simplify validate_height

src/rstd/collections/tree.rs

@@ -70,14 +70,12 @@ pub struct Tree<'a, Ctx: Context<'a>, A: MentionableBase<'a, Ctx>> {
 
 impl<'a, Ctx: Context<'a>, A: MentionableBase<'a, Ctx>> Tree<'a, Ctx, A> {
     fn validate_height(&self) -> Result<(), HeightError> {
-        if let Nullable::Null(_) = self.node {
-            if self.height != 0 {
-                Err(HeightError::LeafHeight(self.height))?
-            }
-        } else if self.height == 0 {
-            Err(HeightError::NodeHeight)?
+        match (&self.node, self.height) {
+            (Nullable::NotNull(_), 0) => Err(HeightError::NodeHeight),
+            (_, 0) => Ok(()),
+            (Nullable::Null(_), height) => Err(HeightError::LeafHeight(height)),
+            _ => Ok(()),
         }
-        Ok(())
     }
 }