Implicit field initialization in struct constructors

docs/Language Feature Status.md

@@ -29,6 +29,7 @@ efforts behind them.
 | [Utf8 String Literals](https://github.com/dotnet/csharplang/issues/184) | [Utf8StringLiterals](https://github.com/dotnet/roslyn/tree/features/Utf8StringLiterals) | [In Progress](https://github.com/dotnet/roslyn/issues/58848) | [AlekseyTs](https://github.com/AlekseyTs) | [cston](https://github.com/cston), [RikkiGibson](https://github.com/RikkiGibson) | [MadsTorgersen](https://github.com/MadsTorgersen) |
 | [ref fields](https://github.com/dotnet/csharplang/blob/main/proposals/low-level-struct-improvements.md) | [ref-fields](https://github.com/dotnet/roslyn/tree/features/ref-fields) | [In Progress](https://github.com/dotnet/roslyn/issues/59194) | [cston](https://github.com/cston) | [RikkiGibson](https://github.com/RikkiGibson), [AlekseyTs](https://github.com/AlekseyTs) | [jaredpar](https://github.com/jaredpar) | 
 | [checked operators](https://github.com/dotnet/csharplang/issues/4665) | [CheckedUserDefinedOperators](https://github.com/dotnet/roslyn/tree/features/CheckedUserDefinedOperators) | [In Progress](https://github.com/dotnet/roslyn/issues/59458) | [AlekseyTs](https://github.com/AlekseyTs) | [333fred](https://github.com/333fred), [chsienki](https://github.com/chsienki) | [AlekseyTs](https://github.com/AlekseyTs) | 
+| [auto-default structs](https://github.com/dotnet/csharplang/issues/5737) | main | [In Progress](https://github.com/dotnet/roslyn/issues/60167) | [RikkiGibson](https://github.com/RikkiGibson) | [cston](https://github.com/cston), [jcouv](https://github.com/jcouv) | 
 
