[vfs] Don't confuse paths with source roots that have the same prefix

crates/vfs/src/file_set.rs

@@ -170,6 +170,12 @@ impl FileSetConfigBuilder {
                 for p in paths {
                     let mut buf = Vec::new();
                     p.encode(&mut buf);
+
+                    // A root is a directory, so append a trailing separator to the string
+                    // used in prefix checks. This ensures that r-a doesn't think that
+                    // `/foo/bar.rs` lives in the `/foo/b/` root.
+                    buf.extend(std::path::MAIN_SEPARATOR.to_string().as_bytes());
+
                     entries.push((buf, i as u64));
                 }
             }

crates/vfs/src/file_set/tests.rs

@@ -40,3 +40,26 @@ fn name_prefix() {
     let partition = file_set.partition(&vfs).into_iter().map(|it| it.len()).collect::<Vec<_>>();
     assert_eq!(partition, vec![1, 1, 0]);
 }
+
+/// Ensure that we don't consider `/foo/bar_baz.rs` to be in the
+/// `/foo/bar/` root.
+#[test]
+fn name_prefix_partially_matches() {
+    let mut file_set = FileSetConfig::builder();
+    file_set.add_file_set(vec![VfsPath::new_virtual_path("/foo".into())]);
+    file_set.add_file_set(vec![VfsPath::new_virtual_path("/foo/bar".into())]);
+    let file_set = file_set.build();
+
+    let mut vfs = Vfs::default();
+
+    // These two are both in /foo.
+    vfs.set_file_contents(VfsPath::new_virtual_path("/foo/lib.rs".into()), Some(Vec::new()));
+    vfs.set_file_contents(VfsPath::new_virtual_path("/foo/bar_baz.rs".into()), Some(Vec::new()));
+
+    // Only this file is in /foo/bar.
+    vfs.set_file_contents(VfsPath::new_virtual_path("/foo/bar/biz.rs".into()), Some(Vec::new()));
+
+    let partition = file_set.partition(&vfs).into_iter().map(|it| it.len()).collect::<Vec<_>>();
+
+    assert_eq!(partition, vec![2, 1, 0]);
+}