rename Continue variant to Reschedule, adds more helpful error message

Author: sunny-g

rustler/src/schedule.rs

@@ -21,7 +21,7 @@ pub fn consume_timeslice(env: Env, percent: i32) -> bool {
 /// See `schedule_nif!` for a convenience wrapper around this type.
 pub enum NifScheduleResult<T> {
     Final(T),
-    Continue {
+    Reschedule {
         fun_name: &'static str,
         fun: unsafe extern "C" fn(NIF_ENV, i32, *const NIF_TERM) -> NIF_TERM,
         args: Vec<T>,
@@ -48,7 +48,7 @@ where
             Err(inner) => inner.as_returned(env),
             Ok(inner) => match inner {
                 NifScheduleResult::Final(inner) => inner.as_returned(env),
-                NifScheduleResult::Continue {
+                NifScheduleResult::Reschedule {
                     fun_name,
                     fun,
                     flags,
@@ -92,10 +92,13 @@ macro_rules! schedule_nif {
         schedule_nif!($fun_name, $args, $crate::schedule::SchedulerFlags::Normal)
     };
     ($fun_name:expr, $args:expr, $flags:expr) => {{
-        let func = EXTERN_NIF_MAP.get($fun_name).unwrap();
-        Ok($crate::schedule::NifScheduleResult::Continue {
+        let func = EXTERN_NIF_MAP.get($fun_name);
+        if func.is_none() {
+            panic!("NIF \"{}\" does not exist", $fun_name);
+        }
+        Ok($crate::schedule::NifScheduleResult::Reschedule {
             fun_name: $fun_name,
-            fun: *func,
+            fun: *(func.unwrap()),
             args: $args,
             flags: $flags,
         })