Ignore things in .gitignore in tidy

- Switch from `walkdir` to `ignore`. This required various changes to
  make `skip` thread-safe.
- Ignore `build` anywhere in the source tree, not just at the top-level.
  We support this in bootstrap, we should support it in tidy too.

As a nice side benefit, this also makes tidy a bit faster.

Before:
```
; hyperfine -i '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
  Time (mean ± σ):      1.080 s ±  0.008 s    [User: 2.616 s, System: 3.243 s]
  Range (min … max):    1.069 s …  1.099 s    10 runs
```

After:
```
; hyperfine '"/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"'
Benchmark 1: "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0-tools-bin/rust-tidy" "/home/gh-jyn514/rust2" "/home/gh-jyn514/rust2/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "/home/gh-jyn514/rust2/build" "32"
  Time (mean ± σ):     705.0 ms ±   1.4 ms    [User: 3179.1 ms, System: 1517.5 ms]
  Range (min … max):   702.3 ms … 706.9 ms    10 runs
```

15 files changed, 75 insertions, 85 deletions

diff --git a/.gitignore b/.gitignore
index e5ca3e50313..ce797a7a837 100644
--- a/.gitignore
+++ b/.gitignore
@@ -41,7 +41,7 @@ no_llvm_build
 /inst/
 /llvm/
 /mingw-build/
-/build/
+build/
 /build-rust-analyzer/
 /dist/
 /unicode-downloads
diff --git a/src/tools/replace-version-placeholder/src/main.rs b/src/tools/replace-version-placeholder/src/main.rs
index 33b35d05415..864e68de55d 100644
--- a/src/tools/replace-version-placeholder/src/main.rs
+++ b/src/tools/replace-version-placeholder/src/main.rs
@@ -10,7 +10,7 @@ fn main() {
     let version_str = version_str.trim();
     walk::walk(
         &root_path,
-        &mut |path| {
+        |path| {
             walk::filter_dirs(path)
                 // We exempt these as they require the placeholder
                 // for their operation
diff --git a/src/tools/tidy/src/alphabetical.rs b/src/tools/tidy/src/alphabetical.rs
index f913f6cde38..9bfee1efc0b 100644
--- a/src/tools/tidy/src/alphabetical.rs
+++ b/src/tools/tidy/src/alphabetical.rs
@@ -95,7 +95,7 @@ fn check_section<'a>(
 }
 
 pub fn check(path: &Path, bad: &mut bool) {
-    walk(path, &mut filter_dirs, &mut |entry, contents| {
+    walk(path, filter_dirs, &mut |entry, contents| {
         let file = &entry.path().display();
 
         let mut lines = contents.lines().enumerate().peekable();
diff --git a/src/tools/tidy/src/bins.rs b/src/tools/tidy/src/bins.rs
index b898f20a5d0..2d6abe59343 100644
--- a/src/tools/tidy/src/bins.rs
+++ b/src/tools/tidy/src/bins.rs
@@ -101,54 +101,38 @@ mod os_impl {
 
         const ALLOWED: &[&str] = &["configure", "x"];
 
-        walk_no_read(
-            path,
-            &mut |path| {
-                filter_dirs(path)
-                    || path.ends_with("src/etc")
-                    // This is a list of directories that we almost certainly
-                    // don't need to walk. A future PR will likely want to
-                    // remove these in favor of crate::walk_no_read using git
-                    // ls-files to discover the paths we should check, which
-                    // would naturally ignore all of these directories. It's
-                    // also likely faster than walking the directory tree
-                    // directly (since git is just reading from a couple files
-                    // to produce the results).
-                    || path.ends_with("target")
-                    || path.ends_with("build")
-                    || path.ends_with(".git")
-            },
-            &mut |entry| {
-                let file = entry.path();
-                let extension = file.extension();
-                let scripts = ["py", "sh", "ps1"];
-                if scripts.into_iter().any(|e| extension == Some(OsStr::new(e))) {
-                    return;
-                }
+        // FIXME: we don't need to look at all binaries, only files that have been modified in this branch
+        // (e.g. using `git ls-files`).
+        walk_no_read(path, |path| filter_dirs(path) || path.ends_with("src/etc"), &mut |entry| {
+            let file = entry.path();
+            let extension = file.extension();
+            let scripts = ["py", "sh", "ps1"];
+            if scripts.into_iter().any(|e| extension == Some(OsStr::new(e))) {
+                return;
+            }
 
