Rollup merge of rust-lang#107948 - jieyouxu:issue-107944, r=ozkanonur

Allow shortcuts to directories to be used for ./x.py fmt

Fixes rust-lang#107944.

Maximum recursive search depth is 3 and only accepts shortcuts for directories. If there are no shortcut candidates, the previous behavior to panic is preserved. If there are multiple candidates, the shortcut candidates are ignored.

After this change, `./x.py fmt std` no longer panics and formats `library/std` instead.

src/bootstrap/format.rs

@@ -193,10 +193,46 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
     let (tx, rx): (SyncSender<PathBuf>, _) = std::sync::mpsc::sync_channel(128);
     let walker = match paths.get(0) {
         Some(first) => {
-            let mut walker = WalkBuilder::new(first);
+            let find_shortcut_candidates = |p: &PathBuf| {
+                let mut candidates = Vec::new();
+                for candidate in WalkBuilder::new(src.clone()).max_depth(Some(3)).build() {
+                    if let Ok(entry) = candidate {
+                        if let Some(dir_name) = p.file_name() {
+                            if entry.path().is_dir() && entry.file_name() == dir_name {
+                                candidates.push(entry.into_path());
+                            }
+                        }
+                    }
+                }
+                candidates
+            };
+
+            // Only try to look for shortcut candidates for single component paths like
+            // `std` and not for e.g. relative paths like `../library/std`.
+            let should_look_for_shortcut_dir = |p: &PathBuf| p.components().count() == 1;
+
+            let mut walker = if should_look_for_shortcut_dir(first) {
+                if let [single_candidate] = &find_shortcut_candidates(first)[..] {
+                    WalkBuilder::new(single_candidate)
+                } else {
+                    WalkBuilder::new(first)
+                }
+            } else {
+                WalkBuilder::new(first)
+            };
+
             for path in &paths[1..] {
-                walker.add(path);
+                if should_look_for_shortcut_dir(path) {
+                    if let [single_candidate] = &find_shortcut_candidates(path)[..] {
+                        walker.add(single_candidate);
+                    } else {
+                        walker.add(path);
+                    }
+                } else {
+                    walker.add(path);
+                }
             }
+
             walker
         }
         None => WalkBuilder::new(src.clone()),