match: strengthen visit_children_set invariant, Recursive means "all files"

My previous interpretation of "Recursive" was too relaxed: I thought it
instructed the caller to do something like this:

> you can stop calling `visit_children_set` because you'll need to descend into
> every directory recursively, but you should still check every file if it
> matches or not

Whereas the real instruction seems to be:

> I guarantee that everything in this subtree matches, you can stop
> querying the matcher for all files and dirs altogether.

The evidence to support this:

- the test actually passes with the stronger invariant, revealing no
exceptions from this rule

- the implementation of `visit_children_set` for `DifferenceMatcher`
clearly relies on this requirement, so it must hold for that not to
lead to bugs.

rust/hg-core/src/matchers.rs

@@ -1,2430 +1,2450 b''
 // matchers.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.
 
 //! Structs and types for matching files and directories.
 
 use format_bytes::format_bytes;
 use once_cell::sync::OnceCell;
 
 use crate::{
     dirstate::dirs_multiset::DirsChildrenMultiset,
     filepatterns::{
         build_single_regex, filter_subincludes, get_patterns_from_file,
         PatternFileWarning, PatternResult,
     },
     utils::{
         files::{dir_ancestors, find_dirs},
         hg_path::{HgPath, HgPathBuf, HgPathError},
         Escaped,
     },
     DirsMultiset, FastHashMap, IgnorePattern, PatternError, PatternSyntax,
 };
 
 use crate::dirstate::status::IgnoreFnType;
 use crate::filepatterns::normalize_path_bytes;
 use std::collections::HashSet;
 use std::fmt::{Display, Error, Formatter};
 use std::path::{Path, PathBuf};
 use std::{borrow::ToOwned, collections::BTreeSet};
 
 #[derive(Debug, PartialEq)]
 pub enum VisitChildrenSet {
     /// Don't visit anything
     Empty,
     /// Visit this directory and probably its children
     This,
     /// Only visit the children (both files and directories) if they
     /// are mentioned in this set. (empty set corresponds to [Empty])
     /// TODO Should we implement a `NonEmptyHashSet`?
     Set(HashSet<HgPathBuf>),
     /// Visit this directory and all subdirectories
     /// (you can stop asking about the children set)
     Recursive,
 }
 
 pub trait Matcher: core::fmt::Debug {
     /// Explicitly listed files
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>>;
     /// Returns whether `filename` is in `file_set`
     fn exact_match(&self, filename: &HgPath) -> bool;
     /// Returns whether `filename` is matched by this matcher
     fn matches(&self, filename: &HgPath) -> bool;
     /// Decides whether a directory should be visited based on whether it
     /// has potential matches in it or one of its subdirectories, and
     /// potentially lists which subdirectories of that directory should be
     /// visited. This is based on the match's primary, included, and excluded
     /// patterns.
     ///
     /// # Example
     ///
     /// Assume matchers `['path:foo/bar', 'rootfilesin:qux']`, we would
     /// return the following values (assuming the implementation of
     /// visit_children_set is capable of recognizing this; some implementations
     /// are not).
     ///
     /// ```text
     /// ```ignore
     /// '' -> {'foo', 'qux'}
     /// 'baz' -> set()
     /// 'foo' -> {'bar'}
     /// // Ideally this would be `Recursive`, but since the prefix nature of
     /// // matchers is applied to the entire matcher, we have to downgrade this
     /// // to `This` due to the (yet to be implemented in Rust) non-prefix
     /// // `RootFilesIn'-kind matcher being mixed in.
     /// 'foo/bar' -> 'this'
     /// 'qux' -> 'this'
     /// ```
     /// # Important
     ///
     /// Most matchers do not know if they're representing files or
     /// directories. They see `['path:dir/f']` and don't know whether `f` is a
     /// file or a directory, so `visit_children_set('dir')` for most matchers
     /// will return `HashSet{ HgPath { "f" } }`, but if the matcher knows it's
     /// a file (like the yet to be implemented in Rust `ExactMatcher` does),
     /// it may return `VisitChildrenSet::This`.
     /// Do not rely on the return being a `HashSet` indicating that there are
     /// no files in this dir to investigate (or equivalently that if there are
     /// files to investigate in 'dir' that it will always return
     /// `VisitChildrenSet::This`).
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet;
     /// Matcher will match everything and `files_set()` will be empty:
     /// optimization might be possible.
     fn matches_everything(&self) -> bool;
     /// Matcher will match exactly the files in `files_set()`: optimization
     /// might be possible.
     fn is_exact(&self) -> bool;
 }
 
 /// Matches everything.
 ///```
 /// use hg::{ matchers::{Matcher, AlwaysMatcher}, utils::hg_path::HgPath };
 ///
 /// let matcher = AlwaysMatcher;
 ///
 /// assert_eq!(matcher.matches(HgPath::new(b"whatever")), true);
 /// assert_eq!(matcher.matches(HgPath::new(b"b.txt")), true);
 /// assert_eq!(matcher.matches(HgPath::new(b"main.c")), true);
 /// assert_eq!(matcher.matches(HgPath::new(br"re:.*\.c$")), true);
 /// ```
 #[derive(Debug)]
 pub struct AlwaysMatcher;
 
 impl Matcher for AlwaysMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         None
     }
     fn exact_match(&self, _filename: &HgPath) -> bool {
         false
     }
     fn matches(&self, _filename: &HgPath) -> bool {
         true
     }
     fn visit_children_set(&self, _directory: &HgPath) -> VisitChildrenSet {
         VisitChildrenSet::Recursive
     }
     fn matches_everything(&self) -> bool {
         true
     }
     fn is_exact(&self) -> bool {
         false
     }
 }
 
 /// Matches nothing.
 #[derive(Debug)]
 pub struct NeverMatcher;
 
 impl Matcher for NeverMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         None
     }
     fn exact_match(&self, _filename: &HgPath) -> bool {
         false
     }
     fn matches(&self, _filename: &HgPath) -> bool {
         false
     }
     fn visit_children_set(&self, _directory: &HgPath) -> VisitChildrenSet {
         VisitChildrenSet::Empty
     }
     fn matches_everything(&self) -> bool {
         false
     }
     fn is_exact(&self) -> bool {
         true
     }
 }
 
 /// Matches the input files exactly. They are interpreted as paths, not
 /// patterns.
 ///
 ///```
 /// use hg::{ matchers::{Matcher, FileMatcher}, utils::hg_path::{HgPath, HgPathBuf} };
 ///
 /// let files = vec![HgPathBuf::from_bytes(b"a.txt"), HgPathBuf::from_bytes(br"re:.*\.c$")];
 /// let matcher = FileMatcher::new(files).unwrap();
 ///
 /// assert_eq!(matcher.matches(HgPath::new(b"a.txt")), true);
 /// assert_eq!(matcher.matches(HgPath::new(b"b.txt")), false);
 /// assert_eq!(matcher.matches(HgPath::new(b"main.c")), false);
 /// assert_eq!(matcher.matches(HgPath::new(br"re:.*\.c$")), true);
 /// ```
 #[derive(Debug)]
 pub struct FileMatcher {
     files: HashSet<HgPathBuf>,
     dirs: DirsMultiset,
     sorted_visitchildrenset_candidates: OnceCell<BTreeSet<HgPathBuf>>,
 }
 
