Only pop drive letters at the start of a path

url/src/parser.rs

@@ -1397,6 +1397,9 @@ impl Parser<'_> {
             let segment_start = path_start + slash_position + 1;
             // Don’t pop a Windows drive letter
             if !(scheme_type.is_file()
+                // It's only a drive letter if it's at the beginning of a path,
+                // e.g. in `/a/c:` the `c:` isn't one because it's not at the start of a path.
+                && segment_start == path_start + 1
                 && is_normalized_windows_drive_letter(&self.serialization[segment_start..]))
             {
                 self.serialization.truncate(segment_start);

url/tests/unit.rs

@@ -1351,6 +1351,25 @@ fn test_file_with_drive_and_path() {
     assert_eq!(url2.to_string(), "file:///p:/a");
 }
 
+#[test]
+fn test_file_base_drive_shaped_segment() {
+    let testcases = [
+        ("file:a/c:", "..", "file:///"),
+        (
+            "file:a/c:",
+            "../example-1.0-py3-none-any.whl",
+            "file:///example-1.0-py3-none-any.whl",
+        ),
+        ("file:///w:/c:", "file:..", "file:///w:/"),
+        ("file:///c:/..", "", "file:///c:/"),
+    ];
+
+    for (base, reference, expected) in testcases {
+        let base = Url::parse(base).unwrap();
+        assert_eq!(base.join(reference).unwrap().as_str(), expected);
+    }
+}
+
 #[cfg(feature = "std")]
 #[test]
 fn issue_864() {