Add and apply nullable attributes (#24679)

* Add and apply nullable attributes

* Adapt to API review decisions

* Address PR feedback

3 files changed, 10 insertions, 5 deletions

diff --git a/src/System.Private.CoreLib/src/System/Threading/Interlocked.cs b/src/System.Private.CoreLib/src/System/Threading/Interlocked.cs
index ed6829e228..f23942ee29 100644
--- a/src/System.Private.CoreLib/src/System/Threading/Interlocked.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/Interlocked.cs
@@ -5,6 +5,7 @@
 using System.Runtime.CompilerServices;
 using Internal.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Threading
 {
@@ -59,7 +60,8 @@ namespace System.Threading
         public static extern double Exchange(ref double location1, double value);
 
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
-        public static extern object? Exchange(ref object? location1, object? value);
+        [return: NotNullIfNotNull("value")]
+        public static extern object? Exchange([NotNullIfNotNull("value")] ref object? location1, object? value);
 
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
         public static extern IntPtr Exchange(ref IntPtr location1, IntPtr value);
@@ -67,7 +69,8 @@ namespace System.Threading
         // This whole method reduces to a single call to Exchange(ref object, object) but
         // the JIT thinks that it will generate more native code than it actually does.
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static T Exchange<T>(ref T location1, T value) where T : class?
+        [return: NotNullIfNotNull("value")]
+        public static T Exchange<T>([NotNullIfNotNull("value")] ref T location1, T value) where T : class?
         {
             return Unsafe.As<T>(Exchange(ref Unsafe.As<T, object?>(ref location1), value));
         }
diff --git a/src/System.Private.CoreLib/src/System/Threading/Monitor.cs b/src/System.Private.CoreLib/src/System/Threading/Monitor.cs
index 4385bf9992..1783d4d685 100644
--- a/src/System.Private.CoreLib/src/System/Threading/Monitor.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/Monitor.cs
@@ -21,6 +21,7 @@ using System.Runtime.ConstrainedExecution;
 using System.Runtime.Versioning;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Threading
 {
@@ -52,6 +53,7 @@ namespace System.Threading
             Debug.Assert(lockTaken);
         }
 
+        [DoesNotReturn]
         private static void ThrowLockTakenException()
         {
             throw new ArgumentException(SR.Argument_MustBeFalse, "lockTaken");
diff --git a/src/System.Private.CoreLib/src/System/Threading/ThreadPool.CoreCLR.cs b/src/System.Private.CoreLib/src/System/Threading/ThreadPool.CoreCLR.cs
index 53d47b98cf..ec82665b64 100644
--- a/src/System.Private.CoreLib/src/System/Threading/ThreadPool.CoreCLR.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/ThreadPool.CoreCLR.cs
@@ -52,7 +52,7 @@ namespace System.Threading
             m_internalWaitObject = waitObject;
             if (waitObject != null)
             {
-                m_internalWaitObject.SafeWaitHandle!.DangerousAddRef(ref bReleaseNeeded); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/2384
+                m_internalWaitObject.SafeWaitHandle.DangerousAddRef(ref bReleaseNeeded);
             }
         }
 
@@ -82,7 +82,7 @@ namespace System.Threading
                                 if (bReleaseNeeded)
                                 {
                                     Debug.Assert(m_internalWaitObject != null, "Must be non-null for bReleaseNeeded to be true");
-                                    m_internalWaitObject.SafeWaitHandle!.DangerousRelease(); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/2384
+                                    m_internalWaitObject.SafeWaitHandle.DangerousRelease();
                                     bReleaseNeeded = false;
                                 }
                                 // if result not true don't release/suppress here so finalizer can make another attempt
@@ -144,7 +144,7 @@ namespace System.Threading
                         if (bReleaseNeeded)
                         {
                             Debug.Assert(m_internalWaitObject != null, "Must be non-null for bReleaseNeeded to be true");
-                            m_internalWaitObject.SafeWaitHandle!.DangerousRelease(); // TODO-NULLABLE: https://github.com/dotnet/csharplang/issues/2384
+                            m_internalWaitObject.SafeWaitHandle.DangerousRelease();
                             bReleaseNeeded = false;
                         }
                         SetHandle(InvalidHandle);