 impl FileMatcher {
     pub fn new(files: Vec<HgPathBuf>) -> Result<Self, HgPathError> {
         let dirs = DirsMultiset::from_manifest(&files)?;
         Ok(Self {
             files: HashSet::from_iter(files),
             dirs,
             sorted_visitchildrenset_candidates: OnceCell::new(),
         })
     }
     fn inner_matches(&self, filename: &HgPath) -> bool {
         self.files.contains(filename.as_ref())
     }
 }
 
 impl Matcher for FileMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         Some(&self.files)
     }
     fn exact_match(&self, filename: &HgPath) -> bool {
         self.inner_matches(filename)
     }
     fn matches(&self, filename: &HgPath) -> bool {
         self.inner_matches(filename)
     }
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         if self.files.is_empty() || !self.dirs.contains(directory) {
             return VisitChildrenSet::Empty;
         }
 
         let compute_candidates = || -> BTreeSet<HgPathBuf> {
             let mut candidates: BTreeSet<HgPathBuf> =
                 self.dirs.iter().cloned().collect();
             candidates.extend(self.files.iter().cloned());
             candidates.remove(HgPath::new(b""));
             candidates
         };
         let candidates =
             if directory.as_ref().is_empty() {
                 compute_candidates()
             } else {
                 let sorted_candidates = self
                     .sorted_visitchildrenset_candidates
                     .get_or_init(compute_candidates);
                 let directory_bytes = directory.as_ref().as_bytes();
                 let start: HgPathBuf =
                     format_bytes!(b"{}/", directory_bytes).into();
                 let start_len = start.len();
                 // `0` sorts after `/`
                 let end = format_bytes!(b"{}0", directory_bytes).into();
                 BTreeSet::from_iter(sorted_candidates.range(start..end).map(
                     |c| HgPathBuf::from_bytes(&c.as_bytes()[start_len..]),
                 ))
             };
 
         // `self.dirs` includes all of the directories, recursively, so if
         // we're attempting to match 'foo/bar/baz.txt', it'll have '', 'foo',
         // 'foo/bar' in it. Thus we can safely ignore a candidate that has a
         // '/' in it, indicating it's for a subdir-of-a-subdir; the immediate
         // subdir will be in there without a slash.
         VisitChildrenSet::Set(
             candidates
                 .into_iter()
                 .filter_map(|c| {
                     if c.bytes().all(|b| *b != b'/') {
                         Some(c)
                     } else {
                         None
                     }
                 })
                 .collect(),
         )
     }
     fn matches_everything(&self) -> bool {
         false
     }
     fn is_exact(&self) -> bool {
         true
     }
 }
 
 /// Matches a set of (kind, pat, source) against a 'root' directory.
 /// (Currently the 'root' directory is effectively always empty)
 /// ```
 /// use hg::{
 ///     matchers::{PatternMatcher, Matcher},
 ///     IgnorePattern,
 ///     PatternSyntax,
 ///     utils::hg_path::{HgPath, HgPathBuf}
 /// };
 /// use std::collections::HashSet;
 /// use std::path::Path;
 /// ///
 /// let ignore_patterns : Vec<IgnorePattern> =
 ///     vec![IgnorePattern::new(PatternSyntax::Regexp, br".*\.c$", Path::new("")),
 ///          IgnorePattern::new(PatternSyntax::Path, b"foo/a", Path::new("")),
 ///          IgnorePattern::new(PatternSyntax::RelPath, b"b", Path::new("")),
 ///          IgnorePattern::new(PatternSyntax::Glob, b"*.h", Path::new("")),
 ///     ];
 /// let matcher = PatternMatcher::new(ignore_patterns).unwrap();
 /// ///
 /// assert_eq!(matcher.matches(HgPath::new(b"main.c")), true); // matches re:.*\.c$
 /// assert_eq!(matcher.matches(HgPath::new(b"b.txt")), false);
 /// assert_eq!(matcher.matches(HgPath::new(b"foo/a")), true); // matches path:foo/a
 /// assert_eq!(matcher.matches(HgPath::new(b"a")), false); // does not match path:b, since 'root' is 'foo'
 /// assert_eq!(matcher.matches(HgPath::new(b"b")), true); // matches relpath:b, since 'root' is 'foo'
 /// assert_eq!(matcher.matches(HgPath::new(b"lib.h")), true); // matches glob:*.h
 /// assert_eq!(matcher.file_set().unwrap(),
 ///            &HashSet::from([HgPathBuf::from_bytes(b""), HgPathBuf::from_bytes(b"foo/a"),
 ///                            HgPathBuf::from_bytes(b""), HgPathBuf::from_bytes(b"b")]));
 /// assert_eq!(matcher.exact_match(HgPath::new(b"foo/a")), true);
 /// assert_eq!(matcher.exact_match(HgPath::new(b"b")), true);
 /// assert_eq!(matcher.exact_match(HgPath::new(b"lib.h")), false); // exact matches are for (rel)path kinds
 /// ```
 pub struct PatternMatcher<'a> {
     patterns: Vec<u8>,
     match_fn: IgnoreFnType<'a>,
     /// Whether all the patterns match a prefix (i.e. recursively)
     prefix: bool,
     files: HashSet<HgPathBuf>,
     dirs_explicit: HashSet<HgPathBuf>,
     dirs: DirsMultiset,
 }
 
 impl core::fmt::Debug for PatternMatcher<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("PatternMatcher")
             .field("patterns", &String::from_utf8_lossy(&self.patterns))
             .field("prefix", &self.prefix)
             .field("files", &self.files)
             .field("dirs", &self.dirs)
             .finish()
     }
 }
 
 impl<'a> PatternMatcher<'a> {
     pub fn new(ignore_patterns: Vec<IgnorePattern>) -> PatternResult<Self> {
         let RootsDirsAndParents {
             roots,
             dirs: dirs_explicit,
             parents,
         } = roots_dirs_and_parents(&ignore_patterns)?;
         let files = roots;
         let dirs = parents;
         let files: HashSet<HgPathBuf> = HashSet::from_iter(files);
 
         let prefix = ignore_patterns.iter().all(|k| {
             matches!(k.syntax, PatternSyntax::Path | PatternSyntax::RelPath)
         });
         let (patterns, match_fn) = build_match(ignore_patterns, b"$")?;
 
         Ok(Self {
             patterns,
             match_fn,
             prefix,
             files,
             dirs,
             dirs_explicit,
         })
     }
 }
 
 impl<'a> Matcher for PatternMatcher<'a> {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         Some(&self.files)
     }
 
     fn exact_match(&self, filename: &HgPath) -> bool {
         self.files.contains(filename)
     }
 
     fn matches(&self, filename: &HgPath) -> bool {
         if self.files.contains(filename) {
             return true;
         }
         (self.match_fn)(filename)
     }
 
