feat(cli): check for rustc before doing anything

Addresses #190.

src/main.rs

@@ -7,6 +7,7 @@ use notify::{RecommendedWatcher, RecursiveMode, Watcher};
 use std::ffi::OsStr;
 use std::fs;
 use std::path::Path;
+use std::process::{Command, Stdio};
 use std::sync::mpsc::channel;
 use std::time::Duration;
 
@@ -56,6 +57,13 @@ fn main() {
         std::process::exit(1);
     }
 
+    if !rustc_exists() {
+        println!("We cannot find `rustc`.");
+        println!("Try running `rustc --version` to diagnose your problem.");
+        println!("For instructions on how to install Rust, check the README.");
+        std::process::exit(1);
+    }
+
     let toml_str = &fs::read_to_string("info.toml").unwrap();
     let exercises = toml::from_str::<ExerciseList>(toml_str).unwrap().exercises;
 
@@ -134,3 +142,13 @@ fn watch(exercises: &[Exercise]) -> notify::Result<()> {
         }
     }
 }
+
+fn rustc_exists() -> bool {
+    Command::new("rustc")
+        .args(&["--version"])
+        .stdout(Stdio::null())
+        .spawn()
+        .and_then(|mut child| child.wait())
+        .map(|status| status.success())
+        .unwrap_or(false)
+}