repo_description: An interpreter for Rust's mid-level intermediate representation
code: //! This is a "monotonic `FxHashMap`": A `FxHashMap` that, when shared, can be pushed to but not
  //! otherwise mutated. We also box items in the map. This means we can safely provide
  //! shared references into existing items in the `FxHashMap`, because they will not be dropped
  //! (from being removed) or moved (because they are boxed).
  //! The API is completely tailored to what `memory.rs` needs. It is still in
  //! a separate file to minimize the amount of code that has to care about the unsafety.
  
  use std::borrow::Borrow;
  use std::cell::RefCell;
  use std::collections::hash_map::Entry;
  use std::hash::Hash;
  
  use rustc_data_structures::fx::FxHashMap;
  
  use crate::AllocMap;
  
  #[derive(Debug, Clone)]
  pub struct MonoHashMap<K: Hash + Eq, V>(RefCell<FxHashMap<K, Box<V>>>);
  
  impl<K: Hash + Eq, V> MonoHashMap<K, V> {
      /// This function exists for priroda to be able to iterate over all evaluator memory.
      ///
      /// The function is somewhat roundabout with the closure argument because internally the
      /// `MonoHashMap` uses a `RefCell`. When iterating over the `FxHashMap` inside the `RefCell`,
      /// we need to keep a borrow to the `FxHashMap` inside the iterator. The borrow is only alive
      /// as long as the `Ref` returned by `RefCell::borrow()` is alive. So we can't return the
      /// iterator, as that would drop the `Ref`. We can't return both, as it's not possible in Rust
      /// to have a struct/tuple with a field that refers to another field.
      pub fn iter<T>(&self, f: impl FnOnce(&mut dyn Iterator<Item = (&K, &V)>) -> T) -> T {
          f(&mut self.0.borrow().iter().map(|(k, v)| (k, &**v)))
      }
  }
  
  impl<K: Hash + Eq, V> Default for MonoHashMap<K, V> {
      fn default() -> Self {
          MonoHashMap(RefCell::new(Default::default()))
      }
  }
  
  impl<K: Hash + Eq, V> AllocMap<K, V> for MonoHashMap<K, V> {
      #[inline(always)]
      fn contains_key<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> bool
      where
          K: Borrow<Q>,
      {
          self.0.get_mut().contains_key(k)
      }
  
      #[inline(always)]
      fn contains_key_ref<Q: ?Sized + Hash + Eq>(&self, k: &Q) -> bool
      where
          K: Borrow<Q>,
      {
          self.0.borrow().contains_key(k)
      }
  
      #[inline(always)]
      fn insert(&mut self, k: K, v: V) -> Option<V> {
          self.0.get_mut().insert(k, Box::new(v)).map(|x| *x)
      }
  
      #[inline(always)]
      fn remove<Q: ?Sized + Hash + Eq>(&mut self, k: &Q) -> Option<V>
      where
          K: Borrow<Q>,
      {
          self.0.get_mut().remove(k).map(|x| *x)
      }
  
      #[inline(always)]
      fn filter_map_collect<T>(&self, mut f: impl FnMut(&K, &V) -> Option<T>) -> Vec<T> {
          self.0.borrow().iter().filter_map(move |(k, v)| f(k, v)).collect()
      }
  
      /// The most interesting method: Providing a shared reference without
      /// holding the `RefCell` open, and inserting new data if the key
      /// is not used yet.
      /// `vacant` is called if the key is not found in the map;
      /// if it returns a reference, that is used directly, if it
      /// returns owned data, that is put into the map and returned.
      #[inline(always)]
      fn get_or<E>(&self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&V, E> {
          // We cannot hold borrow_mut while calling `vacant`, since that might have to do lookups in this very map.
          if let Some(v) = self.0.borrow().get(&k) {
              let val: *const V = &**v;
              // This is safe because `val` points into a `Box`, that we know will not move and
              // will also not be dropped as long as the shared reference `self` is live.
              return unsafe { Ok(&*val) };
          }
          let new_val = Box::new(vacant()?);
          let val: *const V = &**self.0.borrow_mut().try_insert(k, new_val).ok().unwrap();
          // This is safe because `val` points into a `Box`, that we know will not move and
          // will also not be dropped as long as the shared reference `self` is live.
          unsafe { Ok(&*val) }
      }
  
      /// Read-only lookup (avoid read-acquiring the RefCell).
      fn get(&self, k: K) -> Option<&V> {
          let val: *const V = match self.0.borrow().get(&k) {
              Some(v) => &**v,
              None => return None,
          };
          // This is safe because `val` points into a `Box`, that we know will not move and
          // will also not be dropped as long as the shared reference `self` is live.
          unsafe { Some(&*val) }
      }
  
      #[inline(always)]
      fn get_mut_or<E>(&mut self, k: K, vacant: impl FnOnce() -> Result<V, E>) -> Result<&mut V, E> {
          match self.0.get_mut().entry(k) {
              Entry::Occupied(e) => Ok(e.into_mut()),
              Entry::Vacant(e) => {
                  let v = vacant()?;
                  Ok(e.insert(Box::new(v)))
              }
          }
      }
  }
  