Quit immediately when current directory is invalid

Thanks-to: @kennytm

Author: spk

src/librustc/session/mod.rs

@@ -776,9 +776,10 @@ pub fn build_session_(sopts: config::Options,
     let print_fuel_crate = sopts.debugging_opts.print_fuel.clone();
     let print_fuel = Cell::new(0);
 
-    let working_dir = env::current_dir()
-        .expect("Could not find current working directory")
-        .to_string_lossy().into_owned();
+    let working_dir = match env::current_dir() {
+        Ok(dir) => dir.to_string_lossy().into_owned(),
+        Err(e) => panic!(p_s.span_diagnostic.fatal(&format!("Current directory is invalid: {}", e))),
+    };
     let working_dir = file_path_mapping.map_prefix(working_dir);
 
     let sess = Session {