rust-status: move to recursive traversal to prepare for parallel traversal

I have looked into traversing the working directory in parallel either
by a recursive or an iterative algorithm. The recursive approach won quite
decisively both in terms of performance and code readability.

You can look at my experiment here:
https://heptapod.octobus.net/Alphare/rayon-recursive-traversal

The chance of a stack overflow happening because the directories get too nested
seems slim.

This change does not yet do anything in parallel.

Differential Revision: https://phab.mercurial-scm.org/D8215

rust/hg-core/src/dirstate/status.rs

@@ -1,770 +1,755 b''
 // status.rs
 //
 // Copyright 2019 Raphaël Gomès <[e-mail]>
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 
 //! Rust implementation of dirstate.status (dirstate.py).
 //! It is currently missing a lot of functionality compared to the Python one
 //! and will only be triggered in narrow cases.
 
 use crate::{
     dirstate::SIZE_FROM_OTHER_PARENT,
     filepatterns::PatternFileWarning,
     matchers::{get_ignore_function, Matcher, VisitChildrenSet},
     utils::{
         files::{find_dirs, HgMetadata},
         hg_path::{
             hg_path_to_path_buf, os_string_to_hg_path_buf, HgPath, HgPathBuf,
             HgPathError,
         },
         path_auditor::PathAuditor,
     },
     CopyMap, DirstateEntry, DirstateMap, EntryState, FastHashMap,
     PatternError,
 };
 use lazy_static::lazy_static;
 use rayon::prelude::*;
-use std::collections::VecDeque;
 use std::{
     borrow::Cow,
     collections::HashSet,
     fs::{read_dir, DirEntry},
     io::ErrorKind,
     ops::Deref,
     path::Path,
 };
 
 /// Wrong type of file from a `BadMatch`
 /// Note: a lot of those don't exist on all platforms.
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone)]
 pub enum BadType {
     CharacterDevice,
     BlockDevice,
     FIFO,
     Socket,
     Directory,
     Unknown,
 }
 
 impl ToString for BadType {
     fn to_string(&self) -> String {
         match self {
             BadType::CharacterDevice => "character device",
             BadType::BlockDevice => "block device",
             BadType::FIFO => "fifo",
             BadType::Socket => "socket",
             BadType::Directory => "directory",
             BadType::Unknown => "unknown",
         }
         .to_string()
     }
 }
 
 /// Was explicitly matched but cannot be found/accessed
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone)]
 pub enum BadMatch {
     OsError(i32),
     BadType(BadType),
 }
 
 /// Marker enum used to dispatch new status entries into the right collections.
 /// Is similar to `crate::EntryState`, but represents the transient state of
 /// entries during the lifetime of a command.
-#[derive(Debug)]
+#[derive(Debug, Copy, Clone)]
 enum Dispatch {
     Unsure,
     Modified,
     Added,
     Removed,
     Deleted,
     Clean,
     Unknown,
     Ignored,
     /// Empty dispatch, the file is not worth listing
     None,
     /// Was explicitly matched but cannot be found/accessed
     Bad(BadMatch),
     Directory {
         /// True if the directory used to be a file in the dmap so we can say
         /// that it's been removed.
         was_file: bool,
     },
 }
 
 type IoResult<T> = std::io::Result<T>;
 
 /// Dates and times that are outside the 31-bit signed range are compared
 /// modulo 2^31. This should prevent hg from behaving badly with very large
 /// files or corrupt dates while still having a high probability of detecting
 /// changes. (issue2608)
 /// TODO I haven't found a way of having `b` be `Into<i32>`, since `From<u64>`
 /// is not defined for `i32`, and there is no `As` trait. This forces the
 /// caller to cast `b` as `i32`.
 fn mod_compare(a: i32, b: i32) -> bool {
     a & i32::max_value() != b & i32::max_value()
 }
 
