Do not compare Span<T> with 'default'

src/libraries/Common/src/Microsoft/Win32/SafeHandles/SafeUnicodeStringHandle.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 namespace Microsoft.Win32.SafeHandles
@@ -23,12 +24,12 @@ public SafeUnicodeStringHandle(string s)
         public unsafe SafeUnicodeStringHandle(ReadOnlySpan<char> s)
             : base(IntPtr.Zero, ownsHandle: true)
         {
-            // If s == default then the span represents the null string,
+            // If s is null ref then the span represents the null string,
             // and handle should be IntPtr.Zero to match Marshal.StringToHGlobalUni.
             //
             // Since that was already done in the base ctor call, we only need to do
-            // work when s != default.
-            if (s != default)
+            // work when s is not null ref.
+            if (!Unsafe.IsNullRef(ref MemoryMarshal.GetReference(s)))
             {
                 int cch = checked(s.Length + 1);
                 int cb = checked(cch * sizeof(char));

src/libraries/Common/src/System/Security/Cryptography/Pkcs12Kdf.cs

@@ -3,6 +3,8 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Text;
 
 namespace System.Security.Cryptography.Pkcs
@@ -115,9 +117,15 @@ private static void Derive(
             // The password is a null-terminated UTF-16BE version of the input.
             int passLen = checked((password.Length + 1) * 2);
 
-            // If password == default then the span represents the null string (as opposed to
+            // If password is null ref then the span represents the null string (as opposed to
             // an empty string), and the P block should then have size 0 in the next step.
+#if NETSTANDARD
+#pragma warning disable CA2265 // Do not compare Span<T> to 'default'
             if (password == default)
+#pragma warning restore CA2265
+#else
+            if (Unsafe.IsNullRef(ref MemoryMarshal.GetReference(password)))
+#endif
             {
                 passLen = 0;
             }

src/libraries/System.Security.Cryptography/src/Microsoft/Win32/SafeHandles/SafePasswordHandle.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Security;
 
@@ -35,8 +36,8 @@ public SafePasswordHandle(string? password, bool passwordProvided)
         public SafePasswordHandle(ReadOnlySpan<char> password, bool passwordProvided)
             : base(ownsHandle: true)
         {
-            // "".AsSpan() is not default, so this is compat for "null tries NULL first".
-            if (password != default)
+            // "".AsSpan() is not null ref, so this is compat for "null tries NULL first".
+            if (!Unsafe.IsNullRef(ref MemoryMarshal.GetReference(password)))
             {
                 int spanLen;
 

src/libraries/System.Text.Json/src/System/Text/Json/Document/JsonElement.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace System.Text.Json
 {
@@ -1243,7 +1244,7 @@ public bool ValueEquals(ReadOnlySpan<byte> utf8Text)
             if (TokenType == JsonTokenType.Null)
             {
                 // This is different than Length == 0, in that it tests true for null, but false for ""
-                return utf8Text == default;
+                return Unsafe.IsNullRef(ref MemoryMarshal.GetReference(utf8Text));
             }
 
             return TextEqualsHelper(utf8Text, isPropertyName: false, shouldUnescape: true);
@@ -1271,7 +1272,7 @@ public bool ValueEquals(ReadOnlySpan<char> text)
             if (TokenType == JsonTokenType.Null)
             {
                 // This is different than Length == 0, in that it tests true for null, but false for ""
-                return text == default;
+                return Unsafe.IsNullRef(ref MemoryMarshal.GetReference(text));
             }
 
             return TextEqualsHelper(text, isPropertyName: false);

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCaseEquivalences.cs

@@ -52,7 +52,7 @@ public static bool TryFindCaseEquivalencesForCharWithIBehavior(char c, CultureIn
                     // Default
                     _ => default
                 };
-                return equivalences != default;
+                return !equivalences.IsEmpty;
             }
             else
             {