cli/completion: Static shell completion for `rad`

Introduce the use of `clap_completion` for generating static completion.
The shells that are supported are the ones that are included in `clap_complete`:
- `bash`,
- `elvish`,
- `zsh`,
- `fish`, and
- `powershell`.

Co-authored-by: Michael Uti <utimichael9@gmail.com>
Co-authored-by: Lorenz Leutgeb <lorenz.leutgeb@radicle.xyz>

Cargo.lock

@@ -477,6 +477,15 @@ dependencies = [
  "strsim",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.5.60"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8e602857739c5a4291dfa33b5a298aeac9006185229a700e5810a3ef7272d971"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.5.41"
@@ -2819,6 +2828,7 @@ dependencies = [
  "anyhow",
  "chrono",
  "clap",
+ "clap_complete",
  "dunce",
  "human-panic",
  "itertools",

crates/radicle-cli/Cargo.toml

@@ -17,6 +17,7 @@ path = "src/main.rs"
 anyhow = "1"
 chrono = { workspace = true, features = ["clock", "std"] }
 clap = { version = "4.5.44", features = ["derive"] }
+clap_complete = "4.5"
 dunce = { workspace = true }
 human-panic.workspace = true
 itertools.workspace = true

crates/radicle-cli/src/main.rs

@@ -7,7 +7,7 @@ use std::{io::ErrorKind, process};
 use anyhow::anyhow;
 use clap::builder::styling::AnsiColor;
 use clap::builder::Styles;
-use clap::{Parser, Subcommand};
+use clap::{CommandFactory as _, Parser, Subcommand};
 
 use radicle::version::Version;
 use radicle_cli::commands::*;
@@ -98,6 +98,13 @@ enum Command {
         json: bool,
     },
 
+    /// Print static completion information for a given shell
+    #[command(hide = true)]
+    Completion {
+        /// The type of shell to output a static completion script for.
+        shell: clap_complete::Shell,
+    },
+
     #[command(external_subcommand)]
     External(Vec<OsString>),
 }
@@ -176,10 +183,23 @@ fn run_command(command: Command, ctx: impl term::Context) -> Result<(), anyhow::
         Command::Unseed(args) => unseed::run(args, ctx),
         Command::Watch(args) => watch::run(args, ctx),
         Command::Version { json } => write_version(json),
+        Command::Completion { shell } => {
+            print_completion(shell, &mut CliArgs::command());
+            Ok(())
+        }
         Command::External(args) => ExternalCommand::new(args).run(),
     }
 }
 
+fn print_completion<G: clap_complete::Generator>(generator: G, cmd: &mut clap::Command) {
+    clap_complete::generate(
+        generator,
+        cmd,
+        cmd.get_name().to_string(),
+        &mut io::stdout(),
+    );
+}
+
 struct ExternalCommand {
     command: OsString,
     args: Vec<OsString>,