Help compiler enforce nullability annotations

src/coreclr/src/System.Private.CoreLib/src/System/Environment.CoreCLR.cs

@@ -16,6 +16,7 @@ public static partial class Environment
 
         // Terminates this process with the given exit code.
         [DllImport(RuntimeHelpers.QCall, CharSet = CharSet.Unicode)]
+        [DoesNotReturn]
         private static extern void _Exit(int exitCode);
 
         [DoesNotReturn]

src/libraries/Common/src/System/Collections/Generic/ReferenceEqualityComparer.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
 namespace System.Collections.Generic
@@ -15,7 +17,7 @@ private ReferenceEqualityComparer()
         {
         }
 
-        public bool Equals(T x, T y)
+        public bool Equals(T? x, T? y)
         {
             return ReferenceEquals(x, y);
         }

src/libraries/System.Collections.Concurrent/src/System/Collections/Concurrent/ConcurrentDictionary.cs

@@ -918,11 +918,13 @@ public TValue this[TKey key]
         // as these are uncommonly needed and when inlined are observed to prevent the inlining
         // of important methods like TryGetValue and ContainsKey.
 
+        [DoesNotReturn]
         private static void ThrowKeyNotFoundException(object key)
         {
             throw new KeyNotFoundException(SR.Format(SR.Arg_KeyNotFoundWithKey, key.ToString()));
         }
 
+        [DoesNotReturn]
         private static void ThrowKeyNullException()
         {
             throw new ArgumentNullException("key");

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/IImmutableListQueries.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Collections.Immutable
 {
@@ -120,6 +121,7 @@ internal interface IImmutableListQueries<T> : IReadOnlyList<T>
         /// The first element that matches the conditions defined by the specified predicate,
         /// if found; otherwise, the default value for type <typeparamref name="T"/>.
         /// </returns>
+        [return: MaybeNull]
         T Find(Predicate<T> match);
 
         /// <summary>
@@ -193,6 +195,7 @@ internal interface IImmutableListQueries<T> : IReadOnlyList<T>
         /// The last element that matches the conditions defined by the specified predicate,
         /// if found; otherwise, the default value for type <typeparamref name="T"/>.
         /// </returns>
+        [return: MaybeNull]
         T FindLast(Predicate<T> match);
 
         /// <summary>

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableHashSet_1.cs

@@ -221,7 +221,7 @@ public ImmutableHashSet<T> Remove(T item)
         [Pure]
         public bool TryGetValue(T equalValue, out T actualValue)
         {
-            int hashCode = _equalityComparer.GetHashCode(equalValue);
+            int hashCode = equalValue != null ? _equalityComparer.GetHashCode(equalValue) : 0;
             HashBucket bucket;
             if (_root.TryGetValue(hashCode, out bucket))
             {
@@ -639,7 +639,7 @@ private static bool IsSupersetOf(IEnumerable<T> other, MutationInput origin)
         private static MutationResult Add(T item, MutationInput origin)
         {
             OperationResult result;
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket = origin.Root.GetValueOrDefault(hashCode);
             var newBucket = bucket.Add(item, origin.EqualityComparer, out result);
             if (result == OperationResult.NoChangeRequired)
@@ -658,7 +658,7 @@ private static MutationResult Add(T item, MutationInput origin)
         private static MutationResult Remove(T item, MutationInput origin)
         {
             var result = OperationResult.NoChangeRequired;
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket;
             var newRoot = origin.Root;
             if (origin.Root.TryGetValue(hashCode, out bucket))
@@ -680,7 +680,7 @@ private static MutationResult Remove(T item, MutationInput origin)
         /// </summary>
         private static bool Contains(T item, MutationInput origin)
         {
-            int hashCode = origin.EqualityComparer.GetHashCode(item);
+            int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
             HashBucket bucket;
             if (origin.Root.TryGetValue(hashCode, out bucket))
             {
@@ -701,7 +701,7 @@ private static MutationResult Union(IEnumerable<T> other, MutationInput origin)
             var newRoot = origin.Root;
             foreach (var item in other.GetEnumerableDisposable<T, Enumerator>())
             {
-                int hashCode = origin.EqualityComparer.GetHashCode(item);
+                int hashCode = item != null ? origin.EqualityComparer.GetHashCode(item) : 0;
                 HashBucket bucket = newRoot.GetValueOrDefault(hashCode);
                 OperationResult result;
                 var newBucket = bucket.Add(item, origin.EqualityComparer, out result);
@@ -812,7 +812,7 @@ private static MutationResult Except(IEnumerable<T> other, IEqualityComparer<T>
             var newRoot = root;
             foreach (var item in other.GetEnumerableDisposable<T, Enumerator>())
             {
-                int hashCode = equalityComparer.GetHashCode(item);
+                int hashCode = item != null ? equalityComparer.GetHashCode(item) : 0;
                 HashBucket bucket;
                 if (newRoot.TryGetValue(hashCode, out bucket))
                 {

src/libraries/System.Collections/src/System/Collections/Generic/HashSetEqualityComparer.cs

