Add 81974

Most likely low-priority; feel free to ignore

Issue: rust-lang/rust#81974

ices/81974.rs

@@ -0,0 +1,51 @@
+#![feature(unboxed_closures)]
+#![feature(fn_traits)]
+
+use std::collections::HashMap;
+use std::hash::Hash;
+
+struct CachedFun<A, B>{
+  cache: HashMap<A, B>,
+  fun: fn(&mut CachedFun<A, B>, A) -> B
+}
+
+impl<A: Eq + Hash, B> CachedFun<A, B> {
+    fn new(fun: fn(&mut Self, A) -> B) -> Self {
+        CachedFun {
+            cache: HashMap::new(),
+            fun
+        }
+    }
+}
+
+impl<A, B> FnOnce<A> for CachedFun<A, B> where
+    A: Eq + Hash + Clone,
+    B: Clone,
+{
+    type Output = B;
+    extern "rust-call" fn call_once(mut self, a: A) -> Self::Output {
+        self.call_mut(a)
+    }
+}
+
+
+impl<A, B> FnMut<A> for CachedFun<A, B> where
+    A: Eq + Hash + Clone,
+    B: Clone,
+{
+    extern "rust-call" fn call_mut(&mut self, a: A) -> Self::Output {
+        self.cache.get(&a)
+            .map(|a| a.clone())
+            .unwrap_or_else(|| {
+                let b = (self.fun)(self, a.clone());
+                self.cache.insert(a, b.clone());
+                b
+            })
+    }
+}
+
+fn main() -> () {
+    let pesce = |y: &mut CachedFun<i32, i32>, x| x + 1;
+    let cachedcoso = CachedFun::new(pesce);
+    cachedcoso.call_once(1);
+}