Refactor command-line handling into modules per subcommands

cli/src/cmd/gc.rs

@@ -0,0 +1,37 @@
+use crate::cmd::{ArgMatchResult, CommandInvocation};
+use clap::{App, ArgMatches, SubCommand as ClapSubCommand};
+use failure::Fallible;
+
+#[derive(Debug)]
+struct Invocation {}
+
+define_subcommand! {
+    fn decorate_app<'a>(&self, app: App<'a, 'a>) -> App<'a, 'a> {
+        app.subcommand(ClapSubCommand::with_name("gc").about("run garbage collection"))
+    }
+
+    fn arg_match<'a>(&self, matches: &ArgMatches<'a>) -> ArgMatchResult {
+        match matches.subcommand() {
+            ("gc", _) => ArgMatchResult::Ok(Box::new(Invocation {})),
+            _ => ArgMatchResult::None,
+        }
+    }
+}
+
+subcommand_invocation! {
+    fn run(&self, command: &CommandInvocation) -> Fallible<()> {
+        command.get_replica().gc()?;
+        println!("garbage collected.");
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn parse_command() {
+        with_subcommand_invocation!(vec!["task", "gc"], |_inv| {});
+    }
+}