Do not Path.Join user directory if it's "C:" (#94587)

Fixes #68503

On netfx we were concatenating user directories ending with volume separator `:` instead of joining with a path separator `\`. That is not the case in .NET [core]. This change brings that back as it is incorrect to join paths like "C:" with a separator in between the enumerated entries as the meaning of "relative to drive's CWD" gets lost.

Author: jozkee

src/libraries/System.Private.CoreLib/src/System/IO/Enumeration/FileSystemEntry.Unix.cs

@@ -168,5 +168,11 @@ public FileSystemInfo ToFileSystemInfo()
         /// </summary>
         public string ToFullPath() =>
             new string(FullPath);
+
+        private static string Join(
+            ReadOnlySpan<char> originalRootDirectory,
+            ReadOnlySpan<char> relativePath,
+            ReadOnlySpan<char> fileName) =>
+            Path.Join(originalRootDirectory, relativePath, fileName);
     }
 }

src/libraries/System.Private.CoreLib/src/System/IO/Enumeration/FileSystemEntry.Windows.cs

@@ -75,5 +75,18 @@ public FileSystemInfo ToFileSystemInfo()
         /// <returns>A string representing the full path.</returns>
         public string ToFullPath() =>
             Path.Join(Directory, FileName);
+
+        private static string Join(
+            ReadOnlySpan<char> originalRootDirectory,
+            ReadOnlySpan<char> relativePath,
+            ReadOnlySpan<char> fileName)
+        {
+            if (originalRootDirectory.Length == 2 && originalRootDirectory[1] == Path.VolumeSeparatorChar)
+            {
+                return string.Concat(originalRootDirectory, Path.Join(relativePath, fileName));
+            }
+
+            return Path.Join(originalRootDirectory, relativePath, fileName);
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/IO/Enumeration/FileSystemEntry.cs

@@ -39,7 +39,7 @@ public string ToSpecifiedFullPath()
             if (Path.EndsInDirectorySeparator(OriginalRootDirectory) && PathInternal.StartsWithDirectorySeparator(relativePath))
                 relativePath = relativePath.Slice(1);
 
-            return Path.Join(OriginalRootDirectory, relativePath, FileName);
+            return Join(OriginalRootDirectory, relativePath, FileName);
         }
     }
 }

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/GetFileSystemEntries_str.cs

@@ -246,6 +246,27 @@ public void WindowsInvalidCharsPath_Core(string invalid)
             Assert.Throws<IOException>(() => GetEntries(invalid));
         }
 
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [PlatformSpecific(TestPlatforms.Windows)]
+        public void WindowsRelativeToCurrentDrivePath()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                string testRootDir = GetTestFilePath();
+                Directory.CreateDirectory(testRootDir);
+                Directory.SetCurrentDirectory(testRootDir);
+
+                const string TestSubdir = "foo";
+                Directory.CreateDirectory(Path.Combine(testRootDir, TestSubdir));
+
+                string currentDrive = testRootDir.Substring(0, 2);
+                string[] results = GetEntries(currentDrive);
+
+                string result = Assert.Single(results);
+                Assert.Equal(currentDrive + TestSubdir, result);
+            }).Dispose();
+        }
+
         [Theory,
             InlineData("         "),
             InlineData(" "),