Patchwork D9397: rhg: add a `debugrequirements` subcommand

diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -115,3 +115,12 @@ 
   $ hg commit -m "add copy of original"
   $ rhg cat -r 1 copy_of_original
   original content
+
+Requirements
+  $ rhg debugrequirements
+  dotencode
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
diff --git a/rust/rhg/src/main.rs b/rust/rhg/src/main.rs
--- a/rust/rhg/src/main.rs
+++ b/rust/rhg/src/main.rs
@@ -83,6 +83,10 @@ 
                         .required(true)
                         .value_name("REV"),
                 ),
+        )
+        .subcommand(
+            SubCommand::with_name("debugrequirements")
+                .about(commands::debugrequirements::HELP_TEXT),
         );
 
     let matches = app.clone().get_matches_safe().unwrap_or_else(|err| {
@@ -124,6 +128,10 @@ 
         ("debugdata", Some(matches)) => {
             commands::debugdata::DebugDataCommand::try_from(matches)?.run(&ui)
         }
+        ("debugrequirements", _) => {
+            commands::debugrequirements::DebugRequirementsCommand::new()
+                .run(&ui)
+        }
         _ => unreachable!(), // Because of AppSettings::SubcommandRequired,
     }
 }
diff --git a/rust/rhg/src/error.rs b/rust/rhg/src/error.rs
--- a/rust/rhg/src/error.rs
+++ b/rust/rhg/src/error.rs
@@ -12,6 +12,9 @@ 
     RootNotFound(PathBuf),
     /// The current directory cannot be found
     CurrentDirNotFound(std::io::Error),
+    /// Error while reading or writing a file
+    // TODO: add the file name/path?
+    FileError(std::io::Error),
     /// The standard output stream cannot be written to
     StdoutError,
     /// The standard error stream cannot be written to
@@ -27,6 +30,7 @@ 
         match self {
             CommandErrorKind::RootNotFound(_) => exitcode::ABORT,
             CommandErrorKind::CurrentDirNotFound(_) => exitcode::ABORT,
+            CommandErrorKind::FileError(_) => exitcode::ABORT,
             CommandErrorKind::StdoutError => exitcode::ABORT,
             CommandErrorKind::StderrError => exitcode::ABORT,
             CommandErrorKind::Abort(_) => exitcode::ABORT,
diff --git a/rust/rhg/src/commands/debugrequirements.rs b/rust/rhg/src/commands/debugrequirements.rs
new file mode 100644
--- /dev/null
+++ b/rust/rhg/src/commands/debugrequirements.rs
@@ -0,0 +1,40 @@ 
+use crate::commands::Command;
+use crate::error::{CommandError, CommandErrorKind};
+use crate::ui::Ui;
+use hg::operations::FindRoot;
+
+pub const HELP_TEXT: &str = "
+Print the current repo requirements.
+";
+
+pub struct DebugRequirementsCommand {}
+
+impl DebugRequirementsCommand {
+    pub fn new() -> Self {
+        DebugRequirementsCommand {}
+    }
+}
+
+impl Command for DebugRequirementsCommand {
+    fn run(&self, ui: &Ui) -> Result<(), CommandError> {
+        let root = FindRoot::new().run()?;
+        let requires = root.join(".hg").join("requires");
+        let requirements = match std::fs::read(requires) {
+            Ok(bytes) => bytes,
+
+            // Treat a missing file the same as an empty file.
+            // From `mercurial/localrepo.py`:
+            // > requires file contains a newline-delimited list of
+            // > features/capabilities the opener (us) must have in order to use
+            // > the repository. This file was introduced in Mercurial 0.9.2,
+            // > which means very old repositories may not have one. We assume
+            // > a missing file translates to no requirements.
+            Err(error) if error.kind() == std::io::ErrorKind::NotFound => Vec::new(),
+
+            Err(error) => Err(CommandErrorKind::FileError(error))?,
+        };
+
+        ui.write_stdout(&requirements)?;
+        Ok(())
+    }
+}
diff --git a/rust/rhg/src/commands.rs b/rust/rhg/src/commands.rs
--- a/rust/rhg/src/commands.rs
+++ b/rust/rhg/src/commands.rs
@@ -1,5 +1,6 @@ 
 pub mod cat;
 pub mod debugdata;
+pub mod debugrequirements;
 pub mod files;
 pub mod root;
 use crate::error::CommandError;