Delete empty dirs in baseline-accept task

Herebyfile.mjs

@@ -852,6 +852,65 @@ function baselineAcceptTask(localBaseline, refBaseline) {
         return path.join(refBaseline, relative);
     }
 
+    /**
+     * Recursively find top-most empty subtrees under root, excluding root itself.
+     * Returns the highest ancestors whose entire subtrees contain only empty
+     * directories, so a single rimraf can remove each whole subtree at once.
+     * @param {string} root
+     * @returns {string[]}
+     */
+    function findEmptySubtrees(root) {
+        /** @type {string[]} */
+        const result = [];
+
+        /**
+         * @param {string} dir
+         * @returns {boolean} true if dir is entirely empty
+         */
+        function visit(dir) {
+            /** @type {import("fs").Dirent[]} */
+            let entries;
+            try {
+                entries = fs.readdirSync(dir, { withFileTypes: true });
+            }
+            catch {
+                return false;
+            }
+
+            let empty = true;
+            /** @type {string[]} */
+            const childEmpty = [];
+            for (const entry of entries) {
+                if (entry.isDirectory()) {
+                    const childPath = path.join(dir, entry.name);
+                    if (visit(childPath)) {
+                        childEmpty.push(childPath);
+                    }
+                    else {
+                        empty = false;
+                    }
+                }
+                else {
+                    empty = false;
+                }
+            }
+
+            if (!empty) {
+                // This dir is not fully empty, so collect the empty children
+                // we found (they are top-most empty subtrees).
+                result.push(...childEmpty);
+            }
+            // If empty, don't collect children — the caller will either
+            // collect this dir or propagate emptiness further up.
+            return empty;
+        }
+
+        // If root itself is entirely empty, there's nothing to collect
+        // (we exclude root). Otherwise, visit already pushed the right subtrees.
+        visit(root);
+        return result;
+    }
+
     return async () => {
         const toCopy = await glob(`${localBaseline}/**`, { nodir: true, ignore: `${localBaseline}/**/*.delete` });
         for (const p of toCopy) {
@@ -865,6 +924,14 @@ function baselineAcceptTask(localBaseline, refBaseline) {
             await rimraf(out);
             await rimraf(p); // also delete the .delete file so that it no longer shows up in a diff tool.
         }
+
+        // Remove empty directory trees left behind by deletions.
+        for (const dir of findEmptySubtrees(refBaseline)) {
+            await rimraf(dir);
+        }
+        for (const dir of findEmptySubtrees(localBaseline)) {
+            await rimraf(dir);
+        }
     };
 }