning with C# 13, a `ref struct` can be used as a type argument for a generic type parameter, if and only if the generic type parameter has the [`allows ref struct`](../../programming-guide/generics/constraints-on-type-parameters.md#allows-ref-struct) anti-constraint. When you use the `allows ref struct` anti-constraint you must follow these rules:
+
+- A `ref struct` is used as a type argument, the type parameter *must* have the `allows ref struct` anti-constraint.- The `allows ref struct` anti-constraint must be last in the `where` clause for that parameter
+- Uses of instances the type parameter must obey ref safety rules.

docs/csharp/language-reference/compiler-messages/ref-struct-errors.md

@@ -18,3 +18,4 @@ You use the `ref` keyword in the following contexts:
 - As the part of a [conditional ref expression](../operators/conditional-operator.md#conditional-ref-expression) or a [ref assignment operator](../operators/assignment-operator.md#ref-assignment).
 - In a `struct` declaration, to declare a `ref struct`. For more information, see the [`ref` structure types](../builtin-types/ref-struct.md) article.
 - In a `ref struct` definition, to declare a `ref` field. For more information, see the [`ref` fields](../builtin-types/ref-struct.md#ref-fields) section of the [`ref` structure types](../builtin-types/ref-struct.md) article.
+- In a generic type declaration to specify that a type parameter [`allows ref struct`](../../programming-guide/generics/constraints-on-type-parameters.md#allows-ref-struct) types.

docs/csharp/language-reference/keywords/ref.md

@@ -437,7 +437,7 @@ items:
       CS0171, CS0188, CS0843, CS8904, CS1738, CS8022, CS8023, CS8024, CS8025, CS8026, CS8058, CS8059, CS8107, CS8192, CS8302,
       CS8303, CS8304, CS8305, CS8306, CS8314, CS8320, CS8370, CS8371, CS8400, CS8401, CS8511, CS8627, CS8630, CS8652, CS8703,
       CS8704, CS8706, CS8773, CS8912, CS8919, CS8929, CS8936, CS8957, CS8967, CS9014, CS9015, CS9016, CS9017, CS9058, CS9064,
-      CS9103, CS9171, CS9194, CS9202, CS9204
+      CS9103, CS9171, CS9194, CS9202, CS9204, CS9240
   - name: Assembly references
     href: ./compiler-messages/assembly-references.md
     displayName: >
@@ -460,10 +460,15 @@ items:
     displayName: >
       ref safety,
       CS0192, CS0199, CS0206, CS0631, CS0767, CS1510, CS1605, CS1623, CS1649, CS1651, CS1655, CS1657, CS1741, CS1939, CS1988,
-      CS7084, CS8166, CS8167, CS8168, CS8169. CS8325, CS8326, CS8327, CS8329, CS8330, CS8331, CS8332, CS8337, CS8338, CS8345,
-      CS8351, CS8373, CS8374, CS8388, CS8977, CS9072, CS9077, CS9078, CS9079, CS9085, CS9086, CS9087, CS9089, CS9091, CS9092,
-      CS9093, CS9094, CS9095, CS9096, CS9097, CS9101, CS9102, CS9104, CS9190, CS9191, CS9192, CS9193, CS9195, CS9196, CS9197,
-      CS9198, CS9199, CS9200, CS9201
+      CS7084, CS8166, CS8167, CS8168, CS8169. CS8325, CS8326, CS8327, CS8329, CS8330, CS8331, CS8332, CS8337, CS8338, CS8351,
+      CS8373, CS8374, CS8388, CS8977, CS9072, CS9077, CS9078, CS9079, CS9085, CS9086, CS9087, CS9089, CS9091, CS9092, CS9093,
+      CS9094, CS9095, CS9096, CS9097, CS9101, CS9102, CS9104, CS9190, CS9191, CS9192, CS9193, CS9195, CS9196, CS9197, CS9198,
+      CS9199, CS9200, CS9201
+  - name: "`ref struct` types"
+    href: ./compiler-messages/ref-struct-errors.md
+    displayName: > 
+      ref struct,
+      CS8343, CS8344, CS8345, CS9048, CS9050, CS9059, CS9241, CS9242, CS9243, CS9244, CS9245, CS9246, CS9247
   - name: Iterator methods
     href: ./compiler-messages/iterator-yield.md
     displayName: > 
@@ -2012,8 +2017,6 @@ items:
       href: ./compiler-messages/CS8817.md
     - name: CS9043
       href: ./compiler-messages/cs9043.md
-    - name: CS9050
-      href: ./compiler-messages/cs9050.md
   - name: Level 1 warning messages
     items:
     - name: CS0183

docs/csharp/language-reference/toc.yml

@@ -10,7 +10,7 @@ ms.assetid: 94eac5a8-7344-44d2-9d0c-a9954993603d
 ---
 # Compiler Error CS0401
 
-The new() constraint must be the last constraint specified  
+The new() constraint must be the last restrictive constraint specified  
 
  When using multiple constraints, list all other constraints before the new() constraint.  
 

docs/csharp/misc/cs0401.md

@@ -304,8 +304,6 @@ f1_keywords:
   - "CS8340"
   - "CS8341"
   - "CS8342"
-  - "CS8343"
-  - "CS8344"
   - "CS8346"
   - "CS8347"
   - "CS8348"
@@ -584,15 +582,13 @@ f1_keywords:
   - "CS9045"
   - "CS9046"
   - "CS9047"
-  - "CS9048"
   - "CS9049"
   - "CS9051"
   - "CS9052"
   - "CS9053"
   - "CS9054"
   - "CS9056"
   - "CS9057"
-  - "CS9059"
   - "CS9060"
   - "CS9061"
   - "CS9062"