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         if self.prefix && self.files.contains(directory) {
             return VisitChildrenSet::Recursive;
         }
         if self.dirs.contains(directory) {
             return VisitChildrenSet::This;
         }
         if dir_ancestors(directory).any(|parent_dir| {
             self.files.contains(parent_dir)
                 || self.dirs_explicit.contains(parent_dir)
         }) {
             VisitChildrenSet::This
         } else {
             VisitChildrenSet::Empty
         }
     }
 
     fn matches_everything(&self) -> bool {
         false
     }
 
     fn is_exact(&self) -> bool {
         false
     }
 }
 
 /// Matches files that are included in the ignore rules.
 /// ```
 /// use hg::{
 ///     matchers::{IncludeMatcher, Matcher},
 ///     IgnorePattern,
 ///     PatternSyntax,
 ///     utils::hg_path::HgPath
 /// };
 /// use std::path::Path;
 /// ///
 /// let ignore_patterns =
 /// vec![IgnorePattern::new(PatternSyntax::RootGlob, b"this*", Path::new(""))];
 /// let matcher = IncludeMatcher::new(ignore_patterns).unwrap();
 /// ///
 /// assert_eq!(matcher.matches(HgPath::new(b"testing")), false);
 /// assert_eq!(matcher.matches(HgPath::new(b"this should work")), true);
 /// assert_eq!(matcher.matches(HgPath::new(b"this also")), true);
 /// assert_eq!(matcher.matches(HgPath::new(b"but not this")), false);
 /// ///
 /// let ignore_patterns =
 /// vec![IgnorePattern::new(PatternSyntax::RootFilesIn, b"dir/subdir", Path::new(""))];
 /// let matcher = IncludeMatcher::new(ignore_patterns).unwrap();
 /// ///
 /// assert!(!matcher.matches(HgPath::new(b"file")));
 /// assert!(!matcher.matches(HgPath::new(b"dir/file")));
 /// assert!(matcher.matches(HgPath::new(b"dir/subdir/file")));
 /// assert!(!matcher.matches(HgPath::new(b"dir/subdir/subsubdir/file")));
 /// ```
 pub struct IncludeMatcher<'a> {
     patterns: Vec<u8>,
     match_fn: IgnoreFnType<'a>,
     /// Whether all the patterns match a prefix (i.e. recursively)
     prefix: bool,
     roots: HashSet<HgPathBuf>,
     dirs: HashSet<HgPathBuf>,
     parents: DirsMultiset,
 }
 
 impl core::fmt::Debug for IncludeMatcher<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         f.debug_struct("IncludeMatcher")
             .field("patterns", &String::from_utf8_lossy(&self.patterns))
             .field("prefix", &self.prefix)
             .field("roots", &self.roots)
             .field("dirs", &self.dirs)
             .field("parents", &self.parents)
             .finish()
     }
 }
 
 impl<'a> Matcher for IncludeMatcher<'a> {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         None
     }
 
     fn exact_match(&self, _filename: &HgPath) -> bool {
         false
     }
 
     fn matches(&self, filename: &HgPath) -> bool {
         (self.match_fn)(filename)
     }
 
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         let dir = directory;
         if self.prefix && self.roots.contains(dir) {
             return VisitChildrenSet::Recursive;
         }
         if self.roots.contains(HgPath::new(b""))
             || self.roots.contains(dir)
             || self.dirs.contains(dir)
             || find_dirs(dir).any(|parent_dir| self.roots.contains(parent_dir))
         {
             return VisitChildrenSet::This;
         }
 
         if self.parents.contains(dir.as_ref()) {
             let multiset = self.get_all_parents_children();
             if let Some(children) = multiset.get(dir) {
                 return VisitChildrenSet::Set(
                     children.iter().map(HgPathBuf::from).collect(),
                 );
             }
         }
         VisitChildrenSet::Empty
     }
 
     fn matches_everything(&self) -> bool {
         false
     }
 
     fn is_exact(&self) -> bool {
         false
     }
 }
 
 /// The union of multiple matchers. Will match if any of the matchers match.
 #[derive(Debug)]
 pub struct UnionMatcher {
     matchers: Vec<Box<dyn Matcher + Sync>>,
 }
 
 impl Matcher for UnionMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         None
     }
 
     fn exact_match(&self, _filename: &HgPath) -> bool {
         false
     }
 
     fn matches(&self, filename: &HgPath) -> bool {
         self.matchers.iter().any(|m| m.matches(filename))
     }
 
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         let mut result = HashSet::new();
         let mut this = false;
         for matcher in self.matchers.iter() {
             let visit = matcher.visit_children_set(directory);
             match visit {
                 VisitChildrenSet::Empty => continue,
                 VisitChildrenSet::This => {
                     this = true;
                     // Don't break, we might have an 'all' in here.
                     continue;
                 }
                 VisitChildrenSet::Set(set) => {
                     result.extend(set);
                 }
                 VisitChildrenSet::Recursive => {
                     return visit;
                 }
             }
         }
         if this {
             return VisitChildrenSet::This;
         }
         if result.is_empty() {
             VisitChildrenSet::Empty
         } else {
             VisitChildrenSet::Set(result)
         }
     }
 
     fn matches_everything(&self) -> bool {
         // TODO Maybe if all are AlwaysMatcher?
         false
     }
 
     fn is_exact(&self) -> bool {
         false
     }
 }
 
 impl UnionMatcher {
     pub fn new(matchers: Vec<Box<dyn Matcher + Sync>>) -> Self {
         Self { matchers }
     }
 }
 
 #[derive(Debug)]
 pub struct IntersectionMatcher {
     m1: Box<dyn Matcher + Sync>,
     m2: Box<dyn Matcher + Sync>,
     files: Option<HashSet<HgPathBuf>>,
 }
 
 impl Matcher for IntersectionMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         self.files.as_ref()
     }
 
     fn exact_match(&self, filename: &HgPath) -> bool {
         self.files.as_ref().map_or(false, |f| f.contains(filename))
     }
 
     fn matches(&self, filename: &HgPath) -> bool {
         self.m1.matches(filename) && self.m2.matches(filename)
     }
 
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         let m1_set = self.m1.visit_children_set(directory);
         if m1_set == VisitChildrenSet::Empty {
             return VisitChildrenSet::Empty;
         }
         let m2_set = self.m2.visit_children_set(directory);
         if m2_set == VisitChildrenSet::Empty {
             return VisitChildrenSet::Empty;
         }
 
         if m1_set == VisitChildrenSet::Recursive {
             return m2_set;
         } else if m2_set == VisitChildrenSet::Recursive {
             return m1_set;
         }
 
         match (&m1_set, &m2_set) {
             (VisitChildrenSet::Recursive, _) => m2_set,
             (_, VisitChildrenSet::Recursive) => m1_set,
             (VisitChildrenSet::This, _) | (_, VisitChildrenSet::This) => {
                 VisitChildrenSet::This
             }
             (VisitChildrenSet::Set(m1), VisitChildrenSet::Set(m2)) => {
                 let set: HashSet<_> = m1.intersection(m2).cloned().collect();
                 if set.is_empty() {
                     VisitChildrenSet::Empty
                 } else {
                     VisitChildrenSet::Set(set)
                 }
             }
             _ => unreachable!(),
         }
     }
 
     fn matches_everything(&self) -> bool {
         self.m1.matches_everything() && self.m2.matches_everything()
     }
 
     fn is_exact(&self) -> bool {
         self.m1.is_exact() || self.m2.is_exact()
     }
 }
 
 impl IntersectionMatcher {
     pub fn new(
         mut m1: Box<dyn Matcher + Sync>,
         mut m2: Box<dyn Matcher + Sync>,
     ) -> Self {
         let files = if m1.is_exact() || m2.is_exact() {
             if !m1.is_exact() {
                 std::mem::swap(&mut m1, &mut m2);
             }
             m1.file_set().map(|m1_files| {
                 m1_files.iter().cloned().filter(|f| m2.matches(f)).collect()
             })
         } else {
             // without exact input file sets, we can't do an exact
             // intersection, so we must over-approximate by
             // unioning instead
             m1.file_set().map(|m1_files| match m2.file_set() {
                 Some(m2_files) => m1_files.union(m2_files).cloned().collect(),
                 None => m1_files.iter().cloned().collect(),
             })
         };
         Self { m1, m2, files }
     }
 }
 