 /// Return a sorted list containing information about the entries
 /// in the directory.
 ///
 /// * `skip_dot_hg` - Return an empty vec if `path` contains a `.hg` directory
 fn list_directory(
     path: impl AsRef<Path>,
     skip_dot_hg: bool,
 ) -> std::io::Result<Vec<(HgPathBuf, DirEntry)>> {
     let mut results = vec![];
     let entries = read_dir(path.as_ref())?;
 
     for entry in entries {
         let entry = entry?;
         let filename = os_string_to_hg_path_buf(entry.file_name())?;
         let file_type = entry.file_type()?;
         if skip_dot_hg && filename.as_bytes() == b".hg" && file_type.is_dir() {
             return Ok(vec![]);
         } else {
             results.push((HgPathBuf::from(filename), entry))
         }
     }
 
     results.sort_unstable_by_key(|e| e.0.clone());
     Ok(results)
 }
 
 /// The file corresponding to the dirstate entry was found on the filesystem.
 fn dispatch_found(
     filename: impl AsRef<HgPath>,
     entry: DirstateEntry,
     metadata: HgMetadata,
     copy_map: &CopyMap,
     options: StatusOptions,
 ) -> Dispatch {
     let DirstateEntry {
         state,
         mode,
         mtime,
         size,
     } = entry;
 
     let HgMetadata {
         st_mode,
         st_size,
         st_mtime,
         ..
     } = metadata;
 
     match state {
         EntryState::Normal => {
             let size_changed = mod_compare(size, st_size as i32);
             let mode_changed =
                 (mode ^ st_mode as i32) & 0o100 != 0o000 && options.check_exec;
             let metadata_changed = size >= 0 && (size_changed || mode_changed);
             let other_parent = size == SIZE_FROM_OTHER_PARENT;
             if metadata_changed
                 || other_parent
                 || copy_map.contains_key(filename.as_ref())
             {
                 Dispatch::Modified
             } else if mod_compare(mtime, st_mtime as i32) {
                 Dispatch::Unsure
             } else if st_mtime == options.last_normal_time {
                 // the file may have just been marked as normal and
                 // it may have changed in the same second without
                 // changing its size. This can happen if we quickly
                 // do multiple commits. Force lookup, so we don't
                 // miss such a racy file change.
                 Dispatch::Unsure
             } else if options.list_clean {
                 Dispatch::Clean
             } else {
                 Dispatch::None
             }
         }
         EntryState::Merged => Dispatch::Modified,
         EntryState::Added => Dispatch::Added,
         EntryState::Removed => Dispatch::Removed,
         EntryState::Unknown => Dispatch::Unknown,
     }
 }
 
 /// The file corresponding to this Dirstate entry is missing.
 fn dispatch_missing(state: EntryState) -> Dispatch {
     match state {
         // File was removed from the filesystem during commands
         EntryState::Normal | EntryState::Merged | EntryState::Added => {
             Dispatch::Deleted
         }
         // File was removed, everything is normal
         EntryState::Removed => Dispatch::Removed,
         // File is unknown to Mercurial, everything is normal
         EntryState::Unknown => Dispatch::Unknown,
     }
 }
 
 lazy_static! {
     static ref DEFAULT_WORK: HashSet<&'static HgPath> = {
         let mut h = HashSet::new();
         h.insert(HgPath::new(b""));
         h
     };
 }
 
 /// Get stat data about the files explicitly specified by match.
 /// TODO subrepos
 fn walk_explicit<'a>(
     files: Option<&'a HashSet<&HgPath>>,
     dmap: &'a DirstateMap,
     root_dir: impl AsRef<Path> + Sync + Send + 'a,
     options: StatusOptions,
 ) -> impl ParallelIterator<Item = IoResult<(&'a HgPath, Dispatch)>> {
     files
         .unwrap_or(&DEFAULT_WORK)
         .par_iter()
         .map(move |filename| {
             // TODO normalization
             let normalized = filename.as_ref();
 
             let buf = match hg_path_to_path_buf(normalized) {
                 Ok(x) => x,
                 Err(e) => return Some(Err(e.into())),
             };
             let target = root_dir.as_ref().join(buf);
             let st = target.symlink_metadata();
             let in_dmap = dmap.get(normalized);
             match st {
                 Ok(meta) => {
                     let file_type = meta.file_type();
                     return if file_type.is_file() || file_type.is_symlink() {
                         if let Some(entry) = in_dmap {
                             return Some(Ok((
                                 normalized,
                                 dispatch_found(
                                     &normalized,
                                     *entry,
                                     HgMetadata::from_metadata(meta),
                                     &dmap.copy_map,
                                     options,
                                 ),
                             )));
                         }
                         Some(Ok((normalized, Dispatch::Unknown)))
                     } else {
                         if file_type.is_dir() {
                             Some(Ok((
                                 normalized,
                                 Dispatch::Directory {
                                     was_file: in_dmap.is_some(),
                                 },
                             )))
                         } else {
                             Some(Ok((
                                 normalized,
                                 Dispatch::Bad(BadMatch::BadType(
                                     // TODO do more than unknown
                                     // Support for all `BadType` variant
                                     // varies greatly between platforms.
                                     // So far, no tests check the type and
                                     // this should be good enough for most
                                     // users.
                                     BadType::Unknown,
                                 )),
                             )))
                         }
                     };
                 }
                 Err(_) => {
                     if let Some(entry) = in_dmap {
                         return Some(Ok((
                             normalized,
                             dispatch_missing(entry.state),
                         )));
                     }
                 }
             };
             None
         })
         .flatten()
 }
 