 # C# 10.0
 

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -864,14 +864,14 @@
   <data name="WRN_UseDefViolationProperty_Title" xml:space="preserve">
     <value>Use of possibly unassigned auto-implemented property</value>
   </data>
-  <data name="ERR_UnassignedThis" xml:space="preserve">
-    <value>Field '{0}' must be fully assigned before control is returned to the caller</value>
+  <data name="ERR_UnassignedThisUnsupportedVersion" xml:space="preserve">
+    <value>Field '{0}' must be fully assigned before control is returned to the caller. Consider updating to language version '{1}' to auto-default the field.</value>
   </data>
-  <data name="WRN_UnassignedThis" xml:space="preserve">
-    <value>Field '{0}' must be fully assigned before control is returned to the caller</value>
+  <data name="WRN_UnassignedThisUnsupportedVersion" xml:space="preserve">
+    <value>Field '{0}' must be fully assigned before control is returned to the caller. Consider updating to language version '{1}' to auto-default the field.</value>
   </data>
-  <data name="WRN_UnassignedThis_Title" xml:space="preserve">
-    <value>Fields of a struct must be fully assigned in a constructor before control is returned to the caller</value>
+  <data name="WRN_UnassignedThisUnsupportedVersion_Title" xml:space="preserve">
+    <value>Fields of a struct must be fully assigned in a constructor before control is returned to the caller. Consider updating the language version to auto-default the field.</value>
   </data>
   <data name="ERR_AmbigQM" xml:space="preserve">
     <value>Type of conditional expression cannot be determined because '{0}' and '{1}' implicitly convert to one another</value>
@@ -939,14 +939,14 @@
   <data name="ERR_DefaultLiteralNotValid" xml:space="preserve">
     <value>Use of default literal is not valid in this context</value>
   </data>
-  <data name="ERR_UseDefViolationThis" xml:space="preserve">
-    <value>The 'this' object cannot be used before all of its fields have been assigned</value>
+  <data name="ERR_UseDefViolationThisUnsupportedVersion" xml:space="preserve">
+    <value>The 'this' object cannot be used before all of its fields have been assigned. Consider updating to language version '{0}' to auto-default the unassigned fields.</value>
   </data>
-  <data name="WRN_UseDefViolationThis" xml:space="preserve">
-    <value>The 'this' object cannot be used before all of its fields have been assigned</value>
+  <data name="WRN_UseDefViolationThisUnsupportedVersion" xml:space="preserve">
+    <value>The 'this' object cannot be used before all of its fields have been assigned. Consider updating to language version '{0}' to auto-default the unassigned fields.</value>
   </data>
-  <data name="WRN_UseDefViolationThis_Title" xml:space="preserve">
-    <value>The 'this' object cannot be used in a constructor before all of its fields have been assigned</value>
+  <data name="WRN_UseDefViolationThisUnsupportedVersion_Title" xml:space="preserve">
+    <value>The 'this' object cannot be used in a constructor before all of its fields have been assigned. Consider updating the language version to auto-default the unassigned fields.</value>
   </data>
   <data name="ERR_ArgsInvalid" xml:space="preserve">
     <value>The __arglist construct is valid only within a variable argument method</value>
@@ -2318,14 +2318,14 @@ If such a class is used as a base class and if the deriving class defines a dest
   <data name="ERR_RecursivelyTypedVariable" xml:space="preserve">
     <value>Type of '{0}' cannot be inferred since its initializer directly or indirectly refers to the definition.</value>
   </data>
-  <data name="ERR_UnassignedThisAutoProperty" xml:space="preserve">
-    <value>Auto-implemented property '{0}' must be fully assigned before control is returned to the caller.</value>
+  <data name="ERR_UnassignedThisAutoPropertyUnsupportedVersion" xml:space="preserve">
+    <value>Auto-implemented property '{0}' must be fully assigned before control is returned to the caller. Consider updating to language version '{1}' to auto-default the property.</value>
   </data>
-  <data name="WRN_UnassignedThisAutoProperty" xml:space="preserve">
-    <value>Auto-implemented property '{0}' must be fully assigned before control is returned to the caller.</value>
+  <data name="WRN_UnassignedThisAutoPropertyUnsupportedVersion" xml:space="preserve">
+    <value>Auto-implemented property '{0}' must be fully assigned before control is returned to the caller. Consider updating to language version '{1}' to auto-default the property.</value>
   </data>
-  <data name="WRN_UnassignedThisAutoProperty_Title" xml:space="preserve">
-    <value>An auto-implemented property must be fully assigned before control is returned to the caller.</value>
+  <data name="WRN_UnassignedThisAutoPropertyUnsupportedVersion_Title" xml:space="preserve">