 #[derive(Debug)]
 pub struct DifferenceMatcher {
     base: Box<dyn Matcher + Sync>,
     excluded: Box<dyn Matcher + Sync>,
     files: Option<HashSet<HgPathBuf>>,
 }
 
 impl Matcher for DifferenceMatcher {
     fn file_set(&self) -> Option<&HashSet<HgPathBuf>> {
         self.files.as_ref()
     }
 
     fn exact_match(&self, filename: &HgPath) -> bool {
         self.files.as_ref().map_or(false, |f| f.contains(filename))
     }
 
     fn matches(&self, filename: &HgPath) -> bool {
         self.base.matches(filename) && !self.excluded.matches(filename)
     }
 
     fn visit_children_set(&self, directory: &HgPath) -> VisitChildrenSet {
         let excluded_set = self.excluded.visit_children_set(directory);
         if excluded_set == VisitChildrenSet::Recursive {
             return VisitChildrenSet::Empty;
         }
         let base_set = self.base.visit_children_set(directory);
         // Possible values for base: 'recursive', 'this', set(...), set()
         // Possible values for excluded:          'this', set(...), set()
         // If excluded has nothing under here that we care about, return base,
         // even if it's 'recursive'.
         if excluded_set == VisitChildrenSet::Empty {
             return base_set;
         }
         match base_set {
             VisitChildrenSet::This | VisitChildrenSet::Recursive => {
                 // Never return 'recursive' here if excluded_set is any kind of
                 // non-empty (either 'this' or set(foo)), since excluded might
                 // return set() for a subdirectory.
                 VisitChildrenSet::This
             }
             set => {
                 // Possible values for base:         set(...), set()
                 // Possible values for excluded: 'this', set(...)
                 // We ignore excluded set results. They're possibly incorrect:
                 //  base = path:dir/subdir
                 //  excluded=rootfilesin:dir,
                 //  visit_children_set(''):
                 //   base returns {'dir'}, excluded returns {'dir'}, if we
                 //   subtracted we'd return set(), which is *not* correct, we
                 //   still need to visit 'dir'!
                 set
             }
         }
     }
 
     fn matches_everything(&self) -> bool {
         false
     }
 
     fn is_exact(&self) -> bool {
         self.base.is_exact()
     }
 }
 
 impl DifferenceMatcher {
     pub fn new(
         base: Box<dyn Matcher + Sync>,
         excluded: Box<dyn Matcher + Sync>,
     ) -> Self {
         let base_is_exact = base.is_exact();
         let base_files = base.file_set().map(ToOwned::to_owned);
         let mut new = Self {
             base,
             excluded,
             files: None,
         };
         if base_is_exact {
             new.files = base_files.map(|files| {
                 files.iter().cloned().filter(|f| new.matches(f)).collect()
             });
         }
         new
     }
 }
 
 /// Wraps [`regex::bytes::Regex`] to improve performance in multithreaded
 /// contexts.
 ///
 /// The `status` algorithm makes heavy use of threads, and calling `is_match`
 /// from many threads at once is prone to contention, probably within the
 /// scratch space needed as the regex DFA is built lazily.
 ///
 /// We are in the process of raising the issue upstream, but for now
 /// the workaround used here is to store the `Regex` in a lazily populated
 /// thread-local variable, sharing the initial read-only compilation, but
 /// not the lazy dfa scratch space mentioned above.
 ///
 /// This reduces the contention observed with 16+ threads, but does not
 /// completely remove it. Hopefully this can be addressed upstream.
 struct RegexMatcher {
     /// Compiled at the start of the status algorithm, used as a base for
     /// cloning in each thread-local `self.local`, thus sharing the expensive
     /// first compilation.
     base: regex::bytes::Regex,
     /// Thread-local variable that holds the `Regex` that is actually queried
     /// from each thread.
     local: thread_local::ThreadLocal<regex::bytes::Regex>,
 }
 
 impl RegexMatcher {
     /// Returns whether the path matches the stored `Regex`.
     pub fn is_match(&self, path: &HgPath) -> bool {
         self.local
             .get_or(|| self.base.clone())
             .is_match(path.as_bytes())
     }
 }
 
 /// Return a `RegexBuilder` from a bytes pattern
 ///
 /// This works around the fact that even if it works on byte haysacks,
 /// [`regex::bytes::Regex`] still uses UTF-8 patterns.
 pub fn re_bytes_builder(pattern: &[u8]) -> regex::bytes::RegexBuilder {
     use std::io::Write;
 
     // The `regex` crate adds `.*` to the start and end of expressions if there
     // are no anchors, so add the start anchor.
     let mut escaped_bytes = vec![b'^', b'(', b'?', b':'];
     for byte in pattern {
         if *byte > 127 {
             write!(escaped_bytes, "\\x{:x}", *byte).unwrap();
         } else {
             escaped_bytes.push(*byte);
         }
     }
     escaped_bytes.push(b')');
 
     // Avoid the cost of UTF8 checking
     //
     // # Safety
     // This is safe because we escaped all non-ASCII bytes.
     let pattern_string = unsafe { String::from_utf8_unchecked(escaped_bytes) };
     regex::bytes::RegexBuilder::new(&pattern_string)
 }
 
 /// Returns a function that matches an `HgPath` against the given regex
 /// pattern.
 ///
 /// This can fail when the pattern is invalid or not supported by the
 /// underlying engine (the `regex` crate), for instance anything with
 /// back-references.
 #[logging_timer::time("trace")]
 fn re_matcher(pattern: &[u8]) -> PatternResult<RegexMatcher> {
     let re = re_bytes_builder(pattern)
         .unicode(false)
         // Big repos with big `.hgignore` will hit the default limit and
         // incur a significant performance hit. One repo's `hg status` hit
         // multiple *minutes*.
         .dfa_size_limit(50 * (1 << 20))
         .build()
         .map_err(|e| PatternError::UnsupportedSyntax(e.to_string()))?;
 
     Ok(RegexMatcher {
         base: re,
         local: Default::default(),
     })
 }
 
 /// Returns the regex pattern and a function that matches an `HgPath` against
 /// said regex formed by the given ignore patterns.
 fn build_regex_match<'a>(
     ignore_patterns: &[IgnorePattern],
     glob_suffix: &[u8],
 ) -> PatternResult<(Vec<u8>, IgnoreFnType<'a>)> {
     let mut regexps = vec![];
     let mut exact_set = HashSet::new();
 
     for pattern in ignore_patterns {
         if let Some(re) = build_single_regex(pattern, glob_suffix)? {
             regexps.push(re);
         } else {
             let exact = normalize_path_bytes(&pattern.pattern);
             exact_set.insert(HgPathBuf::from_bytes(&exact));
         }
     }
 
     let full_regex = regexps.join(&b'|');
 
