Produce warnings for definite assignment issues with private fields from metadata.

docs/compilers/CSharp/Warnversion Warning Waves.md

@@ -22,3 +22,11 @@ The table below describes all of the warnings controlled by warning levels `5` o
 | CS7023 | 5 | [A static type is used in an 'is' or 'as' expression](https://github.com/dotnet/roslyn/issues/30198) |
 | CS8073 | 5 | [Expression always true (or false) when comparing a struct to null](https://github.com/dotnet/roslyn/issues/45744) |
 | CS8848 | 5 | [Diagnose precedence error with query expression](https://github.com/dotnet/roslyn/issues/30231) |
+| CS8880 | 5 | [Struct constructor does not assign auto property (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8881 | 5 | [Struct constructor does not assign field (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8882 | 5 | [Out parameter not assigned (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8883 | 5 | [Auto-property used before assigned in struct constructor (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8884 | 5 | [Field used before assigned in struct constructor (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8885 | 5 | [Struct constructor reads 'this' before assigning all fields (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8886 | 5 | [Out parameter used before being assigned (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |
+| CS8887 | 5 | [Local variable used before being assigned (imported struct type with private fields)](https://github.com/dotnet/roslyn/issues/30194) |

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -819,6 +819,12 @@
   <data name="ERR_UseDefViolation" xml:space="preserve">
     <value>Use of unassigned local variable '{0}'</value>
   </data>
+  <data name="WRN_UseDefViolation" xml:space="preserve">
+    <value>Use of unassigned local variable '{0}'</value>
+  </data>
+  <data name="WRN_UseDefViolation_Title" xml:space="preserve">
+    <value>Use of unassigned local variable</value>
+  </data>
   <data name="WRN_UnreferencedVar" xml:space="preserve">
     <value>The variable '{0}' is declared but never used</value>
   </data>
@@ -834,12 +840,30 @@
   <data name="ERR_UseDefViolationField" xml:space="preserve">
     <value>Use of possibly unassigned field '{0}'</value>
   </data>
+  <data name="WRN_UseDefViolationField" xml:space="preserve">
+    <value>Use of possibly unassigned field '{0}'</value>
+  </data>
+  <data name="WRN_UseDefViolationField_Title" xml:space="preserve">
+    <value>Use of possibly unassigned field</value>
+  </data>
   <data name="ERR_UseDefViolationProperty" xml:space="preserve">
     <value>Use of possibly unassigned auto-implemented property '{0}'</value>
   </data>
+  <data name="WRN_UseDefViolationProperty" xml:space="preserve">
+    <value>Use of possibly unassigned auto-implemented property '{0}'</value>
+  </data>
+  <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>
+  <data name="WRN_UnassignedThis" xml:space="preserve">
+    <value>Field '{0}' must be fully assigned before control is returned to the caller</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>
   <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>
   </data>
@@ -858,6 +882,12 @@
   <data name="ERR_ParamUnassigned" xml:space="preserve">
     <value>The out parameter '{0}' must be assigned to before control leaves the current method</value>
   </data>
+  <data name="WRN_ParamUnassigned" xml:space="preserve">
+    <value>The out parameter '{0}' must be assigned to before control leaves the current method</value>
+  </data>
+  <data name="WRN_ParamUnassigned_Title" xml:space="preserve">
+    <value>An out parameter must be assigned to before control leaves the method</value>
+  </data>
   <data name="ERR_InvalidArray" xml:space="preserve">
     <value>Invalid rank specifier: expected ',' or ']'</value>
   </data>
@@ -901,7 +931,13 @@
     <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 are assigned to</value>
+    <value>The 'this' object cannot be used before all of its fields have been assigned</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>
+  <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>
   <data name="ERR_ArgsInvalid" xml:space="preserve">
     <value>The __arglist construct is valid only within a variable argument method</value>
@@ -1116,6 +1152,12 @@
   <data name="ERR_UseDefViolationOut" xml:space="preserve">
     <value>Use of unassigned out parameter '{0}'</value>
   </data>
+  <data name="WRN_UseDefViolationOut" xml:space="preserve">
+    <value>Use of unassigned out parameter '{0}'</value>
+  </data>
+  <data name="WRN_UseDefViolationOut_Title" xml:space="preserve">
+    <value>Use of unassigned out parameter</value>
+  </data>
   <data name="ERR_ArraySizeInDeclaration" xml:space="preserve">
     <value>Array size cannot be specified in a variable declaration (try initializing with a 'new' expression)</value>
   </data>
@@ -2249,6 +2291,12 @@ If such a class is used as a base class and if the deriving class defines a dest
   <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>
+  <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>
+  <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>
   <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>
   </data>