-                if t!(is_executable(&file), file) {
-                    let rel_path = file.strip_prefix(path).unwrap();
-                    let git_friendly_path = rel_path.to_str().unwrap().replace("\\", "/");
+            if t!(is_executable(&file), file) {
+                let rel_path = file.strip_prefix(path).unwrap();
+                let git_friendly_path = rel_path.to_str().unwrap().replace("\\", "/");
 
-                    if ALLOWED.contains(&git_friendly_path.as_str()) {
-                        return;
-                    }
+                if ALLOWED.contains(&git_friendly_path.as_str()) {
+                    return;
+                }
 
-                    let output = Command::new("git")
-                        .arg("ls-files")
-                        .arg(&git_friendly_path)
-                        .current_dir(path)
-                        .stderr(Stdio::null())
-                        .output()
-                        .unwrap_or_else(|e| {
-                            panic!("could not run git ls-files: {e}");
-                        });
-                    let path_bytes = rel_path.as_os_str().as_bytes();
-                    if output.status.success() && output.stdout.starts_with(path_bytes) {
-                        tidy_error!(bad, "binary checked into source: {}", file.display());
-                    }
+                let output = Command::new("git")
+                    .arg("ls-files")
+                    .arg(&git_friendly_path)
+                    .current_dir(path)
+                    .stderr(Stdio::null())
+                    .output()
+                    .unwrap_or_else(|e| {
+                        panic!("could not run git ls-files: {e}");
+                    });
+                let path_bytes = rel_path.as_os_str().as_bytes();
+                if output.status.success() && output.stdout.starts_with(path_bytes) {
+                    tidy_error!(bad, "binary checked into source: {}", file.display());
                 }
-            },
-        )
+            }
+        })
     }
 }
diff --git a/src/tools/tidy/src/debug_artifacts.rs b/src/tools/tidy/src/debug_artifacts.rs
index 0dd9c1e160c..2241375eaae 100644
--- a/src/tools/tidy/src/debug_artifacts.rs
+++ b/src/tools/tidy/src/debug_artifacts.rs
@@ -6,7 +6,7 @@ use std::path::Path;
 const GRAPHVIZ_POSTFLOW_MSG: &str = "`borrowck_graphviz_postflow` attribute in test";
 
 pub fn check(test_dir: &Path, bad: &mut bool) {
-    walk(test_dir, &mut filter_dirs, &mut |entry, contents| {
+    walk(test_dir, filter_dirs, &mut |entry, contents| {
         let filename = entry.path();
         let is_rust = filename.extension().map_or(false, |ext| ext == "rs");
         if !is_rust {
diff --git a/src/tools/tidy/src/edition.rs b/src/tools/tidy/src/edition.rs
index 8172e3d292b..ae8233aa910 100644
--- a/src/tools/tidy/src/edition.rs
+++ b/src/tools/tidy/src/edition.rs
@@ -11,7 +11,7 @@ fn is_edition_2021(mut line: &str) -> bool {
 pub fn check(path: &Path, bad: &mut bool) {
     walk(
         path,
-        &mut |path| {
+        |path| {
             filter_dirs(path)
                 || (path.ends_with("tests") && path.join("COMPILER_TESTS.md").exists())
         },
diff --git a/src/tools/tidy/src/error_codes.rs b/src/tools/tidy/src/error_codes.rs
index c60caa0d49c..d90ad5abbf9 100644
--- a/src/tools/tidy/src/error_codes.rs
+++ b/src/tools/tidy/src/error_codes.rs
@@ -127,7 +127,7 @@ fn check_error_codes_docs(
 
     let mut no_longer_emitted_codes = Vec::new();
 
-    walk(&docs_path, &mut |_| false, &mut |entry, contents| {
+    walk(&docs_path, |_| false, &mut |entry, contents| {
         let path = entry.path();
 
         // Error if the file isn't markdown.
@@ -319,7 +319,7 @@ fn check_error_codes_used(
 
     let mut found_codes = Vec::new();
 
-    walk_many(search_paths, &mut filter_dirs, &mut |entry, contents| {
+    walk_many(search_paths, filter_dirs, &mut |entry, contents| {
         let path = entry.path();
 
