Readonly members prototype cleanup

docs/contributing/Compiler Test Plan.md

@@ -61,6 +61,7 @@ This document provides guidance for thinking about language interactions and tes
 - NoPIA
 - Dynamic
 - Ref structs, Readonly structs
+- Readonly members on structs (methods, property/indexer accessors, custom event accessors)
 
 # Code
 - Operators (see Eric's list below)

docs/features/readonly-instance-members.md

@@ -176,6 +176,11 @@ The compiler would emit the instance member, as usual, and would additionally em
 
 This would allow the user to safely call said instance method without the compiler needing to make a copy.
 
+Some more "edge" cases that are explicitly permitted:
+
+* Explicit interface implementations are allowed to be `readonly`.
+* Partial methods are allowed to be `readonly`. Both signatures or neither must have the `readonly` keyword.
+
 The restrictions would include:
 
 * The `readonly` modifier cannot be applied to static methods, constructors or destructors.

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5448,6 +5448,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_FieldLikeEventCantBeReadOnly" xml:space="preserve">
     <value>Field-like event '{0}' cannot be 'readonly'.</value>
   </data>
+  <data name="ERR_PartialMethodReadOnlyDifference" xml:space="preserve">
+    <value>Both partial method declarations must be readonly or neither may be readonly</value>
+  </data>
   <data name="WRN_NullabilityMismatchInArgument" xml:space="preserve">
     <value>Argument of type '{0}' cannot be used for parameter '{2}' of type '{1}' in '{3}' due to differences in the nullability of reference types.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1689,7 +1689,8 @@ internal enum ErrorCode
         ERR_AutoPropertyWithSetterCantBeReadOnly = 8658,
         ERR_InvalidPropertyReadOnlyMods = 8659,
         ERR_DuplicatePropertyReadOnlyMods = 8660,
-        ERR_FieldLikeEventCantBeReadOnly = 8661
+        ERR_FieldLikeEventCantBeReadOnly = 8661,
+        ERR_PartialMethodReadOnlyDifference = 8662,
 
         #endregion diagnostics introduced for C# 8.0
 

src/Compilers/CSharp/Portable/Symbols/Source/SourceOrdinaryMethodSymbol.cs

@@ -1076,6 +1076,11 @@ private static void PartialMethodChecks(SourceOrdinaryMethodSymbol definition, S
                 diagnostics.Add(ErrorCode.ERR_PartialMethodStaticDifference, implementation.Locations[0]);
             }
 
+            if (definition.IsDeclaredReadOnly != implementation.IsDeclaredReadOnly)
+            {
+                diagnostics.Add(ErrorCode.ERR_PartialMethodReadOnlyDifference, implementation.Locations[0]);
+            }
+
             if (definition.IsExtensionMethod != implementation.IsExtensionMethod)
             {
                 diagnostics.Add(ErrorCode.ERR_PartialMethodExtensionDifference, implementation.Locations[0]);

src/Compilers/CSharp/Portable/Symbols/Source/ThisParameterSymbol.cs

@@ -50,9 +50,7 @@ public override RefKind RefKind
                     return RefKind.Out;
                 }
 
-                // PROTOTYPE: examine remaining locations where TypeSymbol.IsReadOnly is tested on 'this'.
-                // Determine whether it needs to be replaced with e.g. ContainingMethod.IsEffectivelyReadOnly.
-                if (ContainingType.IsReadOnly)
+                if (_containingMethod?.IsEffectivelyReadOnly == true)
                 {
                     return RefKind.In;
                 }

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Výstupní proměnná nemůže být deklarovaná jako lokální proměnná podle odkazu.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">Porovnávání vzorů není povolené pro typy ukazatelů.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Eine out-Variable kann nicht als lokales ref-Element deklariert werden.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">Der Musterabgleich ist für Zeigertypen unzulässig.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Una variable out no se puede declarar como ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">No se permite la coincidencia de patrones para tipos de puntero.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Impossible de déclarer une variable out en tant que variable locale ref</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">Les critères spéciaux ne sont pas autorisés pour les types de pointeur.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Non è possibile dichiarare una variabile out come variabile locale ref</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">I criteri di ricerca non sono consentiti per i tipi di puntatore.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -267,6 +267,11 @@
         <target state="translated">out 変数を ref ローカルと宣言することはできません</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">ポインター型でパターン マッチングを使用することはできません。</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -267,6 +267,11 @@
         <target state="translated">출력 변수는 참조 로컬로 선언할 수 없습니다.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">포인터 형식에 대해 패턴 일치가 허용되지 않습니다.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Zmiennej out nie można zadeklarować jako lokalnej zmiennej ref</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">Dopasowanie wzorca jest niedozwolone dla typów wskaźnika.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Uma variável out não pode ser declarada como uma referência local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">A correspondência de padrões não é permitida para tipos de ponteiro.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Выходная переменная не может быть объявлена как локальная переменная ref</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">Сопоставление шаблонов запрещено для типов указателей.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -267,6 +267,11 @@
         <target state="translated">Bir out değişkeni ref yerel değeri olarak bildirilemez</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">İşaretçi türleri için desen eşleştirmeye izin verilmez.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -267,6 +267,11 @@
         <target state="translated">out 变量无法声明为 ref 局部变量</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">指针类型不允许进行模式匹配。</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -267,6 +267,11 @@
         <target state="translated">out 變數不可宣告為 ref local</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_PartialMethodReadOnlyDifference">
+        <source>Both partial method declarations must be readonly or neither may be readonly</source>
+        <target state="new">Both partial method declarations must be readonly or neither may be readonly</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_PointerTypeInPatternMatching">
         <source>Pattern-matching is not permitted for pointer types.</source>
         <target state="translated">指標類型不允許進行模式比對。</target>