Patchwork D8612: rhg: add RootCommand using hg-core FindRoot operation to prepare `hg root`

diff --git a/rust/rhg/src/exitcode.rs b/rust/rhg/src/exitcode.rs
--- a/rust/rhg/src/exitcode.rs
+++ b/rust/rhg/src/exitcode.rs
@@ -1,4 +1,10 @@ 
 pub type ExitCode = i32;
 
+/// Successful exit
+pub const OK: ExitCode = 0;
+
+/// Generic abort
+pub const ABORT: ExitCode = 255;
+
 /// Command not implemented by rhg
 pub const UNIMPLEMENTED_COMMAND: ExitCode = 252;
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
@@ -1,4 +1,32 @@ 
+use crate::exitcode;
+use std::convert::From;
+
+/// The kind of command error
+#[derive(Debug, PartialEq)]
+pub enum CommandErrorKind {
+    /// The command finished without error
+    Ok,
+    /// The root of the repository cannot be found
+    RootNotFound,
+}
+
+impl CommandErrorKind {
+    pub fn get_exit_code(&self) -> exitcode::ExitCode {
+        match self {
+            CommandErrorKind::Ok => exitcode::OK,
+            CommandErrorKind::RootNotFound => exitcode::ABORT,
+        }
+    }
+}
 
 /// The error type for the Command trait
 #[derive(Debug, PartialEq)]
-pub struct CommandError {}
+pub struct CommandError {
+    pub kind: CommandErrorKind,
+}
+
+impl From<CommandErrorKind> for CommandError {
+    fn from(kind: CommandErrorKind) -> Self {
+        CommandError { kind }
+    }
+}
diff --git a/rust/rhg/src/commands/root.rs b/rust/rhg/src/commands/root.rs
new file mode 100644
--- /dev/null
+++ b/rust/rhg/src/commands/root.rs
@@ -0,0 +1,79 @@ 
+use crate::commands::Command;
+use crate::error::{CommandError, CommandErrorKind};
+use hg::operations::{FindRoot, FindRootError, FindRootErrorKind, Operation};
+use hg::utils::files::get_bytes_from_path;
+use std::io::Write;
+use std::path::PathBuf;
+
+pub const HELP_TEXT: &str = "
+Print the root directory of the current repository.
+
+Returns 0 on success.
+";
+
+pub struct RootCommand;
+
+impl RootCommand {
+    fn display_found_path(path_buf: PathBuf) -> Result<(), CommandError> {
+        // TODO use formating macro
+        let mut stdout = std::io::stdout();
+        let bytes = get_bytes_from_path(path_buf);
+
+        stdout
+            .write_all(&[bytes.as_slice(), b"\n"].concat())
+            .expect("Should be able to write to stdout");
+
+        Err(CommandErrorKind::Ok.into())
+    }
+
+    fn display_error(error: FindRootError) -> Result<(), CommandError> {
+        match error.kind {
+            FindRootErrorKind::RootNotFound => {
+                let current_dir = match error.current_dir {
+                    Some(path) => path,
+                    None => panic!("This should not happen"),
+                };
+
+                // TODO use formating macro
+                let mut stderr = std::io::stderr();
+                let bytes = get_bytes_from_path(current_dir);
+
+                stderr
+                    .write_all(
+                        &[
+                            b"abandon : no repository found in ",
+                            bytes.as_slice(),
+                            b" (.hg not found) !\n",
+                        ]
+                        .concat(),
+                    )
+                    .expect("Should be able to write to stderr");
+            }
+            FindRootErrorKind::GetCurrentDirError => {
+                // TODO use formating macro
+                let mut stderr = std::io::stderr();
+
+                stderr
+                    .write_all(
+                        &concat!(
+                        "Current directory does not exists",
+                        "or permissions are insufficient to get access to it"
+                    )
+                        .as_bytes(),
+                    )
+                    .expect("Should be able to write to stderr");
+            }
+        }
+        Err(CommandErrorKind::RootNotFound.into())
+    }
+}
+
+impl Command for RootCommand {
+    fn run() -> Result<(), CommandError> {
+        let operation = FindRoot::new();
+        match operation.run() {
+            Ok(path_buf) => RootCommand::display_found_path(path_buf),
+            Err(e) => RootCommand::display_error(e),
+        }
+    }
+}
diff --git a/rust/rhg/src/commands/mod.rs b/rust/rhg/src/commands/mod.rs
--- a/rust/rhg/src/commands/mod.rs
+++ b/rust/rhg/src/commands/mod.rs
@@ -1,3 +1,4 @@ 
+pub mod root;
 use crate::error::CommandError;
 
 /// The common trait for rhg commands
diff --git a/rust/rhg/Cargo.toml b/rust/rhg/Cargo.toml
--- a/rust/rhg/Cargo.toml
+++ b/rust/rhg/Cargo.toml
@@ -5,4 +5,5 @@ 
 edition = "2018"
 
 [dependencies]
+hg-core = { path = "../hg-core"}
 
diff --git a/rust/Cargo.lock b/rust/Cargo.lock
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -489,6 +489,9 @@ 
 [[package]]
 name = "rhg"
 version = "0.1.0"
+dependencies = [
+ "hg-core 0.1.0",
+]
 
 [[package]]
 name = "rustc_version"