         // Return early if we aren't looking at a source file.
diff --git a/src/tools/tidy/src/features.rs b/src/tools/tidy/src/features.rs
index af92e6eb863..f0f13628dc7 100644
--- a/src/tools/tidy/src/features.rs
+++ b/src/tools/tidy/src/features.rs
@@ -101,7 +101,7 @@ pub fn check(
             &tests_path.join("rustdoc-ui"),
             &tests_path.join("rustdoc"),
         ],
-        &mut filter_dirs,
+        filter_dirs,
         &mut |entry, contents| {
             let file = entry.path();
             let filename = file.file_name().unwrap().to_string_lossy();
@@ -477,11 +477,11 @@ fn get_and_check_lib_features(
 
 fn map_lib_features(
     base_src_path: &Path,
-    mf: &mut dyn FnMut(Result<(&str, Feature), &str>, &Path, usize),
+    mf: &mut (dyn Send + Sync + FnMut(Result<(&str, Feature), &str>, &Path, usize)),
 ) {
     walk(
         base_src_path,
-        &mut |path| filter_dirs(path) || path.ends_with("tests"),
+        |path| filter_dirs(path) || path.ends_with("tests"),
         &mut |entry, contents| {
             let file = entry.path();
             let filename = file.file_name().unwrap().to_string_lossy();
diff --git a/src/tools/tidy/src/pal.rs b/src/tools/tidy/src/pal.rs
index f4592fdcff9..33938ac9a0a 100644
--- a/src/tools/tidy/src/pal.rs
+++ b/src/tools/tidy/src/pal.rs
@@ -68,7 +68,7 @@ pub fn check(path: &Path, bad: &mut bool) {
     // Sanity check that the complex parsing here works.
     let mut saw_target_arch = false;
     let mut saw_cfg_bang = false;
-    walk(path, &mut filter_dirs, &mut |entry, contents| {
+    walk(path, filter_dirs, &mut |entry, contents| {
         let file = entry.path();
         let filestr = file.to_string_lossy().replace("\\", "/");
         if !filestr.ends_with(".rs") {
diff --git a/src/tools/tidy/src/rustdoc_gui_tests.rs b/src/tools/tidy/src/rustdoc_gui_tests.rs
index feb513df34b..d7db5c02297 100644
--- a/src/tools/tidy/src/rustdoc_gui_tests.rs
+++ b/src/tools/tidy/src/rustdoc_gui_tests.rs
@@ -5,7 +5,7 @@ use std::path::Path;
 pub fn check(path: &Path, bad: &mut bool) {
     crate::walk::walk(
         &path.join("rustdoc-gui"),
-        &mut |p| {
+        |p| {
             // If there is no extension, it's very likely a folder and we want to go into it.
             p.extension().map(|e| e != "goml").unwrap_or(false)
         },
diff --git a/src/tools/tidy/src/style.rs b/src/tools/tidy/src/style.rs
index 6a0855405ec..9ecb30529cc 100644
--- a/src/tools/tidy/src/style.rs
+++ b/src/tools/tidy/src/style.rs
@@ -235,7 +235,7 @@ pub fn check(path: &Path, bad: &mut bool) {
         .chain(PROBLEMATIC_CONSTS.iter().map(|v| format!("{:X}", v)))
         .collect();
     let problematic_regex = RegexSet::new(problematic_consts_strings.as_slice()).unwrap();
-    walk(path, &mut skip, &mut |entry, contents| {
+    walk(path, skip, &mut |entry, contents| {
         let file = entry.path();
         let filename = file.file_name().unwrap().to_string_lossy();
         let extensions =
diff --git a/src/tools/tidy/src/target_specific_tests.rs b/src/tools/tidy/src/target_specific_tests.rs
index d7a157672cf..f41fa4fcce1 100644
--- a/src/tools/tidy/src/target_specific_tests.rs
+++ b/src/tools/tidy/src/target_specific_tests.rs
@@ -37,7 +37,7 @@ struct RevisionInfo<'a> {
 pub fn check(path: &Path, bad: &mut bool) {
     crate::walk::walk(
         path,
-        &mut |path| path.extension().map(|p| p == "rs") == Some(false),
+        |path| path.extension().map(|p| p == "rs") == Some(false),
         &mut |entry, content| {
             let file = entry.path().display();
             let mut header_map = BTreeMap::new();
diff --git a/src/tools/tidy/src/ui_tests.rs b/src/tools/tidy/src/ui_tests.rs
index 409f7563184..15c36923e88 100644
--- a/src/tools/tidy/src/ui_tests.rs
+++ b/src/tools/tidy/src/ui_tests.rs
@@ -54,7 +54,7 @@ fn check_entries(path: &Path, bad: &mut bool) {
 pub fn check(path: &Path, bad: &mut bool) {
     check_entries(&path, bad);
     for path in &[&path.join("ui"), &path.join("ui-fulldeps")] {
-        crate::walk::walk_no_read(path, &mut |_| false, &mut |entry| {
+        crate::walk::walk_no_read(path, |_| false, &mut |entry| {
             let file_path = entry.path();
             if let Some(ext) = file_path.extension() {
                 if ext == "stderr" || ext == "stdout" {
diff --git a/src/tools/tidy/src/unit_tests.rs b/src/tools/tidy/src/unit_tests.rs
index 27f36c85561..24ab81587db 100644
--- a/src/tools/tidy/src/unit_tests.rs
+++ b/src/tools/tidy/src/unit_tests.rs
@@ -11,14 +11,16 @@ use crate::walk::{filter_dirs, walk};
 use std::path::Path;
 