+    <value>An auto-implemented property must be fully assigned before control is returned to the caller. Consider updating the language version to auto-default the property.</value>
   </data>
   <data name="ERR_VariableUsedBeforeDeclarationAndHidesField" xml:space="preserve">
     <value>Cannot use local variable '{0}' before it is declared. The declaration of the local variable hides the field '{1}'.</value>
@@ -6697,6 +6697,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureLineSpanDirective" xml:space="preserve">
     <value>line span directive</value>
   </data>
+  <data name="IDS_FeatureAutoDefaultStructs" xml:space="preserve">
+    <value>auto default struct fields</value>
+  </data>
   <data name="ERR_LineSpanDirectiveInvalidValue" xml:space="preserve">
     <value>The #line directive value is missing or out of range</value>
   </data>
@@ -6769,6 +6772,63 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ListPatternRequiresLength" xml:space="preserve">
     <value>List patterns may not be used for a value of type '{0}'. No suitable 'Length' or 'Count' property was found.</value>
   </data>
+
+  <data name="WRN_UseDefViolationPropertySupportedVersion" xml:space="preserve">
+    <value>Auto-implemented property '{0}' is read before being explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+  <data name="WRN_UseDefViolationPropertySupportedVersion_Title" xml:space="preserve">
+    <value>Auto-implemented property is read before being explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+
+  <data name="WRN_UseDefViolationFieldSupportedVersion" xml:space="preserve">
+    <value>Field '{0}' is read before being explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+  <data name="WRN_UseDefViolationFieldSupportedVersion_Title" xml:space="preserve">
+    <value>Field is read before being explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+
+  <data name="WRN_UseDefViolationThisSupportedVersion" xml:space="preserve">
+    <value>The 'this' object is read before all of its fields have been assigned, causing preceding implicit assignments of 'default' to non-explicitly assigned fields.</value>
+  </data>
+  <data name="WRN_UseDefViolationThisSupportedVersion_Title" xml:space="preserve">
+    <value>The 'this' object is read before all of its fields have been assigned, causing preceding implicit assignments of 'default' to non-explicitly assigned fields.</value>
+  </data>
+
+  <data name="WRN_UnassignedThisAutoPropertySupportedVersion" xml:space="preserve">
+    <value>Control is returned to caller before auto-implemented property '{0}' is explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+  <data name="WRN_UnassignedThisAutoPropertySupportedVersion_Title" xml:space="preserve">
+    <value>Control is returned to caller before auto-implemented property is explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+
+  <data name="WRN_UnassignedThisSupportedVersion" xml:space="preserve">
+    <value>Control is returned to caller before field '{0}' is explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+  <data name="WRN_UnassignedThisSupportedVersion_Title" xml:space="preserve">
+    <value>Control is returned to caller before field is explicitly assigned, causing a preceding implicit assignment of 'default'.</value>
+  </data>
+
+  <data name="ERR_UseDefViolationFieldUnsupportedVersion" xml:space="preserve">
+    <value>Use of possibly unassigned field '{0}'. Consider updating to language version '{1}' to auto-default the field.</value>
+  </data>
+  <data name="ERR_UseDefViolationPropertyUnsupportedVersion" xml:space="preserve">
+    <value>Use of possibly unassigned auto-implemented property '{0}'. Consider updating to language version '{1}' to auto-default the property.</value>
+  </data>
+
+  <data name="WRN_UseDefViolationFieldUnsupportedVersion" xml:space="preserve">
+    <value>Use of possibly unassigned field '{0}'. Consider updating to language version '{1}' to auto-default the field.</value>
+  </data>
+  <data name="WRN_UseDefViolationFieldUnsupportedVersion_Title" xml:space="preserve">
+    <value>Use of possibly unassigned field. Consider updating the language version to auto-default the field.</value>
+  </data>
+
+  <data name="WRN_UseDefViolationPropertyUnsupportedVersion" xml:space="preserve">
+    <value>Use of possibly unassigned auto-implemented property '{0}'. Consider updating to language version '{1}' to auto-default the property.</value>
+  </data>
+  <data name="WRN_UseDefViolationPropertyUnsupportedVersion_Title" xml:space="preserve">
+    <value>Use of possibly unassigned auto-implemented property. Consider updating the language version to auto-default the property.</value>
+  </data>
+
   <data name="ERR_UnsupportedTypeForSlicePattern" xml:space="preserve">
     <value>Slice patterns may not be used for a value of type '{0}'.</value>
   </data>

