fix: install completions per shell, and re-run when asked

build.rs

@@ -1,22 +1,55 @@
 use clap::{CommandFactory, ValueEnum};
 use clap_complete::{Shell, generate_to};
-use std::{env, io::Error};
+use std::{env, ffi::OsString, fs, io::Error, path::PathBuf};
 
 include!("src/cli/mod.rs");
 
-fn main() -> Result<(), Error> {
-    let outdir = env::var_os("SHELL_COMPLETIONS_DIR").or_else(|| env::var_os("OUT_DIR"));
+fn shell_name(shell: Shell) -> String {
+    shell
+        .to_possible_value()
+        .map(|v| v.get_name().to_owned())
+        .unwrap_or_default()
+}
 
-    let Some(outdir) = outdir else {
-        return Ok(());
-    };
+fn non_empty(name: &str) -> Option<OsString> {
+    env::var_os(name).filter(|v| !v.is_empty())
+}
+
+fn install_dir(shell: Shell) -> Option<(PathBuf, bool)> {
+    let per_shell = format!("SHELL_COMPLETIONS_DIR_{}", shell_name(shell).to_uppercase());
+
+    if let Some(dir) = non_empty(&per_shell).or_else(|| non_empty("SHELL_COMPLETIONS_DIR")) {
+        return Some((PathBuf::from(dir), true));
+    }
+    non_empty("OUT_DIR").map(|dir| (PathBuf::from(dir), false))
+}
+
+fn main() -> Result<(), Error> {
+    println!("cargo::rerun-if-env-changed=SHELL_COMPLETIONS_DIR");
+    for shell in Shell::value_variants() {
+        println!(
+            "cargo::rerun-if-env-changed=SHELL_COMPLETIONS_DIR_{}",
+            shell_name(*shell).to_uppercase()
+        );
+    }
 
     let mut cmd = ahab::Ahab::command();
 
     for shell in Shell::value_variants() {
-        let path = generate_to(*shell, &mut cmd, env!("CARGO_PKG_NAME"), &outdir)?;
+        let Some((dir, requested)) = install_dir(*shell) else {
+            continue;
+        };
 
-        println!("cargo:warning=completion file is generated: {path:?}");
+        fs::create_dir_all(&dir)?;
+        let path = generate_to(*shell, &mut cmd, env!("CARGO_PKG_NAME"), &dir)?;
+
+        if requested {
+            println!(
+                "cargo::warning=installed {} completion: {}",
+                shell_name(*shell),
+                path.display()
+            );
+        }
     }
 
     Ok(())