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

@@ -1874,6 +1874,17 @@ internal enum ErrorCode
         ERR_CopyConstructorWrongAccessibility = 8878,
         ERR_NonPrivateAPIInRecord = 8879,
 
+        // 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_ParamUnassigned = 8882,
+        WRN_UseDefViolationProperty = 8883,
+        WRN_UseDefViolationField = 8884,
+        WRN_UseDefViolationThis = 8885,
+        WRN_UseDefViolationOut = 8886,
+        WRN_UseDefViolation = 8887,
+
         #endregion diagnostics introduced for C# 9.0
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)
     }

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

@@ -220,6 +220,14 @@ 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_ParamUnassigned:
+                case ErrorCode.WRN_UseDefViolationProperty:
+                case ErrorCode.WRN_UseDefViolationField:
+                case ErrorCode.WRN_UseDefViolationThis:
+                case ErrorCode.WRN_UseDefViolationOut:
+                case ErrorCode.WRN_UseDefViolation:
                     // Warning level 5 is exclusively for warnings introduced in the compiler
                     // shipped with dotnet 5 (C# 9) and that can be reported for pre-existing code.
                     return 5;

src/Compilers/CSharp/Portable/FlowAnalysis/DefiniteAssignment.cs

@@ -17,10 +17,10 @@
 #define REFERENCE_STATE
 #endif
 
-using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
@@ -123,13 +123,14 @@ internal DefiniteAssignmentPass(
             CSharpCompilation compilation,
             Symbol member,
             BoundNode node,
+            bool strictAnalysis,
             bool trackUnassignments = false,
             HashSet<PrefixUnaryExpressionSyntax> unassignedVariableAddressOfSyntaxes = null,
             bool requireOutParamsAssigned = true,
             bool trackClassFields = false,
             bool trackStaticMembers = false)
             : base(compilation, member, node,
-                   compilation.FeatureStrictEnabled ? EmptyStructTypeCache.CreatePrecise() : EmptyStructTypeCache.CreateForDev12Compatibility(compilation),
+                   strictAnalysis ? EmptyStructTypeCache.CreatePrecise() : EmptyStructTypeCache.CreateForDev12Compatibility(compilation),
                    trackUnassignments)
         {
             this.initiallyAssignedVariables = null;
@@ -149,9 +150,7 @@ internal DefiniteAssignmentPass(
             EmptyStructTypeCache emptyStructs,
             bool trackUnassignments = false,
             HashSet<Symbol> initiallyAssignedVariables = null)
-            : base(compilation, member, node,
-                   emptyStructs ?? (compilation.FeatureStrictEnabled ? EmptyStructTypeCache.CreatePrecise() : EmptyStructTypeCache.CreateForDev12Compatibility(compilation)),
-                   trackUnassignments)
+            : base(compilation, member, node, emptyStructs, trackUnassignments)
         {
             this.initiallyAssignedVariables = initiallyAssignedVariables;
             _sourceAssembly = ((object)member == null) ? null : (SourceAssemblySymbol)member.ContainingAssembly;
@@ -378,33 +377,123 @@ protected virtual void ReportUnassignedOutParameter(ParameterSymbol parameter, S
         /// <summary>
         /// Perform data flow analysis, reporting all necessary diagnostics.
         /// </summary>
-        public static void Analyze(CSharpCompilation compilation, MethodSymbol member, BoundNode node, DiagnosticBag diagnostics, bool requireOutParamsAssigned = true)
+        public static void Analyze(
+            CSharpCompilation compilation,
+            MethodSymbol member,
+            BoundNode node,
+            DiagnosticBag diagnostics,
+            bool requireOutParamsAssigned = true)
         {
             Debug.Assert(diagnostics != null);
 