src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs

@@ -1015,7 +1015,7 @@ private void CompileMethod(
                     }
 
                     var unusedDiagnostics = DiagnosticBag.GetInstance();
-                    DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, initializerStatements, unusedDiagnostics, requireOutParamsAssigned: false);
+                    DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, initializerStatements, unusedDiagnostics, out _, requireOutParamsAssigned: false);
                     DiagnosticsPass.IssueDiagnostics(_compilation, initializerStatements, BindingDiagnosticBag.Discarded, methodSymbol);
                     unusedDiagnostics.Free();
                 }
@@ -1077,7 +1077,7 @@ processedInitializers.AfterInitializersState is null &&
                                 // Flow analysis over the initializers is necessary in order to find assignments to fields.
                                 // Bodies of implicit constructors do not get flow analysis later, so the initializers
                                 // are analyzed here.
-                                DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, analyzedInitializers, diagsForCurrentMethod.DiagnosticBag, requireOutParamsAssigned: false);
+                                DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, analyzedInitializers, diagsForCurrentMethod.DiagnosticBag, out _, requireOutParamsAssigned: false);
                             }
 
                             // In order to get correct diagnostics, we need to analyze initializers and the body together.
@@ -1089,7 +1089,7 @@ processedInitializers.AfterInitializersState is null &&
                         {
                             // These analyses check for diagnostics in lambdas.
                             // Control flow analysis and implicit return insertion are unnecessary.
-                            DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, analyzedInitializers, diagsForCurrentMethod.DiagnosticBag, requireOutParamsAssigned: false);
+                            DefiniteAssignmentPass.Analyze(_compilation, methodSymbol, analyzedInitializers, diagsForCurrentMethod.DiagnosticBag, out _, requireOutParamsAssigned: false);
                             DiagnosticsPass.IssueDiagnostics(_compilation, analyzedInitializers, diagsForCurrentMethod, methodSymbol);
                         }
                     }
@@ -1122,7 +1122,7 @@ processedInitializers.AfterInitializersState is null &&
                 BoundBlock flowAnalyzedBody = null;
                 if (body != null)
                 {
-                    flowAnalyzedBody = FlowAnalysisPass.Rewrite(methodSymbol, body, diagsForCurrentMethod.DiagnosticBag, hasTrailingExpression: hasTrailingExpression, originalBodyNested: originalBodyNested);
+                    flowAnalyzedBody = FlowAnalysisPass.Rewrite(methodSymbol, body, compilationState, diagsForCurrentMethod, hasTrailingExpression: hasTrailingExpression, originalBodyNested: originalBodyNested);
                 }
 
                 bool hasErrors = _hasDeclarationErrors || diagsForCurrentMethod.HasAnyErrors() || processedInitializers.HasErrors;

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

@@ -145,7 +145,7 @@ internal enum ErrorCode
         WRN_UnreferencedVar = 168,
         WRN_UnreferencedField = 169,
         ERR_UseDefViolationField = 170,
-        ERR_UnassignedThis = 171,
+        ERR_UnassignedThisUnsupportedVersion = 171,
         ERR_AmbigQM = 172,
         ERR_InvalidQM = 173, // Requires SymbolDistinguisher.
         ERR_NoBaseClass = 174,
@@ -161,7 +161,7 @@ internal enum ErrorCode
         WRN_IsAlwaysFalse = 184,
         ERR_LockNeedsReference = 185,
         ERR_NullNotValid = 186,
-        ERR_UseDefViolationThis = 188,
+        ERR_UseDefViolationThisUnsupportedVersion = 188,
         ERR_ArgsInvalid = 190,
         ERR_AssgReadonly = 191,
         ERR_RefReadonly = 192,
@@ -582,7 +582,7 @@ internal enum ErrorCode
         //ERR_AutoPropertiesMustHaveBothAccessors = 840,
         ERR_VariableUsedBeforeDeclaration = 841,
         //ERR_ExplicitLayoutAndAutoImplementedProperty = 842,
-        ERR_UnassignedThisAutoProperty = 843,
+        ERR_UnassignedThisAutoPropertyUnsupportedVersion = 843,
         ERR_VariableUsedBeforeDeclarationAndHidesField = 844,
         ERR_ExpressionTreeContainsBadCoalesce = 845,
         ERR_ArrayInitializerExpected = 846,
@@ -1885,12 +1885,12 @@ internal enum ErrorCode
 
         // The following warnings correspond to errors of the same name, but are reported
         // when a definite assignment issue is reported due to private fields imported from metadata.
-        WRN_UnassignedThisAutoProperty = 8880,
-        WRN_UnassignedThis = 8881,
+        WRN_UnassignedThisAutoPropertyUnsupportedVersion = 8880,
+        WRN_UnassignedThisUnsupportedVersion = 8881,
         WRN_ParamUnassigned = 8882,
         WRN_UseDefViolationProperty = 8883,
         WRN_UseDefViolationField = 8884,
-        WRN_UseDefViolationThis = 8885,
+        WRN_UseDefViolationThisUnsupportedVersion = 8885,
         WRN_UseDefViolationOut = 8886,
         WRN_UseDefViolation = 8887,
 
