--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/rhg/src/commands/debugrequirements.rs	Tue Nov 24 15:11:58 2020 +0100
@@ -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(())
+    }
+}