Add shell completions

- Add a `build.rs` (plus dependencies) to generate the shell completions
  clap knows.

build.rs

@@ -0,0 +1,50 @@
+//! build
+
+use anyhow::Result;
+use anyhow;
+use clap::CommandFactory;
+use clap::ValueEnum;
+use clap_complete::Shell;
+
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+
+// include the cli definition
+pub mod cli {
+    include!("src/cli/cli_args.rs");
+}
+
+const ASSET_OUT_ENV_NAME: &str = "ASSET_OUT_DIR";
+
+/// Returns a path to a directory suitable as a target for generated
+/// files.
+fn out_dir(section: &str) -> Result<PathBuf> {
+    println!("cargo:rerun-if-env-changed={}", ASSET_OUT_ENV_NAME);
+
+    let outdir: PathBuf = env::var_os(ASSET_OUT_ENV_NAME)
+        .unwrap_or_else(|| env::var_os("OUT_DIR")
+            .expect("OUT_DIR not set")
+        ).into();
+
+    if outdir.exists() && ! outdir.is_dir() {
+        return Err(anyhow::anyhow!("{:?} is not a directory", outdir));
+    }
+    let path = outdir.join(section);
+    fs::create_dir_all(&path)?;
+    Ok(path)
+}
+
+fn main() -> Result<()> {
+    let mut husk = cli::CliArgs::command();
+
+    // shell completions
+    let shell_completion_path = out_dir("shell_completions")?;
+    for shell in Shell::value_variants() {
+        clap_complete::generate_to(*shell, &mut husk, "husk", &shell_completion_path)?;
+    }
+
+    println!("cargo:warning=shell completions written to {}", shell_completion_path.display());
+
+    Ok(())
+}