 #[derive(Debug, Copy, Clone)]
 pub struct StatusOptions {
     /// Remember the most recent modification timeslot for status, to make
     /// sure we won't miss future size-preserving file content modifications
     /// that happen within the same timeslot.
     pub last_normal_time: i64,
     /// Whether we are on a filesystem with UNIX-like exec flags
     pub check_exec: bool,
     pub list_clean: bool,
     pub list_unknown: bool,
     pub list_ignored: bool,
 }
 
-/// Dispatch a single file found during `traverse`.
-/// If `file` is a folder that needs to be traversed, it will be pushed into
+/// Dispatch a single entry (file, folder, symlink...) found during `traverse`.
+/// If the entry is a folder that needs to be traversed, it will be pushed into
 /// `work`.
-fn traverse_worker<'a>(
-    work: &mut VecDeque<HgPathBuf>,
-    matcher: &impl Matcher,
+fn handle_traversed_entry<'a>(
+    dir_entry: &DirEntry,
+    matcher: &(impl Matcher + Sync),
+    root_dir: impl AsRef<Path>,
     dmap: &DirstateMap,
     filename: impl AsRef<HgPath>,
-    dir_entry: &DirEntry,
-    ignore_fn: &impl for<'r> Fn(&'r HgPath) -> bool,
-    dir_ignore_fn: &impl for<'r> Fn(&'r HgPath) -> bool,
+    old_results: &FastHashMap<Cow<'a, HgPath>, Dispatch>,
+    ignore_fn: &(impl for<'r> Fn(&'r HgPath) -> bool + Sync),
+    dir_ignore_fn: &(impl for<'r> Fn(&'r HgPath) -> bool + Sync),
     options: StatusOptions,
-) -> Option<IoResult<(Cow<'a, HgPath>, Dispatch)>> {
-    let file_type = match dir_entry.file_type() {
-        Ok(x) => x,
-        Err(e) => return Some(Err(e.into())),
-    };
+) -> IoResult<Vec<(Cow<'a, HgPath>, Dispatch)>> {
+    let file_type = dir_entry.file_type()?;
     let filename = filename.as_ref();
     let entry_option = dmap.get(filename);
 
     if file_type.is_dir() {
         // Do we need to traverse it?
         if !ignore_fn(&filename) || options.list_ignored {
-            work.push_front(filename.to_owned());
+            return traverse_dir(
+                matcher,
+                root_dir,
+                dmap,
+                filename.to_owned(),
+                &old_results,
+                ignore_fn,
+                dir_ignore_fn,
+                options,
+            );
         }
         // Nested `if` until `rust-lang/rust#53668` is stable
         if let Some(entry) = entry_option {
             // Used to be a file, is now a folder
             if matcher.matches_everything() || matcher.matches(&filename) {
-                return Some(Ok((
+                return Ok(vec![(
                     Cow::Owned(filename.to_owned()),
                     dispatch_missing(entry.state),
-                )));
+                )]);
             }
         }
     } else if file_type.is_file() || file_type.is_symlink() {
         if let Some(entry) = entry_option {
             if matcher.matches_everything() || matcher.matches(&filename) {
-                let metadata = match dir_entry.metadata() {
-                    Ok(x) => x,
-                    Err(e) => return Some(Err(e.into())),
-                };
-                return Some(Ok((
+                let metadata = dir_entry.metadata()?;
+                return Ok(vec![(
                     Cow::Owned(filename.to_owned()),
                     dispatch_found(
                         &filename,
                         *entry,
                         HgMetadata::from_metadata(metadata),
                         &dmap.copy_map,
                         options,
                     ),
-                )));
+                )]);
             }
         } else if (matcher.matches_everything() || matcher.matches(&filename))
             && !ignore_fn(&filename)
         {
             if (options.list_ignored || matcher.exact_match(&filename))
                 && dir_ignore_fn(&filename)
             {
                 if options.list_ignored {
-                    return Some(Ok((
+                    return Ok(vec![(
                         Cow::Owned(filename.to_owned()),
                         Dispatch::Ignored,
-                    )));
+                    )]);
                 }
             } else {
-                return Some(Ok((
+                return Ok(vec![(
                     Cow::Owned(filename.to_owned()),
                     Dispatch::Unknown,
-                )));
+                )]);
             }
+        } else if ignore_fn(&filename) && options.list_ignored {
+            return Ok(vec![(
+                Cow::Owned(filename.to_owned()),
+                Dispatch::Ignored,
+            )]);
         }
     } else if let Some(entry) = entry_option {
         // Used to be a file or a folder, now something else.
         if matcher.matches_everything() || matcher.matches(&filename) {
-            return Some(Ok((
+            return Ok(vec![(
                 Cow::Owned(filename.to_owned()),
                 dispatch_missing(entry.state),
-            )));
+            )]);
         }
     }
-    None
+    return Ok(vec![]);
 }
 
