Difficult ownership questions

src/utils/cache.rs

@@ -0,0 +1,25 @@
+use std::{collections::HashMap, hash::Hash};
+
+/// Cache the return values of an effectless closure in a hashmap
+/// Inspired by the closure_cacher crate.
+pub struct Cache<I, O, F> where F: FnMut(I) -> O {
+    store: HashMap<I, O>,
+    closure: F
+}
+
+impl<I, O, F> Cache<I, O, F>
+where
+    F: FnMut(I) -> O,
+    I: Eq + Hash + Copy
+{
+    pub fn new(closure: F) -> Self { Self { store: HashMap::new(), closure } }
+    pub fn get(&mut self, i: I) -> &O {
+        // I copied it because I might need `drop` and I prefer `I` to be unconstrained. 
+        let closure = &mut self.closure;
+        self.store.entry(i).or_insert_with(|| closure(i))
+    }
+    /// Forget the output for the given input
+    pub fn drop(&mut self, i: &I) -> bool {
+        self.store.remove(i).is_some()
+    } 
+}

src/utils/mod.rs

@@ -0,0 +1,2 @@
+mod cache;
+pub use cache::Cache;