make Queue::new() a const fn

mino/src/queue.rs

@@ -11,17 +11,21 @@ pub struct Queue<'a, T: Copy> {
 
 impl<'a, T: Copy> Queue<'a, T> {
     /// Constructs a new queue.
-    pub fn new(mut hold: Option<T>, mut next: &'a [T]) -> Self {
-        if hold.is_none() && !next.is_empty() {
-            hold = Some(next[0]);
-            next = &next[1..];
+    pub const fn new(mut hold: Option<T>, mut next: &'a [T]) -> Self {
+        // SIGH. so `hold.is_none()` is not valid in `const fn`, for some obscure reason.
+        #[allow(clippy::redundant_pattern_matching)]
+        if let None = hold {
+            if let Some((hd, tl)) = next.split_first() {
+                hold = Some(*hd);
+                next = tl;
+            }
         }
         Self { hold, next }
     }
 
     /// Returns `true` if the queue is completely empty (i.e., `reachable()` would yield
     /// no values).
-    pub fn is_empty(&self) -> bool {
+    pub const fn is_empty(&self) -> bool {
         // HACK: its impossible for `next` to be non-empty and `hold` to be None, because
         // if `next` was non-empty then the `new` constructor would automatically move
         // some value into the hold slot.
@@ -113,15 +117,15 @@ mod test {
     #[test]
     fn test_queue_remove() {
         assert_eq!(
-            Queue::new(None, &["A", "B", "C"]).remove(&"A"),
+            Queue::new(None, &["A", "B", "C"]).remove("A"),
             Queue::new(Some("B"), &["C"])
         );
         assert_eq!(
-            Queue::new(Some("D"), &["A", "B", "C"]).remove(&"A"),
+            Queue::new(Some("D"), &["A", "B", "C"]).remove("A"),
             Queue::new(Some("D"), &["B", "C"])
         );
         assert_eq!(
-            Queue::new(None, &["A", "B", "C"]).remove(&"B"),
+            Queue::new(None, &["A", "B", "C"]).remove("B"),
             Queue::new(None, &["A", "C"])
         );
     }
@@ -129,12 +133,12 @@ mod test {
     #[test]
     #[should_panic]
     fn test_queue_bad_remove_1() {
-        Queue::new(None, &["A", "B", "C"]).remove(&"C");
+        Queue::new(None, &["A", "B", "C"]).remove("C");
     }
 
     #[test]
     #[should_panic]
     fn test_queue_bad_remove_2() {
-        Queue::new(Some("D"), &["A", "B", "C"]).remove(&"B");
+        Queue::new(Some("D"), &["A", "B", "C"]).remove("B");
     }
 }