-/// Walk the working directory recursively to look for changes compared to the
-/// current `DirstateMap`.
-fn traverse<'a>(
+/// Decides whether the directory needs to be listed, and if so dispatches its
+/// entries
+fn traverse_dir<'a>(
     matcher: &(impl Matcher + Sync),
     root_dir: impl AsRef<Path>,
     dmap: &DirstateMap,
     path: impl AsRef<HgPath>,
-    old_results: FastHashMap<Cow<'a, HgPath>, Dispatch>,
+    old_results: &FastHashMap<Cow<'a, HgPath>, Dispatch>,
     ignore_fn: &(impl for<'r> Fn(&'r HgPath) -> bool + Sync),
     dir_ignore_fn: &(impl for<'r> Fn(&'r HgPath) -> bool + Sync),
     options: StatusOptions,
-) -> IoResult<FastHashMap<Cow<'a, HgPath>, Dispatch>> {
-    let root_dir = root_dir.as_ref();
-    let mut new_results = FastHashMap::default();
+) -> IoResult<Vec<(Cow<'a, HgPath>, Dispatch)>> {
+    let directory = path.as_ref();
+    if directory.as_bytes() == b".hg" {
+        return Ok(vec![]);
+    }
+    let visit_entries = match matcher.visit_children_set(directory) {
+        VisitChildrenSet::Empty => return Ok(vec![]),
+        VisitChildrenSet::This | VisitChildrenSet::Recursive => None,
+        VisitChildrenSet::Set(set) => Some(set),
+    };
+    let buf = hg_path_to_path_buf(directory)?;
+    let dir_path = root_dir.as_ref().join(buf);
 
-    let mut work = VecDeque::new();
-    work.push_front(path.as_ref().to_owned());
+    let skip_dot_hg = !directory.as_bytes().is_empty();
+    let entries = match list_directory(dir_path, skip_dot_hg) {
+        Err(e) => match e.kind() {
+            ErrorKind::NotFound | ErrorKind::PermissionDenied => {
+                return Ok(vec![(
+                    Cow::Owned(directory.to_owned()),
+                    Dispatch::Bad(BadMatch::OsError(
+                        // Unwrapping here is OK because the error always
+                        // is a real os error
+                        e.raw_os_error().unwrap(),
+                    )),
+                )]);
+            }
+            _ => return Err(e),
+        },
+        Ok(entries) => entries,
+    };
 
-    while let Some(ref directory) = work.pop_front() {
-        if directory.as_bytes() == b".hg" {
-            continue;
+    let mut new_results = vec![];
+    for (filename, dir_entry) in entries {
+        if let Some(ref set) = visit_entries {
+            if !set.contains(filename.deref()) {
+                continue;
+            }
         }
-        let visit_entries = match matcher.visit_children_set(directory) {
-            VisitChildrenSet::Empty => continue,
-            VisitChildrenSet::This | VisitChildrenSet::Recursive => None,
-            VisitChildrenSet::Set(set) => Some(set),
-        };
-        let buf = hg_path_to_path_buf(directory)?;
-        let dir_path = root_dir.join(buf);
-
-        let skip_dot_hg = !directory.as_bytes().is_empty();
-        let entries = match list_directory(dir_path, skip_dot_hg) {
-            Err(e) => match e.kind() {
-                ErrorKind::NotFound | ErrorKind::PermissionDenied => {
-                    new_results.insert(
-                        Cow::Owned(directory.to_owned()),
-                        Dispatch::Bad(BadMatch::OsError(
-                            // Unwrapping here is OK because the error always
-                            // is a real os error
-                            e.raw_os_error().unwrap(),
-                        )),
-                    );
-                    continue;
-                }
-                _ => return Err(e),
-            },
-            Ok(entries) => entries,
+        // TODO normalize
+        let filename = if directory.is_empty() {
+            filename.to_owned()
+        } else {
+            directory.join(&filename)
         };
 
