One more thing...

src/libraries/System.Private.CoreLib/src/System/Environment.Unix.cs

@@ -74,5 +74,41 @@ public static unsafe string UserName
 
             }
         }
+
+        private static unsafe bool TryGetUserNameFromPasswd(byte* buf, int bufLen, out string? username)
+        {
+            // Call getpwuid_r to get the passwd struct
+            Interop.Sys.Passwd passwd;
+            int error = Interop.Sys.GetPwUidR(Interop.Sys.GetEUid(), out passwd, buf, bufLen);
+
+            // If the call succeeds, give back the user name retrieved
+            if (error == 0)
+            {
+                Debug.Assert(passwd.Name != null);
+                username = Marshal.PtrToStringAnsi((IntPtr)passwd.Name);
+                return true;
+            }
+
+            // If the current user's entry could not be found, give back null,
+            // but still return true (false indicates the buffer was too small).
+            if (error == -1)
+            {
+                username = null;
+                return true;
+            }
+
+            var errorInfo = new Interop.ErrorInfo(error);
+
+            // If the call failed because the buffer was too small, return false to
+            // indicate the caller should try again with a larger buffer.
+            if (errorInfo.Error == Interop.Error.ERANGE)
+            {
+                username = null;
+                return false;
+            }
+
+            // Otherwise, fail.
+            throw new IOException(errorInfo.GetErrorMessage(), errorInfo.RawErrno);
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/Environment.UnixOrBrowser.cs

@@ -56,42 +56,6 @@ private static string ExpandEnvironmentVariablesCore(string name)
 
         public static int SystemPageSize => CheckedSysConf(Interop.Sys.SysConfName._SC_PAGESIZE);
 
-        private static unsafe bool TryGetUserNameFromPasswd(byte* buf, int bufLen, out string? username)
-        {
-            // Call getpwuid_r to get the passwd struct
-            Interop.Sys.Passwd passwd;
-            int error = Interop.Sys.GetPwUidR(Interop.Sys.GetEUid(), out passwd, buf, bufLen);
-
-            // If the call succeeds, give back the user name retrieved
-            if (error == 0)
-            {
-                Debug.Assert(passwd.Name != null);
-                username = Marshal.PtrToStringAnsi((IntPtr)passwd.Name);
-                return true;
-            }
-
-            // If the current user's entry could not be found, give back null,
-            // but still return true (false indicates the buffer was too small).
-            if (error == -1)
-            {
-                username = null;
-                return true;
-            }
-
-            var errorInfo = new Interop.ErrorInfo(error);
-
-            // If the call failed because the buffer was too small, return false to
-            // indicate the caller should try again with a larger buffer.
-            if (errorInfo.Error == Interop.Error.ERANGE)
-            {
-                username = null;
-                return false;
-            }
-
-            // Otherwise, fail.
-            throw new IOException(errorInfo.GetErrorMessage(), errorInfo.RawErrno);
-        }
-
         public static string UserDomainName => MachineName;
 
         /// <summary>Invoke <see cref="Interop.Sys.SysConf"/>, throwing if it fails.</summary>