Avoid panic for unknown shell (#1241)

Downgrade this from a panic to a command error which will cause the CLI
to exit non-zero.

qlty-cli/src/commands/completions.rs

@@ -17,20 +17,26 @@ pub struct Completions {
 impl Completions {
     pub fn execute(&self, _args: &Arguments) -> Result<CommandSuccess, CommandError> {
         let bin_name = self.bin_name();
-        let mut shell = crate::shell::build(self.shell);
 
-        let mut command = Arguments::command();
-        let completions = shell.generate(&bin_name, &mut command)?;
+        if let Some(mut shell) = crate::shell::build(self.shell) {
+            let mut command = Arguments::command();
+            let completions = shell.generate(&bin_name, &mut command)?;
 
-        if self.install {
-            let path = shell.install(&completions)?;
-            shell.update_rc(&path)?;
-            eprintln!("Completions installed in {}", path.display());
+            if self.install {
+                let path = shell.install(&completions)?;
+                shell.update_rc(&path)?;
+                eprintln!("Completions installed in {}", path.display());
+            } else {
+                println!("{}", completions.completions);
+            }
+
+            CommandSuccess::ok()
         } else {
-            println!("{}", completions.completions);
+            eprintln!("Unsupported shell: {:?}", self.shell);
+            Err(CommandError::InvalidOptions {
+                message: "Unsupported shell".to_owned(),
+            })
         }
-
-        CommandSuccess::ok()
     }
 
     fn bin_name(&self) -> String {

qlty-cli/src/errors.rs

@@ -4,7 +4,6 @@ use crate::CommandSuccess;
 
 #[derive(Error, Debug)]
 pub enum CommandError {
-    // TODO: Start using CommandError::Config
     #[error("Config error")]
     Config,
 

qlty-cli/src/shell.rs

@@ -13,6 +13,7 @@ mod zsh;
 
 pub use bash::Bash;
 pub use fish::Fish;
+use tracing::warn;
 pub use zsh::Zsh;
 
 #[derive(Clone, Debug)]
@@ -56,13 +57,16 @@ pub trait TabCompletingShell {
     fn clap_shell(&self) -> Shell;
 }
 
-pub fn build(specified: Option<Shell>) -> Box<dyn TabCompletingShell> {
+pub fn build(specified: Option<Shell>) -> Option<Box<dyn TabCompletingShell>> {
     let shell = specified.unwrap_or_else(|| Shell::from_env().unwrap_or(Shell::Bash));
 
     match shell {
-        Shell::Bash => Box::new(Bash),
-        Shell::Fish => Box::new(Fish),
-        Shell::Zsh => Box::new(Zsh),
-        _ => panic!("Unsupported shell: {}", shell),
+        Shell::Bash => Some(Box::new(Bash)),
+        Shell::Fish => Some(Box::new(Fish)),
+        Shell::Zsh => Some(Box::new(Zsh)),
+        _ => {
+            warn!("Unsupported shell: {:?}", shell);
+            None
+        }
     }
 }