-        for (filename, dir_entry) in entries {
-            if let Some(ref set) = visit_entries {
-                if !set.contains(filename.deref()) {
-                    continue;
-                }
-            }
-            // TODO normalize
-            let filename = if directory.is_empty() {
-                filename.to_owned()
-            } else {
-                directory.join(&filename)
-            };
-
-            if !old_results.contains_key(filename.deref()) {
-                if let Some((res, dispatch)) = traverse_worker(
-                    &mut work,
-                    matcher,
-                    &dmap,
-                    &filename,
-                    &dir_entry,
-                    &ignore_fn,
-                    &dir_ignore_fn,
-                    options,
-                )
-                .transpose()?
-                {
-                    new_results.insert(res, dispatch);
-                }
-            }
+        if !old_results.contains_key(filename.deref()) {
+            new_results.extend(handle_traversed_entry(
+                &dir_entry,
+                matcher,
+                root_dir.as_ref(),
+                &dmap,
+                &filename,
+                old_results,
+                ignore_fn,
+                dir_ignore_fn,
+                options,
+            )?);
         }
     }
-
-    new_results.extend(old_results.into_iter());
-
     Ok(new_results)
 }
 
 /// Stat all entries in the `DirstateMap` and mark them for dispatch.
 fn stat_dmap_entries(
     dmap: &DirstateMap,
     root_dir: impl AsRef<Path> + Sync + Send,
     options: StatusOptions,
 ) -> impl ParallelIterator<Item = IoResult<(&HgPath, Dispatch)>> {
     dmap.par_iter().map(move |(filename, entry)| {
         let filename: &HgPath = filename;
         let filename_as_path = hg_path_to_path_buf(filename)?;
         let meta = root_dir.as_ref().join(filename_as_path).symlink_metadata();
 
         match meta {
             Ok(ref m)
                 if !(m.file_type().is_file()
                     || m.file_type().is_symlink()) =>
             {
                 Ok((filename, dispatch_missing(entry.state)))
             }
             Ok(m) => Ok((
                 filename,
                 dispatch_found(
                     filename,
                     *entry,
                     HgMetadata::from_metadata(m),
                     &dmap.copy_map,
                     options,
                 ),
             )),
             Err(ref e)
                 if e.kind() == ErrorKind::NotFound
                     || e.raw_os_error() == Some(20) =>
             {
                 // Rust does not yet have an `ErrorKind` for
                 // `NotADirectory` (errno 20)
                 // It happens if the dirstate contains `foo/bar` and
                 // foo is not a directory
                 Ok((filename, dispatch_missing(entry.state)))
             }
             Err(e) => Err(e),
         }
     })
 }
 
 pub struct DirstateStatus<'a> {
     pub modified: Vec<Cow<'a, HgPath>>,
     pub added: Vec<Cow<'a, HgPath>>,
     pub removed: Vec<Cow<'a, HgPath>>,
     pub deleted: Vec<Cow<'a, HgPath>>,
     pub clean: Vec<Cow<'a, HgPath>>,
     pub ignored: Vec<Cow<'a, HgPath>>,
     pub unknown: Vec<Cow<'a, HgPath>>,
     pub bad: Vec<(Cow<'a, HgPath>, BadMatch)>,
 }
 
 fn build_response<'a>(
     results: impl IntoIterator<Item = (Cow<'a, HgPath>, Dispatch)>,
 ) -> (Vec<Cow<'a, HgPath>>, DirstateStatus<'a>) {
     let mut lookup = vec![];
     let mut modified = vec![];
     let mut added = vec![];
     let mut removed = vec![];
     let mut deleted = vec![];
     let mut clean = vec![];
     let mut ignored = vec![];
     let mut unknown = vec![];
     let mut bad = vec![];
 