     // An empty pattern would cause the regex engine to incorrectly match the
     // (empty) root directory
     let func = if !(regexps.is_empty()) {
         let matcher = re_matcher(&full_regex)?;
         let func = move |filename: &HgPath| {
             exact_set.contains(filename) || matcher.is_match(filename)
         };
         Box::new(func) as IgnoreFnType
     } else {
         let func = move |filename: &HgPath| exact_set.contains(filename);
         Box::new(func) as IgnoreFnType
     };
 
     Ok((full_regex, func))
 }
 
 /// Returns roots and directories corresponding to each pattern.
 ///
 /// This calculates the roots and directories exactly matching the patterns and
 /// returns a tuple of (roots, dirs). It does not return other directories
 /// which may also need to be considered, like the parent directories.
 fn roots_and_dirs(
     ignore_patterns: &[IgnorePattern],
 ) -> (Vec<HgPathBuf>, Vec<HgPathBuf>) {
     let mut roots = Vec::new();
     let mut dirs = Vec::new();
 
     for ignore_pattern in ignore_patterns {
         let IgnorePattern {
             syntax, pattern, ..
         } = ignore_pattern;
         match syntax {
             PatternSyntax::RootGlob | PatternSyntax::Glob => {
                 let mut root = HgPathBuf::new();
                 for p in pattern.split(|c| *c == b'/') {
                     if p.iter()
                         .any(|c| matches!(*c, b'[' | b'{' | b'*' | b'?'))
                     {
                         break;
                     }
                     root.push(HgPathBuf::from_bytes(p).as_ref());
                 }
                 roots.push(root);
             }
             PatternSyntax::Path
             | PatternSyntax::RelPath
             | PatternSyntax::FilePath => {
                 let pat = HgPath::new(if pattern == b"." {
                     &[] as &[u8]
                 } else {
                     pattern
                 });
                 roots.push(pat.to_owned());
             }
             PatternSyntax::RootFilesIn => {
                 let pat = if pattern == b"." {
                     &[] as &[u8]
                 } else {
                     pattern
                 };
                 dirs.push(HgPathBuf::from_bytes(pat));
             }
             _ => {
                 roots.push(HgPathBuf::new());
             }
         }
     }
     (roots, dirs)
 }
 
 /// Paths extracted from patterns
 #[derive(Debug, PartialEq)]
 struct RootsDirsAndParents {
     /// Directories to match recursively
     pub roots: HashSet<HgPathBuf>,
     /// Directories to match non-recursively
     pub dirs: HashSet<HgPathBuf>,
     /// Implicitly required directories to go to items in either roots or dirs
     pub parents: DirsMultiset,
 }
 
 /// Extract roots, dirs and parents from patterns.
 fn roots_dirs_and_parents(
     ignore_patterns: &[IgnorePattern],
 ) -> PatternResult<RootsDirsAndParents> {
     let (roots, dirs) = roots_and_dirs(ignore_patterns);
 
     let mut parents = DirsMultiset::from_manifest(&dirs)?;
 
     for path in &roots {
         parents.add_path(path)?
     }
 
     Ok(RootsDirsAndParents {
         roots: HashSet::from_iter(roots),
         dirs: HashSet::from_iter(dirs),
         parents,
     })
 }
 
 /// Returns a function that checks whether a given file (in the general sense)
 /// should be matched.
 fn build_match<'a>(
     ignore_patterns: Vec<IgnorePattern>,
     glob_suffix: &[u8],
 ) -> PatternResult<(Vec<u8>, IgnoreFnType<'a>)> {
     let mut match_funcs: Vec<IgnoreFnType<'a>> = vec![];
     // For debugging and printing
     let mut patterns = vec![];
 
     let (subincludes, ignore_patterns) = filter_subincludes(ignore_patterns)?;
 
     if !subincludes.is_empty() {
         // Build prefix-based matcher functions for subincludes
         let mut submatchers = FastHashMap::default();
         let mut prefixes = vec![];
 
         for sub_include in subincludes {
             let matcher = IncludeMatcher::new(sub_include.included_patterns)?;
             let match_fn =
                 Box::new(move |path: &HgPath| matcher.matches(path));
             prefixes.push(sub_include.prefix.clone());
             submatchers.insert(sub_include.prefix.clone(), match_fn);
         }
 
         let match_subinclude = move |filename: &HgPath| {
             for prefix in prefixes.iter() {
                 if let Some(rel) = filename.relative_to(prefix) {
                     if (submatchers[prefix])(rel) {
                         return true;
                     }
                 }
             }
             false
         };
 
         match_funcs.push(Box::new(match_subinclude));
     }
 
     if !ignore_patterns.is_empty() {
         // Either do dumb matching if all patterns are rootfiles, or match
         // with a regex.
         if ignore_patterns
             .iter()
             .all(|k| k.syntax == PatternSyntax::RootFilesIn)
         {
             let dirs: HashSet<_> = ignore_patterns
                 .iter()
                 .map(|k| k.pattern.to_owned())
                 .collect();
             let mut dirs_vec: Vec<_> = dirs.iter().cloned().collect();
 
             let match_func = move |path: &HgPath| -> bool {
                 let path = path.as_bytes();
                 let i = path.iter().rposition(|a| *a == b'/');
                 let dir = if let Some(i) = i { &path[..i] } else { b"." };
                 dirs.contains(dir)
             };
             match_funcs.push(Box::new(match_func));
 
             patterns.extend(b"rootfilesin: ");
             dirs_vec.sort();
             patterns.extend(dirs_vec.escaped_bytes());
         } else {
             let (new_re, match_func) =
                 build_regex_match(&ignore_patterns, glob_suffix)?;
             patterns = new_re;
             match_funcs.push(match_func)
         }
     }
 
     Ok(if match_funcs.len() == 1 {
         (patterns, match_funcs.remove(0))
     } else {
         (
             patterns,
             Box::new(move |f: &HgPath| -> bool {
                 match_funcs.iter().any(|match_func| match_func(f))
             }),
         )
     })
 }
 
 /// Parses all "ignore" files with their recursive includes and returns a
 /// function that checks whether a given file (in the general sense) should be
 /// ignored.
 pub fn get_ignore_matcher<'a>(
     mut all_pattern_files: Vec<PathBuf>,
     root_dir: &Path,
     inspect_pattern_bytes: &mut impl FnMut(&Path, &[u8]),
 ) -> PatternResult<(IncludeMatcher<'a>, Vec<PatternFileWarning>)> {
     let mut all_patterns = vec![];
     let mut all_warnings = vec![];
 
     // Sort to make the ordering of calls to `inspect_pattern_bytes`
     // deterministic even if the ordering of `all_pattern_files` is not (such
     // as when a iteration order of a Python dict or Rust HashMap is involved).
     // Sort by "string" representation instead of the default by component
     // (with a Rust-specific definition of a component)
     all_pattern_files
         .sort_unstable_by(|a, b| a.as_os_str().cmp(b.as_os_str()));
 
     for pattern_file in &all_pattern_files {
         let (patterns, warnings) = get_patterns_from_file(
             pattern_file,
             root_dir,
             inspect_pattern_bytes,
         )?;
 
         all_patterns.extend(patterns.to_owned());
         all_warnings.extend(warnings);
     }
     let matcher = IncludeMatcher::new(all_patterns)?;
     Ok((matcher, all_warnings))
 }
 