-            var walker = new DefiniteAssignmentPass(
-                compilation,
-                member,
-                node,
-                requireOutParamsAssigned: requireOutParamsAssigned);
-            walker._convertInsufficientExecutionStackExceptionToCancelledByStackGuardException = true;
+            // Run the strongest version of analysis
+            DiagnosticBag strictDiagnostics = analyze(strictAnalysis: true);
+            if (strictDiagnostics.IsEmptyWithoutResolution)
+            {
+                // If it reports nothing, there is nothing to report and we are done.
+                strictDiagnostics.Free();
+                return;
+            }
 
-            try
+            // Also run the compat (weaker) version of analysis to see if we get the same diagnostics.
+            // If any are missing, the extra ones from the strong analysis will be downgraded to a warning.
+            DiagnosticBag compatDiagnostics = analyze(strictAnalysis: false);
+
+            // If the compat diagnostics caused a stack overflow, the two analyses might not produce comparable sets of diagnostics.
+            // So we just report the compat ones including that error.
+            if (compatDiagnostics.AsEnumerable().Any(d => (ErrorCode)d.Code == ErrorCode.ERR_InsufficientStack))
             {
-                bool badRegion = false;
-                walker.Analyze(ref badRegion, diagnostics);
-                Debug.Assert(!badRegion);
+                diagnostics.AddRangeAndFree(compatDiagnostics);
+                strictDiagnostics.Free();
+                return;
             }
-            catch (BoundTreeVisitor.CancelledByStackGuardException ex) when (diagnostics != null)
+
+            // If the compat diagnostics did not overflow and we have the same number of diagnostics, we just report the stricter set.
+            // It is OK if the strict analysis had an overflow here, causing the sets to be incomparable: the reported diagnostics will
+            // include the error reporting that fact.
+            if (strictDiagnostics.Count == compatDiagnostics.Count)
             {
-                ex.AddAnError(diagnostics);
+                diagnostics.AddRangeAndFree(strictDiagnostics);
+                compatDiagnostics.Free();
+                return;
             }
-            finally
+
+            HashSet<Diagnostic> compatDiagnosticSet = new HashSet<Diagnostic>(compatDiagnostics.AsEnumerable(), SameDiagnosticComparer.Instance);
+            compatDiagnostics.Free();
+            foreach (var diagnostic in strictDiagnostics.AsEnumerable())
             {
-                walker.Free();
+                // If it is a warning (e.g. WRN_AsyncLacksAwaits), or an error that would be reported by the compatible analysis, just report it.
+                if (diagnostic.Severity != DiagnosticSeverity.Error || compatDiagnosticSet.Contains(diagnostic))
+                {
+                    diagnostics.Add(diagnostic);
+                    continue;
+                }
+
+                // Otherwise downgrade the error to a warning.
+                ErrorCode oldCode = (ErrorCode)diagnostic.Code;
+                ErrorCode newCode = oldCode switch
+                {
+#pragma warning disable format
+                    ErrorCode.ERR_UnassignedThisAutoProperty => ErrorCode.WRN_UnassignedThisAutoProperty,
+                    ErrorCode.ERR_UnassignedThis             => ErrorCode.WRN_UnassignedThis,
+                    ErrorCode.ERR_ParamUnassigned            => ErrorCode.WRN_ParamUnassigned,
+                    ErrorCode.ERR_UseDefViolationProperty    => ErrorCode.WRN_UseDefViolationProperty,
+                    ErrorCode.ERR_UseDefViolationField       => ErrorCode.WRN_UseDefViolationField,
+                    ErrorCode.ERR_UseDefViolationThis        => ErrorCode.WRN_UseDefViolationThis,
+                    ErrorCode.ERR_UseDefViolationOut         => ErrorCode.WRN_UseDefViolationOut,
+                    ErrorCode.ERR_UseDefViolation            => ErrorCode.WRN_UseDefViolation,
+                    _ => oldCode, // rare but possible, e.g. ErrorCode.ERR_InsufficientStack occurring in strict mode only due to needing extra frames
+#pragma warning restore format
+                };
+
+                // We don't know any other way this can happen, but if it does we recover gracefully in production.
+                Debug.Assert(newCode != oldCode || oldCode == ErrorCode.ERR_InsufficientStack, oldCode.ToString());
+
+                var args = diagnostic is DiagnosticWithInfo { Info: { Arguments: var arguments } } ? arguments : diagnostic.Arguments.ToArray();
+                diagnostics.Add(newCode, diagnostic.Location, args);
+            }
+
+            strictDiagnostics.Free();
+            return;
+
+            DiagnosticBag analyze(bool strictAnalysis)
+            {
+                DiagnosticBag result = DiagnosticBag.GetInstance();
+                var walker = new DefiniteAssignmentPass(
+                    compilation,
+                    member,
+                    node,
+                    strictAnalysis: strictAnalysis,
+                    requireOutParamsAssigned: requireOutParamsAssigned);
+                walker._convertInsufficientExecutionStackExceptionToCancelledByStackGuardException = true;
+
+                try
+                {
+                    bool badRegion = false;
+                    walker.Analyze(ref badRegion, result);
+                    Debug.Assert(!badRegion);
+                }
+                catch (BoundTreeVisitor.CancelledByStackGuardException ex) when (diagnostics != null)
+                {
+                    ex.AddAnError(result);
+                }
+                finally
+                {
+                    walker.Free();
+                }
+
+                return result;
             }
         }
 
