Use RefinementCascade to compose pressability and hoverability

Co-Authored-By: Conrad Irwin <conrad@zed.dev>

crates/refineable/src/refineable.rs

@@ -1,7 +1,7 @@
 pub use derive_refineable::Refineable;
 
-pub trait Refineable {
-    type Refinement: Default;
+pub trait Refineable: Clone {
+    type Refinement: Refineable<Refinement = Self::Refinement> + Default;
 
     fn refine(&mut self, refinement: &Self::Refinement);
     fn refined(mut self, refinement: &Self::Refinement) -> Self
@@ -11,4 +11,46 @@ pub trait Refineable {
         self.refine(refinement);
         self
     }
+    fn from_refinement(refinement: &Self::Refinement) -> Self
+    where
+        Self: Default + Sized,
+    {
+        Self::default().refined(refinement)
+    }
+}
+
+pub struct RefinementCascade<S: Refineable>(Vec<Option<S::Refinement>>);
+
+impl<S: Refineable + Default> Default for RefinementCascade<S> {
+    fn default() -> Self {
+        Self(vec![Some(Default::default())])
+    }
+}
+
+#[derive(Copy, Clone)]
+pub struct CascadeSlot(usize);
+
+impl<S: Refineable + Default> RefinementCascade<S> {
+    pub fn reserve(&mut self) -> CascadeSlot {
+        self.0.push(None);
+        return CascadeSlot(self.0.len() - 1);
+    }
+
+    pub fn base(&mut self) -> &mut S::Refinement {
+        self.0[0].as_mut().unwrap()
+    }
+
+    pub fn set(&mut self, slot: CascadeSlot, refinement: Option<S::Refinement>) {
+        self.0[slot.0] = refinement
+    }
+
+    pub fn merged(&self) -> S::Refinement {
+        let mut merged = self.0[0].clone().unwrap();
+        for refinement in self.0.iter().skip(1) {
+            if let Some(refinement) = refinement {
+                merged.refine(refinement);
+            }
+        }
+        merged
+    }
 }