 /// Parses all "ignore" files with their recursive includes and returns a
 /// function that checks whether a given file (in the general sense) should be
 /// ignored.
 pub fn get_ignore_function<'a>(
     all_pattern_files: Vec<PathBuf>,
     root_dir: &Path,
     inspect_pattern_bytes: &mut impl FnMut(&Path, &[u8]),
 ) -> PatternResult<(IgnoreFnType<'a>, Vec<PatternFileWarning>)> {
     let res =
         get_ignore_matcher(all_pattern_files, root_dir, inspect_pattern_bytes);
     res.map(|(matcher, all_warnings)| {
         let res: IgnoreFnType<'a> =
             Box::new(move |path: &HgPath| matcher.matches(path));
 
         (res, all_warnings)
     })
 }
 
 impl<'a> IncludeMatcher<'a> {
     pub fn new(ignore_patterns: Vec<IgnorePattern>) -> PatternResult<Self> {
         let RootsDirsAndParents {
             roots,
             dirs,
             parents,
         } = roots_dirs_and_parents(&ignore_patterns)?;
         let prefix = ignore_patterns.iter().all(|k| {
             matches!(k.syntax, PatternSyntax::Path | PatternSyntax::RelPath)
         });
         let (patterns, match_fn) = build_match(ignore_patterns, b"(?:/|$)")?;
 
         Ok(Self {
             patterns,
             match_fn,
             prefix,
             roots,
             dirs,
             parents,
         })
     }
 
     fn get_all_parents_children(&self) -> DirsChildrenMultiset {
         // TODO cache
         let thing = self
             .dirs
             .iter()
             .chain(self.roots.iter())
             .chain(self.parents.iter());
         DirsChildrenMultiset::new(thing, Some(self.parents.iter()))
     }
 
     pub fn debug_get_patterns(&self) -> &[u8] {
         self.patterns.as_ref()
     }
 }
 
 impl<'a> Display for IncludeMatcher<'a> {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
         // XXX What about exact matches?
         // I'm not sure it's worth it to clone the HashSet and keep it
         // around just in case someone wants to display the matcher, plus
         // it's going to be unreadable after a few entries, but we need to
         // inform in this display that exact matches are being used and are
         // (on purpose) missing from the `includes`.
         write!(
             f,
             "IncludeMatcher(includes='{}')",
             String::from_utf8_lossy(&self.patterns.escaped_bytes())
         )
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
     use pretty_assertions::assert_eq;
     use std::collections::BTreeMap;
     use std::collections::BTreeSet;
     use std::fmt::Debug;
     use std::path::Path;
 
     #[test]
     fn test_roots_and_dirs() {
         let pats = vec![
             IgnorePattern::new(PatternSyntax::Glob, b"g/h/*", Path::new("")),
             IgnorePattern::new(PatternSyntax::Glob, b"g/h", Path::new("")),
             IgnorePattern::new(PatternSyntax::Glob, b"g*", Path::new("")),
         ];
         let (roots, dirs) = roots_and_dirs(&pats);
 
         assert_eq!(
             roots,
             vec!(
                 HgPathBuf::from_bytes(b"g/h"),
                 HgPathBuf::from_bytes(b"g/h"),
                 HgPathBuf::new()
             ),
         );
         assert_eq!(dirs, vec!());
     }
 
     #[test]
     fn test_roots_dirs_and_parents() {
         let pats = vec![
             IgnorePattern::new(PatternSyntax::Glob, b"g/h/*", Path::new("")),
             IgnorePattern::new(PatternSyntax::Glob, b"g/h", Path::new("")),
             IgnorePattern::new(PatternSyntax::Glob, b"g*", Path::new("")),
         ];
 
         let mut roots = HashSet::new();
         roots.insert(HgPathBuf::from_bytes(b"g/h"));
         roots.insert(HgPathBuf::new());
 
         let dirs = HashSet::new();
 
         let parents = DirsMultiset::from_manifest(&[
             HgPathBuf::from_bytes(b"x"),
             HgPathBuf::from_bytes(b"g/x"),
             HgPathBuf::from_bytes(b"g/y"),
         ])
         .unwrap();
 
         assert_eq!(
             roots_dirs_and_parents(&pats).unwrap(),
             RootsDirsAndParents {
                 roots,
                 dirs,
                 parents
             }
         );
     }
 
     #[test]
     fn test_filematcher_visit_children_set() {
         // Visitchildrenset
         let files = vec![HgPathBuf::from_bytes(b"dir/subdir/foo.txt")];
         let matcher = FileMatcher::new(files).unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"foo.txt"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Set(set)
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/foo.txt")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
     }
 
     #[test]
     fn test_filematcher_visit_children_set_files_and_dirs() {
         let files = vec![
             HgPathBuf::from_bytes(b"rootfile.txt"),
             HgPathBuf::from_bytes(b"a/file1.txt"),
             HgPathBuf::from_bytes(b"a/b/file2.txt"),
             // No file in a/b/c
             HgPathBuf::from_bytes(b"a/b/c/d/file4.txt"),
         ];
         let matcher = FileMatcher::new(files).unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"a"));
         set.insert(HgPathBuf::from_bytes(b"rootfile.txt"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"b"));
         set.insert(HgPathBuf::from_bytes(b"file1.txt"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"a")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"c"));
         set.insert(HgPathBuf::from_bytes(b"file2.txt"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"a/b")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"d"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"a/b/c")),
             VisitChildrenSet::Set(set)
         );
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"file4.txt"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"a/b/c/d")),
             VisitChildrenSet::Set(set)
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"a/b/c/d/e")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
     }
 
     #[test]
     fn test_patternmatcher() {
         // VisitdirPrefix
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Path,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         // OPT: This should probably be Recursive if its parent is?
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
 
         // VisitchildrensetPrefix
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Path,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         // OPT: This should probably be Recursive if its parent is?
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
 
         // VisitdirRootfilesin
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RootFilesIn,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
 
         // VisitchildrensetRootfilesin
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RootFilesIn,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         // FIXME: These should probably be {'dir'}, {'subdir'} and This,
         // respectively
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
 
         // VisitdirGlob
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Glob,
             b"dir/z*",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         // OPT: these should probably be False.
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // VisitchildrensetGlob
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Glob,
             b"dir/z*",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         // OPT: these should probably be Empty
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // VisitdirFilepath
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::FilePath,
             b"dir/z",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
 
         // VisitchildrensetFilepath
         let m = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::FilePath,
             b"dir/z",
             Path::new(""),
         )])
         .unwrap();
         assert_eq!(
             m.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             m.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
     }
 
     #[test]
     fn test_includematcher() {
         // VisitchildrensetPrefix
         let matcher = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         // OPT: This should probably be 'all' if its parent is?
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
 
         // VisitchildrensetRootfilesin
         let matcher = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RootFilesIn,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
 
         // VisitchildrensetGlob
         let matcher = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Glob,
             b"dir/z*",
             Path::new(""),
         )])
         .unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         // OPT: these should probably be set().
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // VisitchildrensetFilePath
         let matcher = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::FilePath,
             b"dir/z",
             Path::new(""),
         )])
         .unwrap();
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"z"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
         // OPT: these should probably be set().
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
 
         // Test multiple patterns
         let matcher = IncludeMatcher::new(vec![
             IgnorePattern::new(PatternSyntax::RelPath, b"foo", Path::new("")),
             IgnorePattern::new(PatternSyntax::Glob, b"g*", Path::new("")),
         ])
         .unwrap();
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
 
