: don't panic in relative_path (#36)

Summary:

if in `relative_path` we find `to` and `from` are in different file systems, don't panic but instead just return `to` unmodified. testing indicates that `reindeer buckify` continues to work as normal with that choice in this case (and so we can avoid needing `CARGO_HOME` workarounds).

Differential Revision: D52989646

.github/workflows/build-and-test.yml

@@ -13,8 +13,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: Swatinem/rust-cache@v2
-      - run: echo CARGO_HOME=$GITHUB_WORKSPACE/.cargo >> $GITHUB_ENV
-        shell: bash
       - run: cargo build --locked
       - run: cargo test
       - run: |-

src/buckify.rs

@@ -114,7 +114,23 @@ pub fn relative_path(mut base: &Path, to: &Path) -> PathBuf {
             res.display()
         );
         res.push("..");
-        base = base.parent().expect("root dir not prefix of other?");
+        match base.parent() {
+            Some(parent) => base = parent,
+            None => {
+                // Here, `to` and `base` do not share a common
+                // ancestor.
+
+                // Example:
+                //  - `to`   = 'C:\Users\runneradmin\.cargo\...001f\anyhow-1.0.79\build.rs'
+                //  - `base` = 'D:\a\reindeer\reindeer\shim\third-party\rust'
+
+                // In this case return `to` "as-is" (i.e. without any
+                // modification)`. `reindeer buckify` seems to work
+                // with this choice.
+
+                return to.to_path_buf();
+            }
+        }
     }
 
     res.join(