@@ -30,7 +30,10 @@ public int GetHashCode(HashSet<T>? obj)
             {
                 foreach (T t in obj)
                 {
-                    hashCode = hashCode ^ (_comparer.GetHashCode(t) & 0x7FFFFFFF);
+                    if (t != null)
+                    {
+                        hashCode ^= (_comparer.GetHashCode(t) & 0x7FFFFFFF);
+                    }
                 }
             } // else returns hashcode of 0 for null hashsets
             return hashCode;

src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.TreeSubSet.cs

@@ -341,7 +341,7 @@ internal override int TotalCount()
             // This passes functionality down to the underlying tree, clipping edges if necessary
             // There's nothing gained by having a nested subset. May as well draw it from the base
             // Cannot increase the bounds of the subset, can only decrease it
-            public override SortedSet<T> GetViewBetween(T lowerValue, T upperValue)
+            public override SortedSet<T> GetViewBetween([AllowNull] T lowerValue, [AllowNull] T upperValue)
             {
                 if (_lBoundActive && Comparer.Compare(_min, lowerValue) > 0)
                 {

src/libraries/System.Collections/src/System/Collections/Generic/SortedSet.cs

@@ -806,7 +806,7 @@ public static IEqualityComparer<SortedSet<T>> CreateSetComparer(IEqualityCompare
         /// <param name="set2">The second set.</param>
         /// <param name="comparer">The fallback comparer to use if the sets do not have equal comparers.</param>
         /// <returns><c>true</c> if the sets have equal contents; otherwise, <c>false</c>.</returns>
-        internal static bool SortedSetEquals(SortedSet<T> set1, SortedSet<T> set2, IComparer<T> comparer)
+        internal static bool SortedSetEquals(SortedSet<T>? set1, SortedSet<T>? set2, IComparer<T> comparer)
         {
             if (set1 == null)
             {

src/libraries/System.Collections/src/System/Collections/Generic/SortedSetEqualityComparer.cs

@@ -27,7 +27,7 @@ private SortedSetEqualityComparer(IComparer<T>? comparer, IEqualityComparer<T>?
         }
 
         // Use _comparer to keep equals properties intact; don't want to choose one of the comparers.
-        public bool Equals(SortedSet<T> x, SortedSet<T> y) => SortedSet<T>.SortedSetEquals(x, y, _comparer);
+        public bool Equals(SortedSet<T>? x, SortedSet<T>? y) => SortedSet<T>.SortedSetEquals(x, y, _comparer);
 
         // IMPORTANT: this part uses the fact that GetHashCode() is consistent with the notion of equality in the set.
         public int GetHashCode(SortedSet<T> obj)
@@ -37,7 +37,10 @@ public int GetHashCode(SortedSet<T> obj)
             {
                 foreach (T t in obj)
                 {
-                    hashCode = hashCode ^ (_memberEqualityComparer.GetHashCode(t) & 0x7FFFFFFF);
+                    if (t != null)
+                    {
+                        hashCode ^= (_memberEqualityComparer.GetHashCode(t) & 0x7FFFFFFF);
+                    }
                 }
             }
             // Returns 0 for null sets.

src/libraries/System.ComponentModel.Composition/src/Microsoft/Internal/Collections/CollectionServices.cs

@@ -6,6 +6,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Diagnostics;
 using System.Linq;
 
 namespace Microsoft.Internal.Collections
@@ -144,11 +145,13 @@ private static List<T> FastAppendToListAllowNulls<T>(this List<T>? source, T val
         }
 
         public static List<T>? FastAppendToListAllowNulls<T>(
-                        this List<T>? source, T value,
+                        this List<T>? source, T? value,
                         IEnumerable<T>? second)
+            where T : class
         {
             if (second == null)
             {
+                Debug.Assert(value != null);
                 source = source.FastAppendToListAllowNulls(value);
             }
             else

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/Primitives/ComposablePartDefinition.cs

@@ -103,7 +103,7 @@ protected ComposablePartDefinition()
         /// </remarks>
         public abstract ComposablePart CreatePart();
 
