Make ui_tests non-quadratic

Previously, it would walk each directory twice: once in the main `Walk`
iterator, and once to count the number of entries in the directory. Now
it only walks each directory once.

src/tools/tidy/src/ui_tests.rs

@@ -3,23 +3,29 @@
 //! - there are no stray `.stderr` files
 
 use ignore::Walk;
-use ignore::WalkBuilder;
+use std::collections::HashMap;
 use std::fs;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 const ENTRY_LIMIT: usize = 1000;
 // FIXME: The following limits should be reduced eventually.
 const ROOT_ENTRY_LIMIT: usize = 940;
 const ISSUES_ENTRY_LIMIT: usize = 1978;
 
-fn check_entries(path: &Path, bad: &mut bool) {
-    for dir in Walk::new(&path.join("ui")) {
+fn check_entries(tests_path: &Path, bad: &mut bool) {
+    let mut directories: HashMap<PathBuf, usize> = HashMap::new();
+
+    for dir in Walk::new(&tests_path.join("ui")) {
         if let Ok(entry) = dir {
-            if entry.file_type().map(|ft| ft.is_dir()).unwrap_or(false) {
-                let dir_path = entry.path();
+            let parent = entry.path().parent().unwrap().to_path_buf();
+            *directories.entry(parent).or_default() += 1;
+        }
+    }
+
+    for (dir_path, count) in directories {
         // Use special values for these dirs.
-                let is_root = path.join("ui") == dir_path;
-                let is_issues_dir = path.join("ui/issues") == dir_path;
+        let is_root = tests_path.join("ui") == dir_path;
+        let is_issues_dir = tests_path.join("ui/issues") == dir_path;
         let limit = if is_root {
             ROOT_ENTRY_LIMIT
         } else if is_issues_dir {
@@ -28,14 +34,6 @@ fn check_entries(path: &Path, bad: &mut bool) {
             ENTRY_LIMIT
         };
 
-                let count = WalkBuilder::new(&dir_path)
-                    .max_depth(Some(1))
-                    .build()
-                    .into_iter()
-                    .collect::<Vec<_>>()
-                    .len()
-                    - 1; // remove the dir itself
-
         if count > limit {
             tidy_error!(
                 bad,
@@ -48,8 +46,6 @@ fn check_entries(path: &Path, bad: &mut bool) {
         }
     }
 }
-    }
-}
 
 pub fn check(path: &Path, bad: &mut bool) {
     check_entries(&path, bad);