Keep a Windows drive letter when joining a root-relative path

pkgs/path/CHANGELOG.md

@@ -1,5 +1,6 @@
 ## 1.9.2-wip
 
+- `join('C:', r'\Users\name')` keeps the drive letter.
 - Make `Url` style better at recognizing schemes and authorities.
   Only consider a path as being a schemed URL if it starts with a
   valid scheme.

pkgs/path/lib/src/context.dart

@@ -323,7 +323,13 @@ final class Context {
     var isAbsoluteAndNotRootRelative = false;
 
     for (var part in parts.where((part) => part != '')) {
-      if (isRootRelative(part) && isAbsoluteAndNotRootRelative) {
+      if (isRootRelative(part) && _isWindowsDriveOnly(buffer.toString())) {
+        // `HOMEDRIVE` is `C:` and `HOMEPATH` is `\Users\name`. `C:` is not an
+        // absolute root, so a following root-relative part would otherwise
+        // discard the drive.
+        buffer.write(part);
+        isAbsoluteAndNotRootRelative = true;
+      } else if (isRootRelative(part) && isAbsoluteAndNotRootRelative) {
         // If the new part is root-relative, it preserves the previous root but
         // replaces the path after it.
         final parsed = _parse(part);
@@ -360,6 +366,15 @@ final class Context {
     return buffer.toString();
   }
 
+  /// A Windows path that is only a drive letter, such as `C:`.
+  bool _isWindowsDriveOnly(String path) {
+    if (style.name != 'windows' || path.length != 2) return false;
+    final letter = path.codeUnitAt(0);
+    final isLetter = (letter >= chars.upperA && letter <= chars.upperZ) ||
+        (letter >= chars.lowerA && letter <= chars.lowerZ);
+    return isLetter && path.codeUnitAt(1) == chars.colon;
+  }
+
   /// Splits [path] into its components using the current platform's
   /// [separator]. Example:
   ///

pkgs/path/test/windows_test.dart

@@ -334,6 +334,12 @@ void main() {
       expect(context.join('a/', 'b'), r'a/b');
     });
 
+    test('keeps a drive letter when the next part is root-relative', () {
+      expect(context.join('C:', r'\Users\danny'), r'C:\Users\danny');
+      expect(context.join('c:', r'\Users\danny', 'docs'), r'c:\Users\danny\docs');
+      expect(context.joinAll(['C:', r'\Users\danny']), r'C:\Users\danny');
+    });
+
     test('ignores parts before an absolute path', () {
       expect(context.join('a', r'\b', r'\c', 'd'), r'\c\d');
       expect(context.join('a', '/b', '/c', 'd'), r'/c\d');