-        internal virtual bool TryGetExports(ImportDefinition definition, [NotNullWhen(true)] out Tuple<ComposablePartDefinition, ExportDefinition>? singleMatch, out IEnumerable<Tuple<ComposablePartDefinition, ExportDefinition>>? multipleMatches)
+        internal virtual bool TryGetExports(ImportDefinition definition, out Tuple<ComposablePartDefinition, ExportDefinition>? singleMatch, out IEnumerable<Tuple<ComposablePartDefinition, ExportDefinition>>? multipleMatches)
         {
             singleMatch = null;
             multipleMatches = null;

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ImportType.cs

@@ -23,13 +23,13 @@ internal class ImportType
         private readonly bool _isOpenGeneric = false;
 
         [ThreadStatic]
-        internal static Dictionary<Type, Func<Export, object>>? _castSingleValueCache;
+        internal static Dictionary<Type, Func<Export, object>?>? _castSingleValueCache;
 
-        private static Dictionary<Type, Func<Export, object>> CastSingleValueCache
+        private static Dictionary<Type, Func<Export, object>?> CastSingleValueCache
         {
             get
             {
-                return _castSingleValueCache = _castSingleValueCache ?? new Dictionary<Type, Func<Export, object>>();
+                return _castSingleValueCache = _castSingleValueCache ?? new Dictionary<Type, Func<Export, object>?>();
             }
         }
 
@@ -170,7 +170,7 @@ private static bool IsLazyGenericType(Type genericType)
             return (genericType == LazyOfTType) || (genericType == LazyOfTMType);
         }
 
-        private static bool TryGetCastFunction(Type genericType, bool isOpenGeneric, Type[] arguments, [NotNullWhen(true)] out Func<Export, object>? castFunction)
+        private static bool TryGetCastFunction(Type genericType, bool isOpenGeneric, Type[] arguments, out Func<Export, object>? castFunction)
         {
             castFunction = null;
 

src/libraries/System.Linq.Expressions/src/System/Dynamic/Utils/CollectionExtensions.cs

@@ -86,7 +86,10 @@ public static int ListHashCode<T>(this ReadOnlyCollection<T> list)
             int h = 6551;
             foreach (T t in list)
             {
-                h ^= (h << 5) ^ cmp.GetHashCode(t);
+                if (t != null)
+                {
+                    h ^= (h << 5) ^ cmp.GetHashCode(t);
+                }
             }
             return h;
         }

src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Interpreter/LightCompiler.cs

@@ -108,7 +108,7 @@ internal bool HasHandler(InterpretedFrame frame, Exception exception, [NotNullWh
                 // Unreachable.
                 // Want to assert that this case isn't hit, but an assertion failure here will be eaten because
                 // we are in an exception filter. Therefore return true here and assert in the catch block.
-                handler = null;
+                handler = null!;
                 unwrappedException = exception;
                 return true;
             }
@@ -215,8 +215,9 @@ internal sealed class DebugInfo
         private class DebugInfoComparer : IComparer<DebugInfo>
         {
             //We allow comparison between int and DebugInfo here
-            int IComparer<DebugInfo>.Compare(DebugInfo d1, DebugInfo d2)
+            int IComparer<DebugInfo>.Compare(DebugInfo? d1, DebugInfo? d2)
             {
+                Debug.Assert(d1 != null && d2 != null);
                 if (d1.Index > d2.Index) return 1;
                 else if (d1.Index == d2.Index) return 0;
                 else return -1;

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Partitioning/HashRepartitionStream.cs

@@ -91,19 +91,16 @@ internal int GetHashCode(TInputOutput element)
         {
             return
                 (HashCodeMask &
-                    (_elementComparer == null ?
-                        (element == null ? NULL_ELEMENT_HASH_CODE : element.GetHashCode()) :
-                        _elementComparer.GetHashCode(element)))
-                        % _distributionMod;
+                    (element == null ? NULL_ELEMENT_HASH_CODE : (_elementComparer?.GetHashCode(element) ?? element.GetHashCode())))
+                % _distributionMod;
         }
 
         internal int GetHashCode(THashKey key)
         {
             return
                 (HashCodeMask &
-                    (_keyComparer == null ?
-                        (key == null ? NULL_ELEMENT_HASH_CODE : key.GetHashCode()) :
-                        _keyComparer.GetHashCode(key))) % _distributionMod;
+                    (key == null ? NULL_ELEMENT_HASH_CODE : (_keyComparer?.GetHashCode(key) ?? key.GetHashCode())))
+                % _distributionMod;
         }
     }
 }

