--- a/rust/hg-core/src/repo.rs	Fri Dec 03 14:36:40 2021 +0100
+++ b/rust/hg-core/src/repo.rs	Tue Nov 16 11:53:58 2021 +0000
@@ -248,6 +248,14 @@
             .contains(requirements::DIRSTATE_V2_REQUIREMENT)
     }
 
+    pub fn has_sparse(&self) -> bool {
+        self.requirements.contains(requirements::SPARSE_REQUIREMENT)
+    }
+
+    pub fn has_narrow(&self) -> bool {
+        self.requirements.contains(requirements::NARROW_REQUIREMENT)
+    }
+
     fn dirstate_file_contents(&self) -> Result<Vec<u8>, HgError> {
         Ok(self
             .hg_vfs()

--- a/rust/hg-core/src/requirements.rs	Fri Dec 03 14:36:40 2021 +0100
+++ b/rust/hg-core/src/requirements.rs	Tue Nov 16 11:53:58 2021 +0000
@@ -88,6 +88,10 @@
     // When it starts writing to the repository, it’ll need to either keep the
     // persistent nodemap up to date or remove this entry:
     NODEMAP_REQUIREMENT,
+    // Not all commands support `sparse` and `narrow`. The commands that do
+    // not should opt out by checking `has_sparse` and `has_narrow`.
+    SPARSE_REQUIREMENT,
+    NARROW_REQUIREMENT,
 ];
 
 // Copied from mercurial/requirements.py:

--- a/rust/rhg/src/commands/files.rs	Fri Dec 03 14:36:40 2021 +0100
+++ b/rust/rhg/src/commands/files.rs	Tue Nov 16 11:53:58 2021 +0000
@@ -40,10 +40,38 @@
     let rev = invocation.subcommand_args.value_of("rev");
 
     let repo = invocation.repo?;
+
+    // It seems better if this check is removed: this would correspond to
+    // automatically enabling the extension if the repo requires it.
+    // However we need this check to be in sync with vanilla hg so hg tests
+    // pass.
+    if repo.has_sparse()
+        && invocation.config.get(b"extensions", b"sparse").is_none()
+    {
+        return Err(CommandError::unsupported(
+            "repo is using sparse, but sparse extension is not enabled",
+        ));
+    }
+
     if let Some(rev) = rev {
+        if repo.has_narrow() {
+            return Err(CommandError::unsupported(
+                "rhg files -r <rev> is not supported in narrow clones",
+            ));
+        }
         let files = list_rev_tracked_files(repo, rev).map_err(|e| (e, rev))?;
         display_files(invocation.ui, repo, files.iter())
     } else {
+        // The dirstate always reflects the sparse narrowspec, so if
+        // we only have sparse without narrow all is fine.
+        // If we have narrow, then [hg files] needs to check if
+        // the store narrowspec is in sync with the one of the dirstate,
+        // so we can't support that without explicit code.
+        if repo.has_narrow() {
+            return Err(CommandError::unsupported(
+                "rhg files is not supported in narrow clones",
+            ));
+        }
         let distate = Dirstate::new(repo)?;
         let files = distate.tracked_files()?;
         display_files(invocation.ui, repo, files.into_iter().map(Ok))

--- a/rust/rhg/src/commands/status.rs	Fri Dec 03 14:36:40 2021 +0100
+++ b/rust/rhg/src/commands/status.rs	Tue Nov 16 11:53:58 2021 +0000
@@ -191,6 +191,13 @@
     let no_status = args.is_present("no-status");
 
     let repo = invocation.repo?;
+
+    if repo.has_sparse() || repo.has_narrow() {
+        return Err(CommandError::unsupported(
+            "rhg status is not supported for sparse checkouts or narrow clones yet"
+        ));
+    }
+
     let mut dmap = repo.dirstate_map_mut()?;
 
     let options = StatusOptions {

--- a/rust/rhg/src/main.rs	Fri Dec 03 14:36:40 2021 +0100
+++ b/rust/rhg/src/main.rs	Tue Nov 16 11:53:58 2021 +0000
@@ -593,7 +593,8 @@
     }
 }
 