         // Test multiple patterns
         let matcher = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Glob,
             b"**/*.exe",
             Path::new(""),
         )])
         .unwrap();
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::This
         );
     }
 
     #[test]
     fn test_unionmatcher() {
         // Path + Rootfiles
         let m1 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         let m2 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RootFilesIn,
             b"dir",
             Path::new(""),
         )])
         .unwrap();
         let matcher = UnionMatcher::new(vec![Box::new(m1), Box::new(m2)]);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
 
         // OPT: These next two could be 'all' instead of 'this'.
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // Path + unrelated Path
         let m1 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         let m2 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"folder",
             Path::new(""),
         )])
         .unwrap();
         let matcher = UnionMatcher::new(vec![Box::new(m1), Box::new(m2)]);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"folder"));
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Recursive
         );
         // OPT: These next two could be 'all' instead of 'this'.
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // Path + subpath
         let m1 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"dir/subdir/x",
             Path::new(""),
         )])
         .unwrap();
         let m2 = IncludeMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RelPath,
             b"dir/subdir",
             Path::new(""),
         )])
         .unwrap();
         let matcher = UnionMatcher::new(vec![Box::new(m1), Box::new(m2)]);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Recursive
         );
         // OPT: this should probably be 'all' not 'this'.
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::This
         );
     }
 
     #[test]
     fn test_intersectionmatcher() {
         // Include path + Include rootfiles
         let m1 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let m2 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RootFilesIn,
                 b"dir",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let matcher = IntersectionMatcher::new(m1, m2);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
 
         // Non intersecting paths
         let m1 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let m2 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"folder",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let matcher = IntersectionMatcher::new(m1, m2);
 
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
 
         // Nested paths
         let m1 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir/x",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let m2 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let matcher = IntersectionMatcher::new(m1, m2);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"x"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::Empty
         );
         // OPT: this should probably be 'all' not 'this'.
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
 
         // Diverging paths
         let m1 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir/x",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let m2 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir/z",
                 Path::new(""),
             )])
             .unwrap(),
         );
         let matcher = IntersectionMatcher::new(m1, m2);
 
         // OPT: these next two could probably be Empty as well.
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
         // OPT: these next two could probably be Empty as well.
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::Empty
         );
     }
 
     #[test]
     fn test_differencematcher() {
         // Two alwaysmatchers should function like a nevermatcher
         let m1 = AlwaysMatcher;
         let m2 = AlwaysMatcher;
         let matcher = DifferenceMatcher::new(Box::new(m1), Box::new(m2));
 
         for case in &[
             &b""[..],
             b"dir",
             b"dir/subdir",
             b"dir/subdir/z",
             b"dir/foo",
             b"dir/subdir/x",
             b"folder",
         ] {
             assert_eq!(
                 matcher.visit_children_set(HgPath::new(case)),
                 VisitChildrenSet::Empty
             );
         }
 
         // One always and one never should behave the same as an always
         let m1 = AlwaysMatcher;
         let m2 = NeverMatcher;
         let matcher = DifferenceMatcher::new(Box::new(m1), Box::new(m2));
 
         for case in &[
             &b""[..],
             b"dir",
             b"dir/subdir",
             b"dir/subdir/z",
             b"dir/foo",
             b"dir/subdir/x",
             b"folder",
         ] {
             assert_eq!(
                 matcher.visit_children_set(HgPath::new(case)),
                 VisitChildrenSet::Recursive
             );
         }
 
         // Two include matchers
         let m1 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir",
                 Path::new("/repo"),
             )])
             .unwrap(),
         );
         let m2 = Box::new(
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RootFilesIn,
                 b"dir",
                 Path::new("/repo"),
             )])
             .unwrap(),
         );
 
         let matcher = DifferenceMatcher::new(m1, m2);
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"dir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"")),
             VisitChildrenSet::Set(set)
         );
 
         let mut set = HashSet::new();
         set.insert(HgPathBuf::from_bytes(b"subdir"));
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir")),
             VisitChildrenSet::Set(set)
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir")),
             VisitChildrenSet::Recursive
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/foo")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"folder")),
             VisitChildrenSet::Empty
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/z")),
             VisitChildrenSet::This
         );
         assert_eq!(
             matcher.visit_children_set(HgPath::new(b"dir/subdir/x")),
             VisitChildrenSet::This
         );
     }
 
     mod invariants {
         pub mod visit_children_set {
 
             use crate::{
                 matchers::{tests::Tree, Matcher, VisitChildrenSet},
                 utils::hg_path::HgPath,
             };
 
             #[allow(dead_code)]
             #[derive(Debug)]
             struct Error<'a, M> {
                 matcher: &'a M,
                 path: &'a HgPath,
                 matching: &'a Tree,
                 visit_children_set: &'a VisitChildrenSet,
             }
 
-            fn holds(matching: &Tree, vcs: &VisitChildrenSet) -> bool {
+            fn holds(
+                matching: &Tree,
+                not_matching: &Tree,
+                vcs: &VisitChildrenSet,
+            ) -> bool {
                 match vcs {
                     VisitChildrenSet::Empty => matching.is_empty(),
                     VisitChildrenSet::This => {
                         // `This` does not come with any obligations.
                         true
                     }
                     VisitChildrenSet::Recursive => {
-                        // `Recursive` does not come with any correctness
-                        // obligations.
-                        // It instructs the caller to stop calling
-                        // `visit_children_set` for all
-                        // descendants, so may have negative performance
-                        // implications, but we're not testing against that
-                        // here.
-                        true
+                        // `Recursive` requires that *everything* in the
+                        // subtree matches. This
+                        // requirement is relied on for example in
+                        // DifferenceMatcher implementation.
+                        not_matching.is_empty()
                     }
                     VisitChildrenSet::Set(allowed_children) => {
                         // `allowed_children` does not distinguish between
                         // files and directories: if it's not included, it
                         // must not be matched.
                         for k in matching.dirs.keys() {
                             if !(allowed_children.contains(k)) {
                                 return false;
                             }
                         }
                         for k in matching.files.iter() {
                             if !(allowed_children.contains(k)) {
                                 return false;
                             }
                         }
                         true
                     }
                 }
             }
 
             pub fn check<M: Matcher + std::fmt::Debug>(
                 matcher: &M,
                 path: &HgPath,
                 matching: &Tree,
+                not_matching: &Tree,
                 visit_children_set: &VisitChildrenSet,
             ) {
-                if !holds(matching, visit_children_set) {
+                if !holds(matching, not_matching, visit_children_set) {
                     panic!(
                         "{:#?}",
                         Error {
                             matcher,
                             path,
                             visit_children_set,
                             matching
                         }
                     )
                 }
             }
         }
     }
 
     #[derive(Debug, Clone)]
     pub struct Tree {
         files: BTreeSet<HgPathBuf>,
         dirs: BTreeMap<HgPathBuf, Tree>,
     }
 
     impl Tree {
         fn len(&self) -> usize {
             let mut n = 0;
             n += self.files.len();
             for d in self.dirs.values() {
                 n += d.len();
             }
             n
         }
 
         fn is_empty(&self) -> bool {
             self.files.is_empty() && self.dirs.is_empty()
         }
 
