rhg: add Command trait for subcommands implemented by rhg

Normalizes the interface of the cli's commands

Differential Revision: https://phab.mercurial-scm.org/D8611

Differential Revision: https://phab.mercurial-scm.org/D8648

rust/rhg/src/commands.rs

@@ -0,0 +1,8 b''
+use crate::error::CommandError;
+
+/// The common trait for rhg commands
+///
+/// Normalize the interface of the commands provided by rhg
+pub trait Command {
+    fn run(&self) -> Result<(), CommandError>;
+}

rust/rhg/src/error.rs

@@ -0,0 +1,3 b''
+/// The error type for the Command trait
+#[derive(Debug, PartialEq)]
+pub struct CommandError {}

rust/rhg/src/main.rs

@@ -1,5 +1,7 b''
+mod commands;
+mod error;
 mod exitcode;
 
 fn main() {
     std::process::exit(exitcode::UNIMPLEMENTED_COMMAND)
 }