-const SUPPORTED_EXTENSIONS: &[&[u8]] = &[b"blackbox", b"share"];
+const SUPPORTED_EXTENSIONS: &[&[u8]] =
+    &[b"blackbox", b"share", b"sparse", b"narrow"];
 
 fn check_extensions(config: &Config) -> Result<(), CommandError> {
     let enabled = config.get_section_keys(b"extensions");

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-rhg-sparse-narrow.t	Tue Nov 16 11:53:58 2021 +0000
@@ -0,0 +1,120 @@
+#require rhg
+
+  $ NO_FALLBACK="env RHG_ON_UNSUPPORTED=abort"
+
+Rhg works well when sparse working copy is enabled.
+
+  $ cd "$TESTTMP"
+  $ hg init repo-sparse
+  $ cd repo-sparse
+  $ cat > .hg/hgrc <<EOF
+  > [extensions]
+  > sparse=
+  > EOF
+
+  $ echo a > show
+  $ echo x > hide
+  $ mkdir dir1 dir2
+  $ echo x > dir1/x
+  $ echo y > dir1/y
+  $ echo z > dir2/z
+
+  $ hg ci -Aqm 'initial'
+  $ hg debugsparse --include 'show'
+  $ ls -A
+  .hg
+  show
+
+  $ tip=$(hg log -r . --template '{node}')
+  $ $NO_FALLBACK rhg files -r "$tip"
+  dir1/x
+  dir1/y
+  dir2/z
+  hide
+  show
+  $ $NO_FALLBACK rhg files
+  show
+
+  $ $NO_FALLBACK rhg cat -r "$tip" hide
+  x
+
+  $ cd ..
+
+We support most things when narrow is enabled, too, with a couple of caveats.
+
+  $ . "$TESTDIR/narrow-library.sh"
+  $ real_hg=$RHG_FALLBACK_EXECUTABLE
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > narrow=
+  > EOF
+
+  $ hg clone --narrow  ./repo-sparse repo-narrow --include dir1
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 2 changes to 2 files
+  new changesets 6d714a4a2998
+  updating to branch default
+  2 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ cd repo-narrow
+
+  $ $NO_FALLBACK rhg cat -r "$tip" dir1/x
+  x
+  $ "$real_hg" cat -r "$tip" dir1/x
+  x
+
+TODO: bad error message
+
+  $ $NO_FALLBACK rhg cat -r "$tip" hide
+  abort: invalid revision identifier: 6d714a4a2998cbfd0620db44da58b749f6565d63
+  [255]
+  $ "$real_hg" cat -r "$tip" hide
+  [1]
+
+A naive implementation of [rhg files] leaks the paths that are supposed to be
+hidden by narrow, so we just fall back to hg.
+
+  $ $NO_FALLBACK rhg files -r "$tip"
+  unsupported feature: rhg files -r <rev> is not supported in narrow clones
+  [252]
+  $ "$real_hg" files -r "$tip"
+  dir1/x
+  dir1/y
+
+Hg status needs to do some filtering based on narrow spec, so we don't
+support it in rhg for narrow clones yet.
+
+  $ mkdir dir2
+  $ touch dir2/q
+  $ "$real_hg" status
+  $ $NO_FALLBACK rhg --config rhg.status=true status
+  unsupported feature: rhg status is not supported for sparse checkouts or narrow clones yet
+  [252]
+
+Adding "orphaned" index files:
+
+  $ (cd ..; cp repo-sparse/.hg/store/data/hide.i repo-narrow/.hg/store/data/hide.i)
+  $ (cd ..; mkdir repo-narrow/.hg/store/data/dir2; cp repo-sparse/.hg/store/data/dir2/z.i repo-narrow/.hg/store/data/dir2/z.i)
+  $ "$real_hg" verify
+  checking changesets
+  checking manifests
+  crosschecking files in changesets and manifests
+  checking files
+  checked 1 changesets with 2 changes to 2 files
+
+  $ "$real_hg" files -r "$tip"
+  dir1/x
+  dir1/y
+
+# TODO: even though [hg files] hides the orphaned dir2/z, [hg cat] still shows it.
+# rhg has the same issue, but at least it's not specific to rhg.
+# This is despite [hg verify] succeeding above.
+
+  $ $NO_FALLBACK rhg cat -r "$tip" dir2/z
+  z
+  $ "$real_hg" cat -r "$tip" dir2/z
+  z