diff --git a/src/tools/miri/src/borrow_tracker/tree_borrows/unimap.rs b/src/tools/miri/src/borrow_tracker/tree_borrows/unimap.rs
new file mode 100644
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+++ b/src/tools/miri/src/borrow_tracker/tree_borrows/unimap.rs
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+//! This module implements the `UniMap`, which is a way to get efficient mappings
+//! optimized for the setting of `tree_borrows/tree.rs`.
+//!
+//! A `UniKeyMap<K>` is a (slow) mapping from `K` to `UniIndex`,
+//! and `UniValMap<V>` is a (fast) mapping from `UniIndex` to `V`.
+//! Thus a pair `(UniKeyMap<K>, UniValMap<V>)` acts as a virtual `HashMap<K, V>`.
+//!
+//! Because of the asymmetry in access time, the use-case for `UniMap` is the following:
+//! a tuple `(UniKeyMap<K>, Vec<UniValMap<V>>)` is much more efficient than
+//! the equivalent `Vec<HashMap<K, V>>` it represents if all maps have similar
+//! sets of keys.
+
+#![allow(dead_code)]
+
+use std::hash::Hash;
+
+use rustc_data_structures::fx::FxHashMap;
+
+/// Intermediate key between a UniKeyMap and a UniValMap.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct UniIndex {
+    idx: u32,
+}
+
+/// From K to UniIndex
+#[derive(Debug, Clone, Default)]
+pub struct UniKeyMap<K> {
+    /// Underlying map that does all the hard work.
+    /// Key invariant: the contents of `deassigned` are disjoint from the
+    /// keys of `mapping`, and together they form the set of contiguous integers
+    /// `0 .. (mapping.len() + deassigned.len())`.
+    mapping: FxHashMap<K, u32>,
+    /// Indexes that can be reused: memory gain when the map gets sparse
+    /// due to many deletions.
+    deassigned: Vec<u32>,
+}
+
+/// From UniIndex to V
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct UniValMap<V> {
+    /// The mapping data. Thanks to Vec we get both fast accesses, and
+    /// a memory-optimal representation if there are few deletions.
+    data: Vec<Option<V>>,
+}
+
+impl<V> Default for UniValMap<V> {
+    fn default() -> Self {
+        Self { data: Vec::default() }
+    }
+}
+
+impl<K> UniKeyMap<K>
+where
+    K: Hash + Eq,
+{
+    /// How many keys/index pairs are currently active.
+    pub fn len(&self) -> usize {
+        self.mapping.len()
+    }
+
+    /// Whether this key has an associated index or not.
+    pub fn contains_key(&self, key: &K) -> bool {
+        self.mapping.contains_key(key)
+    }
+
+    /// Assign this key to a new index. Panics if the key is already assigned,
+    /// use `get_or_insert` for a version that instead returns the existing
+    /// assignment.
+    #[track_caller]
+    pub fn insert(&mut self, key: K) -> UniIndex {
+        // We want an unused index. First we attempt to find one from `deassigned`,
+        // and if `deassigned` is empty we generate a fresh index.
+        let idx = self.deassigned.pop().unwrap_or_else(|| {
+            // `deassigned` is empty, so all keys in use are already in `mapping`.
+            // The next available key is `mapping.len()`.
+            self.mapping.len().try_into().expect("UniMap ran out of useable keys")
+        });
+        if self.mapping.insert(key, idx).is_some() {
+            panic!(
+                "This key is already assigned to a different index; either use `get_or_insert` instead if you care about this data, or first call `remove` to undo the preexisting assignment."
+            );
+        };
+        UniIndex { idx }
+    }
+
+    /// If it exists, the index this key maps to.
+    pub fn get(&self, key: &K) -> Option<UniIndex> {
+        self.mapping.get(key).map(|&idx| UniIndex { idx })
+    }
+
+    /// Either get a previously existing entry, or create a new one if it
+    /// is not yet present.
+    pub fn get_or_insert(&mut self, key: K) -> UniIndex {
+        self.get(&key).unwrap_or_else(|| self.insert(key))
+    }
+
+    /// Return whatever index this key was using to the deassigned pool.
+    ///
+    /// Note: calling this function can be dangerous. If the index still exists
+    /// somewhere in a `UniValMap` and is reassigned by the `UniKeyMap` then
+    /// it will inherit the old value of a completely unrelated key.
+    /// If you `UniKeyMap::remove` a key you should make sure to also `UniValMap::remove`
+    /// the associated `UniIndex` from ALL `UniValMap`s.
+    ///
+    /// Example of such behavior:
+    /// ```
+    /// let mut keymap = UniKeyMap::<char>::default();
+    /// let mut valmap = UniValMap::<char>::default();
+    /// // Insert 'a' -> _ -> 'A'
+    /// let idx_a = keymap.insert('a');
+    /// valmap.insert(idx_a, 'A');
+    /// // Remove 'a' -> _, but forget to remove _ -> 'A'
+    /// keymap.remove(&'a');
+    /// // valmap.remove(idx_a); // If we uncomment this line the issue is fixed
+    /// // Insert 'b' -> _
+    /// let idx_b = keymap.insert('b');
+    /// let val_b = valmap.get(idx_b);
+    /// assert_eq!(val_b, Some('A')); // Oh no
+    /// // assert_eq!(val_b, None); // This is what we would have expected
+    /// ```
+    pub fn remove(&mut self, key: &K) {
+        if let Some(idx) = self.mapping.remove(key) {
+            self.deassigned.push(idx);
+        }
+    }
+}
+
+impl<V> UniValMap<V> {
+    /// Whether this index has an associated value.