 pub fn check(root_path: &Path, bad: &mut bool) {
-    let core = &root_path.join("core");
-    let core_tests = &core.join("tests");
-    let core_benches = &core.join("benches");
-    let is_core = |path: &Path| {
-        path.starts_with(core) && !(path.starts_with(core_tests) || path.starts_with(core_benches))
+    let core = root_path.join("core");
+    let core_copy = core.clone();
+    let core_tests = core.join("tests");
+    let core_benches = core.join("benches");
+    let is_core = move |path: &Path| {
+        path.starts_with(&core)
+            && !(path.starts_with(&core_tests) || path.starts_with(&core_benches))
     };
 
-    let mut skip = |path: &Path| {
+    let skip = move |path: &Path| {
         let file_name = path.file_name().unwrap_or_default();
         if path.is_dir() {
             filter_dirs(path)
@@ -35,9 +37,9 @@ pub fn check(root_path: &Path, bad: &mut bool) {
         }
     };
 
-    walk(root_path, &mut skip, &mut |entry, contents| {
+    walk(root_path, skip, &mut |entry, contents| {
         let path = entry.path();
-        let is_core = path.starts_with(core);
+        let is_core = path.starts_with(&core_copy);
         for (i, line) in contents.lines().enumerate() {
             let line = line.trim();
             let is_test = || line.contains("#[test]") && !line.contains("`#[test]");
diff --git a/src/tools/tidy/src/walk.rs b/src/tools/tidy/src/walk.rs
index 4cfb70fa31c..f7f2c647eb4 100644
--- a/src/tools/tidy/src/walk.rs
+++ b/src/tools/tidy/src/walk.rs
@@ -1,9 +1,8 @@
-use std::fs::File;
-use std::io::Read;
-use walkdir::{DirEntry, WalkDir};
+use ignore::DirEntry;
 
-use std::path::Path;
+use std::{fs, path::Path};
 
+/// The default directory filter.
 pub fn filter_dirs(path: &Path) -> bool {
     let skip = [
         "tidy-test-file",
@@ -36,34 +35,39 @@ pub fn filter_dirs(path: &Path) -> bool {
 
 pub fn walk_many(
     paths: &[&Path],
-    skip: &mut dyn FnMut(&Path) -> bool,
+    skip: impl Clone + Send + Sync + 'static + Fn(&Path) -> bool,
     f: &mut dyn FnMut(&DirEntry, &str),
 ) {
     for path in paths {
-        walk(path, skip, f);
+        walk(path, skip.clone(), f);
     }
 }
 
-pub fn walk(path: &Path, skip: &mut dyn FnMut(&Path) -> bool, f: &mut dyn FnMut(&DirEntry, &str)) {
-    let mut contents = String::new();
+pub fn walk(
+    path: &Path,
+    skip: impl Send + Sync + 'static + Fn(&Path) -> bool,
+    f: &mut dyn FnMut(&DirEntry, &str),
+) {
     walk_no_read(path, skip, &mut |entry| {
-        contents.clear();
-        if t!(File::open(entry.path()), entry.path()).read_to_string(&mut contents).is_err() {
-            contents.clear();
-        }
-        f(&entry, &contents);
+        let contents = t!(fs::read(entry.path()), entry.path());
+        let contents_str = match String::from_utf8(contents) {
+            Ok(s) => s,
+            Err(_) => return, // skip this file
+        };
+        f(&entry, &contents_str);
     });
 }
 
 pub(crate) fn walk_no_read(
     path: &Path,
-    skip: &mut dyn FnMut(&Path) -> bool,
+    skip: impl Send + Sync + 'static + Fn(&Path) -> bool,
     f: &mut dyn FnMut(&DirEntry),
 ) {
-    let walker = WalkDir::new(path).into_iter().filter_entry(|e| !skip(e.path()));
-    for entry in walker {
+    let mut walker = ignore::WalkBuilder::new(path);
+    let walker = walker.filter_entry(move |e| !skip(e.path()));
+    for entry in walker.build() {
         if let Ok(entry) = entry {
-            if entry.file_type().is_dir() {
+            if entry.file_type().map_or(true, |kind| kind.is_dir() || kind.is_symlink()) {
                 continue;
             }
             f(&entry);