+        private sealed class SameDiagnosticComparer : EqualityComparer<Diagnostic>
+        {
+            public static readonly SameDiagnosticComparer Instance = new SameDiagnosticComparer();
+            public override bool Equals([AllowNull] Diagnostic x, [AllowNull] Diagnostic y) => x.Equals(y);
+            public override int GetHashCode([DisallowNull] Diagnostic obj) =>
+                Hash.Combine(Hash.CombineValues(obj.Arguments), Hash.Combine(obj.Location.GetHashCode(), obj.Code));
+        }
+
         /// <summary>
         /// Analyze the body, reporting all necessary diagnostics.
         /// </summary>

src/Compilers/CSharp/Portable/FlowAnalysis/LocalDataFlowPass.cs

@@ -65,6 +65,7 @@ protected LocalDataFlowPass(
             int maxSlotDepth = 0)
             : base(compilation, member, node, nonMonotonicTransferFunction: trackUnassignments)
         {
+            Debug.Assert(emptyStructs != null);
             _maxSlotDepth = maxSlotDepth;
             _emptyStructTypeCache = emptyStructs;
         }

src/Compilers/CSharp/Portable/FlowAnalysis/UnassignedAddressTakenVariablesWalker.cs

@@ -4,9 +4,7 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
-using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
-using Microsoft.CodeAnalysis.Text;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
@@ -16,7 +14,7 @@ namespace Microsoft.CodeAnalysis.CSharp
     internal class UnassignedAddressTakenVariablesWalker : DefiniteAssignmentPass
     {
         private UnassignedAddressTakenVariablesWalker(CSharpCompilation compilation, Symbol member, BoundNode node)
-            : base(compilation, member, node)
+            : base(compilation, member, node, strictAnalysis: true)
         {
         }
 

src/Compilers/CSharp/Portable/FlowAnalysis/UnassignedFieldsWalker.cs

@@ -22,7 +22,7 @@ internal sealed class UnassignedFieldsWalker : DefiniteAssignmentPass
         private readonly DiagnosticBag _diagnostics;
 
         private UnassignedFieldsWalker(CSharpCompilation compilation, MethodSymbol method, BoundNode node, DiagnosticBag diagnostics)
-            : base(compilation, method, node, trackClassFields: true, trackStaticMembers: method.MethodKind == MethodKind.StaticConstructor)
+            : base(compilation, method, node, strictAnalysis: true, trackClassFields: true, trackStaticMembers: method.MethodKind == MethodKind.StaticConstructor)
         {
             _diagnostics = diagnostics;
         }