feat(console): add subcommand to gen shell completions (#336)

This PR includes adding subcommand for shell completions. Generating a man page will be done as another PR.

```
❯ tokio-console
--ascii-only            --lang                  --no-duration-colors    --retain-for            -h                      gen-config
--colorterm             --log                   --no-terminated-colors  --version               <TARGET_ADDR>           help
--help                  --no-colors             --palette               -V                      gen-completion

❯ tokio-console gen-completion
--help      --version   -V          -h          bash        elvish      fish        powershell  zsh
```

Related to #325

tokio-console/Cargo.toml

@@ -47,3 +47,4 @@ humantime = "2.1.0"
 serde = { version = "1", features = ["derive"] }
 toml = "0.5"
 dirs = "4"
+clap_complete = "3"

tokio-console/args.example

@@ -95,6 +95,8 @@ OPTIONS:
             Print version information
 
 SUBCOMMANDS:
+    gen-completion
+            Generate shell completions
     gen-config
             Generate a `console.toml` config file with the default configuration values, overridden
             by any provided command-line arguments

tokio-console/src/config.rs

@@ -1,5 +1,6 @@
 use crate::view::Palette;
-use clap::{ArgGroup, Parser as Clap, Subcommand, ValueHint};
+use clap::{ArgGroup, IntoApp, Parser as Clap, Subcommand, ValueHint};
+use clap_complete::Shell;
 use color_eyre::eyre::WrapErr;
 use serde::{Deserialize, Serialize};
 use std::fmt;
@@ -103,6 +104,18 @@ pub enum OptionalCmd {
     ///     $ tokio-console gen-config > console.toml
     ///
     GenConfig,
+
+    /// Generate shell completions
+    ///
+    /// The completion script will be written to stdout.
+    /// The completion script should be saved in the shell's completion directory.
+    /// This depends on which shell is in use.
+    GenCompletion {
+        #[clap(name = "install", long = "install")]
+        install: bool,
+        #[clap(arg_enum)]
+        shell: Shell,
+    },
 }
 
 #[derive(Debug, Clone, Copy, Deserialize)]
@@ -632,6 +645,21 @@ impl ConfigPath {
     }
 }
 
+/// Generete completion scripts for each specified shell.
+pub fn gen_completion(install: bool, shell: Shell) -> color_eyre::Result<()> {
+    let mut app = Config::command();
+    let mut buf: Box<dyn std::io::Write> = if install {
+        color_eyre::eyre::bail!(
+            "Automatically installing completion scripts is not currently supported on {}",
+            shell
+        )
+    } else {
+        Box::new(std::io::stdout())
+    };
+    clap_complete::generate(shell, &mut app, "tokio-console", &mut buf);
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use std::{

tokio-console/src/main.rs

@@ -27,11 +27,17 @@ mod warnings;
 async fn main() -> color_eyre::Result<()> {
     let mut args = config::Config::parse()?;
 
-    if args.subcmd == Some(config::OptionalCmd::GenConfig) {
-        // Generate a default config file and exit.
-        let toml = args.gen_config_file()?;
-        println!("{}", toml);
-        return Ok(());
+    match args.subcmd {
+        Some(config::OptionalCmd::GenConfig) => {
+            // Generate a default config file and exit.
+            let toml = args.gen_config_file()?;
+            println!("{}", toml);
+            return Ok(());
+        }
+        Some(config::OptionalCmd::GenCompletion { install, shell }) => {
+            return config::gen_completion(install, shell);
+        }
+        None => {}
     }
 
     let retain_for = args.retain_for();