don't depend on [!Default; 0]: Default impl

NB: in `&[]` the array is "const promoted", making this reference have
    type `&'static [T; 0]`. this allows us to return that reference from
    any function returning a reference to a slice or array, because
    `&'static _` can coerce to `&'short_lifetime _`.

    it's not necessary to have a field for this.

Author: WaffleLapkin

solver/src/reasoners/cp/mod.rs

@@ -34,7 +34,6 @@ use std::collections::{HashMap, HashSet};
 #[derive(Clone, Default)]
 pub struct Watches {
     propagations: RefMap<SignedVar, Vec<PropagatorId>>,
-    empty: [PropagatorId; 0],
 }
 
 impl Watches {
@@ -71,7 +70,7 @@ impl Watches {
     /// Returns all propagators
     fn get_ub_watches(&self, var: impl Into<SignedVar>) -> &[PropagatorId] {
         let var = var.into();
-        self.propagations.get(var).map(|v| v.as_slice()).unwrap_or(&self.empty)
+        self.propagations.get(var).map(|v| v.as_slice()).unwrap_or(&[])
     }
 }