@@ -2051,6 +2051,18 @@ internal enum ErrorCode
         ERR_MisplacedRecord = 9012,
         ERR_PatternSpanCharCannotBeStringNull = 9013,
 
+        // TODO2: pack before merge
+        ERR_UseDefViolationPropertyUnsupportedVersion = 9020,
+        ERR_UseDefViolationFieldUnsupportedVersion = 9021,
+        WRN_UseDefViolationPropertyUnsupportedVersion = 9022,
+        WRN_UseDefViolationFieldUnsupportedVersion = 9023,
+
+        WRN_UseDefViolationPropertySupportedVersion = 9024,
+        WRN_UseDefViolationFieldSupportedVersion = 9025,
+        WRN_UseDefViolationThisSupportedVersion = 9026,
+        WRN_UnassignedThisAutoPropertySupportedVersion = 9027,
+        WRN_UnassignedThisSupportedVersion = 9028,
+
         #endregion
 
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)

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

@@ -217,12 +217,14 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_NubExprIsConstBool2:
                 case ErrorCode.WRN_StaticInAsOrIs:
                 case ErrorCode.WRN_PrecedenceInversion:
-                case ErrorCode.WRN_UnassignedThisAutoProperty:
-                case ErrorCode.WRN_UnassignedThis:
+                case ErrorCode.WRN_UseDefViolationPropertyUnsupportedVersion:
+                case ErrorCode.WRN_UseDefViolationFieldUnsupportedVersion:
+                case ErrorCode.WRN_UnassignedThisAutoPropertyUnsupportedVersion:
+                case ErrorCode.WRN_UnassignedThisUnsupportedVersion:
                 case ErrorCode.WRN_ParamUnassigned:
                 case ErrorCode.WRN_UseDefViolationProperty:
                 case ErrorCode.WRN_UseDefViolationField:
-                case ErrorCode.WRN_UseDefViolationThis:
+                case ErrorCode.WRN_UseDefViolationThisUnsupportedVersion:
                 case ErrorCode.WRN_UseDefViolationOut:
                 case ErrorCode.WRN_UseDefViolation:
                 case ErrorCode.WRN_SyncAndAsyncEntryPoints:
@@ -494,6 +496,11 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_InterpolatedStringHandlerArgumentAttributeIgnoredOnLambdaParameters:
                 case ErrorCode.WRN_CompileTimeCheckedOverflow:
                 case ErrorCode.WRN_MethGrpToNonDel:
+                case ErrorCode.WRN_UseDefViolationPropertySupportedVersion:
+                case ErrorCode.WRN_UseDefViolationFieldSupportedVersion:
+                case ErrorCode.WRN_UseDefViolationThisSupportedVersion:
+                case ErrorCode.WRN_UnassignedThisAutoPropertySupportedVersion:
+                case ErrorCode.WRN_UnassignedThisSupportedVersion:
                     return 1;
                 default:
                     return 0;

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

@@ -246,6 +246,7 @@ internal enum MessageID
         IDS_FeatureRawStringLiterals = MessageBase + 12817,
         IDS_FeatureSpanCharConstantPattern = MessageBase + 12818,
         IDS_FeatureDisposalPattern = MessageBase + 12819,
+        IDS_FeatureAutoDefaultStructs = MessageBase + 12820,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -363,6 +364,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureCacheStaticMethodGroupConversion: // lowering check
                 case MessageID.IDS_ParameterNullChecking: // syntax check
                 case MessageID.IDS_FeatureSpanCharConstantPattern:
+                case MessageID.IDS_FeatureAutoDefaultStructs: // semantic check
                     return LanguageVersion.Preview;
 
                 // C# 10.0 features.