Use UnsafeAccessor in System.Runtime.Numerics.

src/coreclr/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.Shared.xml

@@ -16,6 +16,14 @@
     <type fullname="System.Collections.Generic.IList`1" />
     <type fullname="System.Collections.Generic.IReadOnlyList`1" />
 
+    <type fullname="System.Number">
+      <!-- Methods are required for System.Runtime.Numerics. -->
+      <method name="TryStringToNumberCore" />
+      <method name="NumberToStringCore" />
+      <method name="FormatNumberCore" />
+      <method name="UInt32ToDecCharsCore" />
+    </type>
+
     <!-- Indicator for illink that all instance methods are instantiated -->
     <type fullname="System.SZArrayHelper">
       <method name=".ctor" />

src/libraries/Common/src/System/Number.Common.cs

@@ -0,0 +1,22 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System
+{
+    internal static partial class Number
+    {
+        private const int CharStackBufferSize = 32;
+        internal enum ParsingStatus
+        {
+            OK,
+            Failed,
+            Overflow
+        }
+
+        private static bool IsWhite(uint ch) => (ch == 0x20) || ((ch - 0x09) <= (0x0D - 0x09));
+
+        private static bool IsDigit(uint ch) => (ch - '0') <= 9;
+
+        private static bool IsSpaceReplacingChar(uint c) => (c == '\u00a0') || (c == '\u202f');
+    }
+}