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Unary/ForAllOperator.cs

@@ -147,7 +147,7 @@ internal ForAllEnumerator(QueryOperatorEnumerator<TInput, TKey> source, Action<T
             // element action for each element.
             //
 
-            internal override bool MoveNext([MaybeNull, AllowNull] ref TInput currentElement, ref int currentKey)
+            internal override bool MoveNext([MaybeNullWhen(false), AllowNull] ref TInput currentElement, ref int currentKey)
             {
                 Debug.Assert(_elementAction != null, "expected a compiled operator");
 
@@ -159,14 +159,13 @@ internal override bool MoveNext([MaybeNull, AllowNull] ref TInput currentElement
                 TInput element = default(TInput)!;
                 TKey keyUnused = default(TKey)!;
                 int i = 0;
-                while (_source.MoveNext(ref element!, ref keyUnused))
+                while (_source.MoveNext(ref element, ref keyUnused))
                 {
                     if ((i++ & CancellationState.POLL_INTERVAL) == 0)
                         CancellationState.ThrowIfCanceled(_cancellationToken);
                     _elementAction(element);
                 }
 
-
                 return false;
             }
 

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/HashLookup.cs

@@ -64,9 +64,7 @@ internal TValue this[TKey key]
         private int GetKeyHashCode(TKey key)
         {
             return HashCodeMask &
-                (comparer == null ?
-                    (key == null ? 0 : key.GetHashCode()) :
-                    comparer.GetHashCode(key));
+                (key == null ? 0 : (comparer?.GetHashCode(key) ?? key.GetHashCode()));
         }
 
         private bool AreKeysEqual(TKey key1, TKey key2)

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/ReverseComparer.cs

@@ -9,6 +9,7 @@
 // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Linq.Parallel
 {
@@ -26,7 +27,7 @@ internal ReverseComparer(IComparer<T> comparer)
             _comparer = comparer;
         }
 
-        public int Compare(T x, T y)
+        public int Compare([AllowNull] T x, [AllowNull] T y)
         {
             return _comparer.Compare(y, x);
         }

src/libraries/System.Linq.Parallel/src/System/Linq/Parallel/Utils/WrapperEqualityComparer.cs

@@ -43,7 +43,8 @@ public bool Equals(Wrapper<T> x, Wrapper<T> y)
         public int GetHashCode(Wrapper<T> x)
         {
             Debug.Assert(_comparer != null);
-            return _comparer.GetHashCode(x.Value);
+            T value = x.Value;
+            return value == null ? 0 : _comparer.GetHashCode(value);
         }
     }
 }

src/libraries/System.Linq/src/System/Linq/Partition.SpeedOpt.cs

@@ -36,7 +36,6 @@ private EmptyPartition()
         public bool MoveNext() => false;
 
         [ExcludeFromCodeCoverage] // Shouldn't be called, and as undefined can return or throw anything anyway.
-        [MaybeNull]
         public TElement Current => default!;
 
         [ExcludeFromCodeCoverage] // Shouldn't be called, and as undefined can return or throw anything anyway.

src/libraries/System.Private.CoreLib/src/System/Action.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace System
 {
     public delegate void Action();
@@ -24,7 +26,7 @@ namespace System
     public delegate TResult Func<in T1, in T2, in T3, in T4, in T5, in T6, in T7, out TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7);
     public delegate TResult Func<in T1, in T2, in T3, in T4, in T5, in T6, in T7, in T8, out TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4, T5 arg5, T6 arg6, T7 arg7, T8 arg8);
 
-    public delegate int Comparison<in T>(T x, T y);
+    public delegate int Comparison<in T>([AllowNull] T x, [AllowNull] T y);
 
     public delegate TOutput Converter<in TInput, out TOutput>(TInput input);
 

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Comparer.cs

@@ -43,7 +43,7 @@ public ComparisonComparer(Comparison<T> comparison)
             _comparison = comparison;
         }
 
-        public override int Compare(T x, T y)
+        public override int Compare([AllowNull] T x, [AllowNull] T y)
         {
             return _comparison(x, y);
         }

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/IEqualityComparer.cs

@@ -8,7 +8,7 @@ namespace System.Collections.Generic
 {
     // The generic IEqualityComparer interface implements methods to if check two objects are equal
     // and generate Hashcode for an object.
-    // It is use in Dictionary class.
+    // It is used in Dictionary class.
     public interface IEqualityComparer<in T>
     {
         bool Equals([AllowNull] T x, [AllowNull] T y);