+        fn make(
+            files: BTreeSet<HgPathBuf>,
+            dirs: BTreeMap<HgPathBuf, Tree>,
+        ) -> Self {
+            Self {
+                files,
+                dirs: dirs
+                    .into_iter()
+                    .filter(|(_k, v)| (!(v.is_empty())))
+                    .collect(),
+            }
+        }
+
         fn filter_and_check<M: Matcher + Debug>(
             &self,
             m: &M,
             path: &HgPath,
-        ) -> Self {
-            let files: BTreeSet<HgPathBuf> = self
-                .files
-                .iter()
-                .filter(|v| m.matches(&path.join(v)))
-                .map(|f| f.to_owned())
-                .collect();
-            let dirs: BTreeMap<HgPathBuf, Tree> = self
+        ) -> (Self, Self) {
+            let (files1, files2): (BTreeSet<HgPathBuf>, BTreeSet<HgPathBuf>) =
+                self.files
+                    .iter()
+                    .map(|v| v.to_owned())
+                    .partition(|v| m.matches(&path.join(v)));
+            let (dirs1, dirs2): (
+                BTreeMap<HgPathBuf, Tree>,
+                BTreeMap<HgPathBuf, Tree>,
+            ) = self
                 .dirs
                 .iter()
-                .filter_map(|(k, v)| {
+                .map(|(k, v)| {
                     let path = path.join(k);
-                    let v = v.filter_and_check(m, &path);
-                    if v.is_empty() {
-                        None
-                    } else {
-                        Some((k.to_owned(), v))
-                    }
+                    let (t1, t2) = v.filter_and_check(m, &path);
+                    ((k.clone(), t1), (k.clone(), t2))
                 })
-                .collect();
-            let matching = Self { files, dirs };
+                .unzip();
+            let matching = Self::make(files1, dirs1);
+            let not_matching = Self::make(files2, dirs2);
             let vcs = m.visit_children_set(path);
-            invariants::visit_children_set::check(m, path, &matching, &vcs);
-            matching
+            invariants::visit_children_set::check(
+                m,
+                path,
+                &matching,
+                &not_matching,
+                &vcs,
+            );
+            (matching, not_matching)
         }
 
         fn check_matcher<M: Matcher + Debug>(
             &self,
             m: &M,
             expect_count: usize,
         ) {
             let res = self.filter_and_check(m, &HgPathBuf::new());
-            if expect_count != res.len() {
+            if expect_count != res.0.len() {
                 eprintln!(
                     "warning: expected {} matches, got {} for {:#?}",
                     expect_count,
-                    res.len(),
+                    res.0.len(),
                     m
                 );
             }
         }
     }
 
     fn mkdir(children: &[(&[u8], &Tree)]) -> Tree {
         let p = HgPathBuf::from_bytes;
         let names = [
             p(b"a"),
             p(b"b.txt"),
             p(b"file.txt"),
             p(b"c.c"),
             p(b"c.h"),
             p(b"dir1"),
             p(b"dir2"),
             p(b"subdir"),
         ];
         let files: BTreeSet<HgPathBuf> = BTreeSet::from(names);
         let dirs = children
             .iter()
             .map(|(name, t)| (p(name), (*t).clone()))
             .collect();
         Tree { files, dirs }
     }
 
     fn make_example_tree() -> Tree {
         let leaf = mkdir(&[]);
         let abc = mkdir(&[(b"d", &leaf)]);
         let ab = mkdir(&[(b"c", &abc)]);
         let a = mkdir(&[(b"b", &ab)]);
         let dir = mkdir(&[(b"subdir", &leaf), (b"subdir.c", &leaf)]);
         mkdir(&[(b"dir", &dir), (b"dir1", &dir), (b"dir2", &dir), (b"a", &a)])
     }
 
     #[test]
     fn test_pattern_matcher_visit_children_set() {
         let tree = make_example_tree();
         let pattern_dir1_glob_c =
             PatternMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::Glob,
                 b"dir1/*.c",
                 Path::new(""),
             )])
             .unwrap();
         let pattern_dir1 = || {
             PatternMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::Path,
                 b"dir1",
                 Path::new(""),
             )])
             .unwrap()
         };
         let pattern_dir1_a = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::Glob,
             b"dir1/a",
             Path::new(""),
         )])
         .unwrap();
         let pattern_relglob_c = || {
             PatternMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelGlob,
                 b"*.c",
                 Path::new(""),
             )])
             .unwrap()
         };
         let files = vec![HgPathBuf::from_bytes(b"dir/subdir/b.txt")];
         let file_dir_subdir_b = FileMatcher::new(files).unwrap();
 
         let files = vec![
             HgPathBuf::from_bytes(b"file.txt"),
             HgPathBuf::from_bytes(b"a/file.txt"),
             HgPathBuf::from_bytes(b"a/b/file.txt"),
             // No file in a/b/c
             HgPathBuf::from_bytes(b"a/b/c/d/file.txt"),
         ];
         let file_abcdfile = FileMatcher::new(files).unwrap();
         let rootfilesin_dir = PatternMatcher::new(vec![IgnorePattern::new(
             PatternSyntax::RootFilesIn,
             b"dir",
             Path::new(""),
         )])
         .unwrap();
 
         let pattern_filepath_dir_subdir =
             PatternMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::FilePath,
                 b"dir/subdir",
                 Path::new(""),
             )])
             .unwrap();
 
         let include_dir_subdir =
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::RelPath,
                 b"dir/subdir",
                 Path::new(""),
             )])
             .unwrap();
 
         let more_includematchers = [
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::Glob,
                 b"dir/s*",
                 Path::new(""),
             )])
             .unwrap(),
             // Test multiple patterns
             IncludeMatcher::new(vec![
                 IgnorePattern::new(
                     PatternSyntax::RelPath,
                     b"dir",
                     Path::new(""),
                 ),
                 IgnorePattern::new(PatternSyntax::Glob, b"s*", Path::new("")),
             ])
             .unwrap(),
             // Test multiple patterns
             IncludeMatcher::new(vec![IgnorePattern::new(
                 PatternSyntax::Glob,
                 b"**/*.c",
                 Path::new(""),
             )])
             .unwrap(),
         ];
 
         tree.check_matcher(&pattern_dir1(), 25);
         tree.check_matcher(&pattern_dir1_a, 1);
         tree.check_matcher(&pattern_dir1_glob_c, 2);
         tree.check_matcher(&pattern_relglob_c(), 14);
         tree.check_matcher(&AlwaysMatcher, 112);
         tree.check_matcher(&NeverMatcher, 0);
         tree.check_matcher(
             &IntersectionMatcher::new(
                 Box::new(pattern_relglob_c()),
                 Box::new(pattern_dir1()),
             ),
             3,
         );
         tree.check_matcher(
             &UnionMatcher::new(vec![
                 Box::new(pattern_relglob_c()),
                 Box::new(pattern_dir1()),
             ]),
             36,
         );
         tree.check_matcher(
             &DifferenceMatcher::new(
                 Box::new(pattern_relglob_c()),
                 Box::new(pattern_dir1()),
             ),
             11,
         );
         tree.check_matcher(&file_dir_subdir_b, 1);
         tree.check_matcher(&file_abcdfile, 4);
         tree.check_matcher(&rootfilesin_dir, 8);
         tree.check_matcher(&pattern_filepath_dir_subdir, 1);
         tree.check_matcher(&include_dir_subdir, 9);
         tree.check_matcher(&more_includematchers[0], 17);
         tree.check_matcher(&more_includematchers[1], 25);
         tree.check_matcher(&more_includematchers[2], 35);
     }
 }