     for (filename, dispatch) in results.into_iter() {
         match dispatch {
             Dispatch::Unknown => unknown.push(filename),
             Dispatch::Unsure => lookup.push(filename),
             Dispatch::Modified => modified.push(filename),
             Dispatch::Added => added.push(filename),
             Dispatch::Removed => removed.push(filename),
             Dispatch::Deleted => deleted.push(filename),
             Dispatch::Clean => clean.push(filename),
             Dispatch::Ignored => ignored.push(filename),
             Dispatch::None => {}
             Dispatch::Bad(reason) => bad.push((filename, reason)),
             Dispatch::Directory { .. } => {}
         }
     }
 
     (
         lookup,
         DirstateStatus {
             modified,
             added,
             removed,
             deleted,
             clean,
             ignored,
             unknown,
             bad,
         },
     )
 }
 
 pub enum StatusError {
     IO(std::io::Error),
     Path(HgPathError),
     Pattern(PatternError),
 }
 
 pub type StatusResult<T> = Result<T, StatusError>;
 
 impl From<PatternError> for StatusError {
     fn from(e: PatternError) -> Self {
         StatusError::Pattern(e)
     }
 }
 impl From<HgPathError> for StatusError {
     fn from(e: HgPathError) -> Self {
         StatusError::Path(e)
     }
 }
 impl From<std::io::Error> for StatusError {
     fn from(e: std::io::Error) -> Self {
         StatusError::IO(e)
     }
 }
 
 impl ToString for StatusError {
     fn to_string(&self) -> String {
         match self {
             StatusError::IO(e) => e.to_string(),
             StatusError::Path(e) => e.to_string(),
             StatusError::Pattern(e) => e.to_string(),
         }
     }
 }
 
 /// Get the status of files in the working directory.
 ///
 /// This is the current entry-point for `hg-core` and is realistically unusable
 /// outside of a Python context because its arguments need to provide a lot of
 /// information that will not be necessary in the future.
 pub fn status<'a: 'c, 'b: 'c, 'c>(
     dmap: &'a DirstateMap,
     matcher: &'b (impl Matcher + Sync),
     root_dir: impl AsRef<Path> + Sync + Send + Copy + 'c,
     ignore_files: &[impl AsRef<Path> + 'c],
     options: StatusOptions,
 ) -> StatusResult<(
     (Vec<Cow<'c, HgPath>>, DirstateStatus<'c>),
     Vec<PatternFileWarning>,
 )> {
     let (ignore_fn, warnings) = get_ignore_function(&ignore_files, root_dir)?;
 
     // Is the path or one of its ancestors ignored?
     let dir_ignore_fn = |dir: &_| {
         if ignore_fn(dir) {
             true
         } else {
             for p in find_dirs(dir) {
                 if ignore_fn(p) {
                     return true;
                 }
             }
             false
         }
     };
 
     let files = matcher.file_set();
 
     // Step 1: check the files explicitly mentioned by the user
     let explicit = walk_explicit(files, &dmap, root_dir, options);
-    let (work, mut results): (Vec<_>, FastHashMap<_, _>) = explicit
+
+    // Collect results into a `Vec` because we do very few lookups in most
+    // cases.
+    let (work, mut results): (Vec<_>, Vec<_>) = explicit
         .filter_map(Result::ok)
         .map(|(filename, dispatch)| (Cow::Borrowed(filename), dispatch))
         .partition(|(_, dispatch)| match dispatch {
             Dispatch::Directory { .. } => true,
             _ => false,
         });
 
-    // Step 2: recursively check the working directory for changes if needed
-    for (dir, dispatch) in work {
-        match dispatch {
-            Dispatch::Directory { was_file } => {
-                if was_file {
-                    results.insert(dir.to_owned(), Dispatch::Removed);
+    if !work.is_empty() {
+        // Hashmaps are quite a bit slower to build than vecs, so only build it
+        // if needed.
+        let old_results = results.iter().cloned().collect();
+
+        // Step 2: recursively check the working directory for changes if
+        // needed
+        for (dir, dispatch) in work {
+            match dispatch {
+                Dispatch::Directory { was_file } => {
+                    if was_file {
+                        results.push((dir.to_owned(), Dispatch::Removed));
+                    }
+                    if options.list_ignored
+                        || options.list_unknown && !dir_ignore_fn(&dir)
+                    {
+                        results.par_extend(traverse_dir(
+                            matcher,
+                            root_dir,
+                            &dmap,
+                            &dir,
+                            &old_results,
+                            &ignore_fn,
+                            &dir_ignore_fn,
+                            options,
+                        )?);
+                    }
                 }
-                if options.list_ignored
-                    || options.list_unknown && !dir_ignore_fn(&dir)
-                {
-                    results = traverse(
-                        matcher,
-                        root_dir,
-                        &dmap,
-                        &dir,
-                        results,
-                        &ignore_fn,
-                        &dir_ignore_fn,
-                        options,
-                    )?;
-                }
+                _ => unreachable!("There can only be directories in `work`"),
             }
-            _ => unreachable!("There can only be directories in `work`"),
         }
     }
 
