fix(completion): auto-detect shell from $SHELL when argument omitted

Composer auto-detects the shell from the $SHELL environment variable
when the shell argument is not provided. Mozart previously required
the argument and errored without it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 27 insertions, 3 deletions

diff --git a/crates/mozart/src/commands/completion.rs b/crates/mozart/src/commands/completion.rs
index d095ce6..7cae278 100644
--- a/crates/mozart/src/commands/completion.rs
+++ b/crates/mozart/src/commands/completion.rs
@@ -4,9 +4,9 @@ use clap_complete::aot::Shell;
 
 #[derive(Args)]
 pub struct CompletionArgs {
-    /// The shell to generate completions for
+    /// The shell to generate completions for (auto-detected from $SHELL if omitted)
     #[arg(value_enum)]
-    pub shell: Shell,
+    pub shell: Option<Shell>,
 }
 
 pub async fn execute(
@@ -14,7 +14,31 @@ pub async fn execute(
     _cli: &super::Cli,
     _console: &mozart_core::console::Console,
 ) -> anyhow::Result<()> {
+    let shell = match args.shell {
+        Some(s) => s,
+        None => detect_shell()?,
+    };
     let mut cmd = super::Cli::command();
-    clap_complete::aot::generate(args.shell, &mut cmd, "mozart", &mut std::io::stdout());
+    clap_complete::aot::generate(shell, &mut cmd, "mozart", &mut std::io::stdout());
     Ok(())
 }
+
+fn detect_shell() -> anyhow::Result<Shell> {
+    let shell_env = std::env::var("SHELL")
+        .map_err(|_| anyhow::anyhow!("Could not auto-detect shell. Please specify one of: bash, elvish, fish, powershell, zsh"))?;
+    let basename = std::path::Path::new(&shell_env)
+        .file_name()
+        .and_then(|s| s.to_str())
+        .unwrap_or("");
+    match basename {
+        "bash" => Ok(Shell::Bash),
+        "zsh" => Ok(Shell::Zsh),
+        "fish" => Ok(Shell::Fish),
+        "elvish" => Ok(Shell::Elvish),
+        "pwsh" | "powershell" => Ok(Shell::PowerShell),
+        _ => anyhow::bail!(
+            "Unrecognized shell '{}'. Please specify one of: bash, elvish, fish, powershell, zsh",
+            basename
+        ),
+    }
+}