+    pub fn contains_idx(&self, idx: UniIndex) -> bool {
+        self.data.get(idx.idx as usize).and_then(Option::as_ref).is_some()
+    }
+
+    /// Reserve enough space to insert the value at the right index.
+    fn extend_to_length(&mut self, len: usize) {
+        if len > self.data.len() {
+            let nb = len - self.data.len();
+            self.data.reserve(nb);
+            for _ in 0..nb {
+                self.data.push(None);
+            }
+        }
+    }
+
+    /// Assign a value to the index. Permanently overwrites any previous value.
+    pub fn insert(&mut self, idx: UniIndex, val: V) {
+        self.extend_to_length(idx.idx as usize + 1);
+        self.data[idx.idx as usize] = Some(val)
+    }
+
+    /// Get the value at this index, if it exists.
+    pub fn get(&self, idx: UniIndex) -> Option<&V> {
+        self.data.get(idx.idx as usize).and_then(Option::as_ref)
+    }
+
+    /// Get the value at this index mutably, if it exists.
+    pub fn get_mut(&mut self, idx: UniIndex) -> Option<&mut V> {
+        self.data.get_mut(idx.idx as usize).and_then(Option::as_mut)
+    }
+
+    /// Delete any value associated with this index. Ok even if the index
+    /// has no associated value.
+    pub fn remove(&mut self, idx: UniIndex) {
+        if idx.idx as usize >= self.data.len() {
+            return;
+        }
+        self.data[idx.idx as usize] = None;
+    }
+}
+
+/// An access to a single value of the map.
+pub struct UniEntry<'a, V> {
+    inner: &'a mut Option<V>,
+}
+
+impl<'a, V> UniValMap<V> {
+    /// Get a wrapper around a mutable access to the value corresponding to `idx`.
+    pub fn entry(&'a mut self, idx: UniIndex) -> UniEntry<'a, V> {
+        self.extend_to_length(idx.idx as usize + 1);
+        UniEntry { inner: &mut self.data[idx.idx as usize] }
+    }
+}
+
+impl<'a, V> UniEntry<'a, V> {
+    /// Insert in the map and get the value.
+    pub fn or_insert_with<F>(&mut self, default: F) -> &mut V
+    where
+        F: FnOnce() -> V,
+    {
+        if self.inner.is_none() {
+            *self.inner = Some(default());
+        }
+        self.inner.as_mut().unwrap()
+    }
+}
+
+mod tests {
+    use super::*;
+
+    #[test]
+    fn extend_to_length() {
+        let mut km = UniValMap::<char>::default();
+        km.extend_to_length(10);
+        assert!(km.data.len() == 10);
+        km.extend_to_length(0);
+        assert!(km.data.len() == 10);
+        km.extend_to_length(10);
+        assert!(km.data.len() == 10);
+        km.extend_to_length(11);
+        assert!(km.data.len() == 11);
+    }
+
+    #[derive(Default)]
+    struct MapWitness<K, V> {
+        key: UniKeyMap<K>,
+        val: UniValMap<V>,
+        map: FxHashMap<K, V>,
+    }
+
+    impl<K, V> MapWitness<K, V>
+    where
+        K: Copy + Hash + Eq,
+        V: Copy + Eq + std::fmt::Debug,
+    {
+        fn insert(&mut self, k: K, v: V) {
+            // UniMap
+            let i = self.key.get_or_insert(k);
+            self.val.insert(i, v);
+            // HashMap
+            self.map.insert(k, v);
+            // Consistency: nothing to check
+        }
+
+        fn get(&self, k: &K) {
+            // UniMap
+            let v1 = self.key.get(k).and_then(|i| self.val.get(i));
+            // HashMap
+            let v2 = self.map.get(k);
+            // Consistency
+            assert_eq!(v1, v2);
+        }
+
+        fn get_mut(&mut self, k: &K) {
+            // UniMap
+            let v1 = self.key.get(k).and_then(|i| self.val.get_mut(i));
+            // HashMap
+            let v2 = self.map.get_mut(k);
+            // Consistency
+            assert_eq!(v1, v2);
+        }
+        fn remove(&mut self, k: &K) {
+            // UniMap
+            if let Some(i) = self.key.get(k) {
+                self.val.remove(i);
+            }
+            self.key.remove(k);
+            // HashMap
+            self.map.remove(k);
+            // Consistency: nothing to check
+        }
+    }
+
+    #[test]
+    fn consistency_small() {
+        let mut m = MapWitness::<u64, char>::default();
+        m.insert(1, 'a');
+        m.insert(2, 'b');
+        m.get(&1);
+        m.get_mut(&2);
+        m.remove(&2);
+        m.insert(1, 'c');
+        m.get(&1);
+        m.insert(3, 'd');
+        m.insert(4, 'e');
+        m.insert(4, 'f');
+        m.get(&2);
+        m.get(&3);
+        m.get(&4);
+        m.get(&5);
+        m.remove(&100);
+        m.get_mut(&100);
+        m.get(&100);
+    }
+
+    #[test]
+    fn consistency_large() {
+        use std::collections::hash_map::DefaultHasher;
+        use std::hash::{Hash, Hasher};
+        let mut hasher = DefaultHasher::new();
+        let mut map = MapWitness::<u64, u64>::default();
+        for i in 0..1000 {
+            i.hash(&mut hasher);
+            let rng = hasher.finish();
+            let op = rng % 3 == 0;
+            let key = (rng / 2) % 50;
+            let val = (rng / 100) % 1000;
+            if op {
+                map.insert(key, val);
+            } else {
+                map.get(&key);
+            }
+        }
+    }
+}