     if !matcher.is_exact() {
         // Step 3: Check the remaining files from the dmap.
         // If a dmap file is not in results yet, it was either
         // a) not matched b) ignored, c) missing, or d) under a
         // symlink directory.
 
         if options.list_unknown {
             let to_visit: Box<dyn Iterator<Item = (&HgPath, &DirstateEntry)>> =
                 if results.is_empty() && matcher.matches_everything() {
                     Box::new(dmap.iter().map(|(f, e)| (f.deref(), e)))
                 } else {
-                    Box::new(dmap.iter().filter_map(|(f, e)| {
-                        if !results.contains_key(f.deref())
+                    // Only convert to a hashmap if needed.
+                    let old_results: FastHashMap<_, _> =
+                        results.iter().cloned().collect();
+                    Box::new(dmap.iter().filter_map(move |(f, e)| {
+                        if !old_results.contains_key(f.deref())
                             && matcher.matches(f)
                         {
                             Some((f.deref(), e))
                         } else {
                             None
                         }
                     }))
                 };
             let mut to_visit: Vec<_> = to_visit.collect();
             to_visit.sort_by(|a, b| a.0.cmp(&b.0));
 
             // We walked all dirs under the roots that weren't ignored, and
             // everything that matched was stat'ed and is already in results.
             // The rest must thus be ignored or under a symlink.
             let path_auditor = PathAuditor::new(root_dir);
 
             for (ref filename, entry) in to_visit {
                 // Report ignored items in the dmap as long as they are not
                 // under a symlink directory.
                 if path_auditor.check(filename) {
                     // TODO normalize for case-insensitive filesystems
                     let buf = hg_path_to_path_buf(filename)?;
-                    results.insert(
+                    results.push((
                         Cow::Borrowed(filename),
                         match root_dir.as_ref().join(&buf).symlink_metadata() {
                             // File was just ignored, no links, and exists
                             Ok(meta) => {
                                 let metadata = HgMetadata::from_metadata(meta);
                                 dispatch_found(
                                     filename,
                                     *entry,
                                     metadata,
                                     &dmap.copy_map,
                                     options,
                                 )
                             }
                             // File doesn't exist
                             Err(_) => dispatch_missing(entry.state),
                         },
-                    );
+                    ));
                 } else {
                     // It's either missing or under a symlink directory which
                     // we, in this case, report as missing.
-                    results.insert(
+                    results.push((
                         Cow::Borrowed(filename),
                         dispatch_missing(entry.state),
-                    );
+                    ));
                 }
             }
         } else {
             // We may not have walked the full directory tree above, so stat
             // and check everything we missed.
             let stat_results = stat_dmap_entries(&dmap, root_dir, options);
             results.par_extend(stat_results.flatten().map(
                 |(filename, dispatch)| (Cow::Borrowed(filename), dispatch),
             ));
         }
     }
 
-    let results = results.into_iter().filter_map(|(filename, dispatch)| {
-        match dispatch {
-            Dispatch::Bad(_) => return Some((filename, dispatch)),
-            _ => {}
-        };
-        // TODO do this in //, not at the end
-        if !dmap.contains_key(filename.deref()) {
-            if (options.list_ignored || matcher.exact_match(&filename))
-                && dir_ignore_fn(&filename)
-            {
-                if options.list_ignored {
-                    return Some((filename.to_owned(), Dispatch::Ignored));
-                }
-            } else {
-                if !ignore_fn(&filename) {
-                    return Some((filename.to_owned(), Dispatch::Unknown));
-                }
-            }
-            return None;
-        }
-        Some((filename, dispatch))
-    });
-
     Ok((build_response(results), warnings))
 }