Throw an error instead of panicking when can't infer shell (#306)

src/commands/completions.rs

@@ -1,7 +1,8 @@
 use super::command::Command;
 use crate::cli::Cli;
 use crate::config::FnmConfig;
-use crate::shell::infer_shell;
+use crate::shell::{infer_shell, AVAILABLE_SHELLS};
+use snafu::{OptionExt, Snafu};
 use structopt::clap::Shell;
 use structopt::StructOpt;
 
@@ -17,11 +18,31 @@ impl Command for Completions {
 
     fn apply(self, _config: &FnmConfig) -> Result<(), Self::Error> {
         let mut stdio = std::io::stdout();
-        let shell = self.shell.unwrap_or_else(|| infer_shell().into());
+        let shell = self
+            .shell
+            .or_else(|| infer_shell().map(Into::into))
+            .context(CantInferShell)?;
         Cli::clap().gen_completions_to(env!("CARGO_PKG_NAME"), shell, &mut stdio);
         Ok(())
     }
 }
 
-#[derive(snafu::Snafu, Debug)]
-pub enum Error {}
+#[derive(Snafu, Debug)]
+pub enum Error {
+    #[snafu(display(
+        "{}\n{}\n{}\n{}",
+        "Can't infer shell!",
+        "fnm can't infer your shell based on the process tree.",
+        "Maybe it is unsupported? we support the following shells:",
+        shells_as_string()
+    ))]
+    CantInferShell,
+}
+
+fn shells_as_string() -> String {
+    AVAILABLE_SHELLS
+        .iter()
+        .map(|x| format!("* {}", x))
+        .collect::<Vec<_>>()
+        .join("\n")
+}

src/commands/env.rs

@@ -4,6 +4,7 @@ use crate::fs::symlink_dir;
 use crate::outln;
 use crate::shell::{infer_shell, Shell, AVAILABLE_SHELLS};
 use colored::Colorize;
+use snafu::{OptionExt, Snafu};
 use std::fmt::Debug;
 use structopt::StructOpt;
 
@@ -50,7 +51,7 @@ impl Command for Env {
             outln!(config#Error, "{} {} is deprecated. This is now the default.", "warning:".yellow().bold(), "--multi".italic());
         }
 
-        let shell: Box<dyn Shell> = self.shell.unwrap_or_else(&infer_shell);
+        let shell: Box<dyn Shell> = self.shell.or_else(&infer_shell).context(CantInferShell)?;
         let multishell_path = make_symlink(&config);
         let binary_path = if cfg!(windows) {
             multishell_path.clone()
@@ -81,8 +82,25 @@ impl Command for Env {
     }
 }
 
-#[derive(Debug, snafu::Snafu)]
-pub enum Error {}
+#[derive(Debug, Snafu)]
+pub enum Error {
+    #[snafu(display(
+        "{}\n{}\n{}\n{}",
+        "Can't infer shell!",
+        "fnm can't infer your shell based on the process tree.",
+        "Maybe it is unsupported? we support the following shells:",
+        shells_as_string()
+    ))]
+    CantInferShell,
+}
+
+fn shells_as_string() -> String {
+    AVAILABLE_SHELLS
+        .iter()
+        .map(|x| format!("* {}", x))
+        .collect::<Vec<_>>()
+        .join("\n")
+}
 
 #[cfg(test)]
 mod tests {

src/shell/mod.rs

@@ -15,15 +15,12 @@ pub use shell::{Shell, AVAILABLE_SHELLS};
 pub use windows_cmd::WindowsCmd;
 pub use zsh::Zsh;
 
-/// Always returns WindowsCmd (because this is what we support on Windows)
 #[cfg(windows)]
-pub fn infer_shell() -> Box<dyn Shell> {
-    let inferred = self::infer::windows::infer_shell();
-    inferred.expect("Can't infer shell")
+pub fn infer_shell() -> Option<Box<dyn Shell>> {
+    self::infer::windows::infer_shell()
 }
 
-/// Tries to infer shell or dies trying
 #[cfg(unix)]
-pub fn infer_shell() -> Box<dyn Shell> {
-    infer::unix::infer_shell().expect("Can't infer shell")
+pub fn infer_shell() -> Option<Box<dyn Shell>> {
+    infer::unix::infer_shell()
 }