diff --git a/src/lib.rs b/src/lib.rs
index d6d3ede9..7d88ffef 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -19,7 +19,7 @@
 //! - The [`Equivalent`] trait, which offers more flexible equality definitions
 //!   between borrowed and owned versions of keys.
 //! - The [`MutableKeys`][map::MutableKeys] trait, which gives opt-in mutable
-//!   access to hash map keys.
+//!   access to map keys, and [`MutableValues`][set::MutableValues] for sets.
 //!
 //! ### Feature Flags
 //!
@@ -116,7 +116,6 @@ mod arbitrary;
 mod macros;
 #[cfg(feature = "borsh")]
 mod borsh;
-mod mutable_keys;
 #[cfg(feature = "serde")]
 mod serde;
 mod util;
diff --git a/src/map.rs b/src/map.rs
index 46965a7c..f6d548bf 100644
--- a/src/map.rs
+++ b/src/map.rs
@@ -3,6 +3,7 @@
 
 mod core;
 mod iter;
+mod mutable;
 mod slice;
 
 #[cfg(feature = "serde")]
@@ -17,8 +18,8 @@ pub use self::core::{Entry, IndexedEntry, OccupiedEntry, VacantEntry};
 pub use self::iter::{
     Drain, IntoIter, IntoKeys, IntoValues, Iter, IterMut, Keys, Splice, Values, ValuesMut,
 };
+pub use self::mutable::MutableKeys;
 pub use self::slice::Slice;
-pub use crate::mutable_keys::MutableKeys;
 
 #[cfg(feature = "rayon")]
 pub use crate::rayon::map as rayon;
@@ -808,13 +809,6 @@ impl<K, V, S> IndexMap<K, V, S> {
         self.core.retain_in_order(move |k, v| keep(k, v));
     }
 
-    pub(crate) fn retain_mut<F>(&mut self, keep: F)
-    where
-        F: FnMut(&mut K, &mut V) -> bool,
-    {
-        self.core.retain_in_order(keep);
-    }
-
     /// Sort the map’s key-value pairs by the default ordering of the keys.
     ///
     /// This is a stable sort -- but equivalent keys should not normally coexist in
diff --git a/src/mutable_keys.rs b/src/map/mutable.rs
similarity index 94%
rename from src/mutable_keys.rs
rename to src/map/mutable.rs
index 917af8c6..f45dd5e2 100644
--- a/src/mutable_keys.rs
+++ b/src/map/mutable.rs
@@ -6,7 +6,7 @@ use super::{Bucket, Entries, Equivalent, IndexMap};
 ///
 /// These methods expose `&mut K`, mutable references to the key as it is stored
 /// in the map.
-/// You are allowed to modify the keys in the hashmap **if the modification
+/// You are allowed to modify the keys in the map **if the modification
 /// does not change the key’s hash and equality**.
 ///
 /// If keys are modified erroneously, you can no longer look them up.
@@ -49,7 +49,7 @@ pub trait MutableKeys: private::Sealed {
         F: FnMut(&mut Self::Key, &mut Self::Value) -> bool;
 }
 
-/// Opt-in mutable access to keys.
+/// Opt-in mutable access to [`IndexMap`] keys.
 ///
 /// See [`MutableKeys`] for more information.
 impl<K, V, S> MutableKeys for IndexMap<K, V, S>
@@ -79,7 +79,7 @@ where
     where
         F: FnMut(&mut K, &mut V) -> bool,
     {
-        self.retain_mut(keep)
+        self.core.retain_in_order(keep);
     }
 }
 
diff --git a/src/set.rs b/src/set.rs
index 5774349d..671b2fa9 100644
--- a/src/set.rs
+++ b/src/set.rs
@@ -1,6 +1,7 @@
 //! A hash set implemented using [`IndexMap`]
 
 mod iter;
+mod mutable;
 mod slice;
 
 #[cfg(test)]
@@ -9,6 +10,7 @@ mod tests;
 pub use self::iter::{
     Difference, Drain, Intersection, IntoIter, Iter, Splice, SymmetricDifference, Union,
 };
+pub use self::mutable::MutableValues;
 pub use self::slice::Slice;
 
 #[cfg(feature = "rayon")]
diff --git a/src/set/mutable.rs b/src/set/mutable.rs
new file mode 100644
index 00000000..3acecf3f
--- /dev/null
+++ b/src/set/mutable.rs
@@ -0,0 +1,86 @@
+use core::hash::{BuildHasher, Hash};
+
+use super::{Equivalent, IndexSet};
+use crate::map::MutableKeys;
+
+/// Opt-in mutable access to [`IndexSet`] values.
+///
+/// These methods expose `&mut T`, mutable references to the value as it is stored
+/// in the set.
+/// You are allowed to modify the values in the set **if the modification
+/// does not change the value’s hash and equality**.
+///
+/// If values are modified erroneously, you can no longer look them up.
+/// This is sound (memory safe) but a logical error hazard (just like
+/// implementing `PartialEq`, `Eq`, or `Hash` incorrectly would be).
+///
+/// `use` this trait to enable its methods for `IndexSet`.
+///
+/// This trait is sealed and cannot be implemented for types outside this crate.
+pub trait MutableValues: private::Sealed {
+    type Value;
+
+    /// Return item index and mutable reference to the value
+    ///
+    /// Computes in **O(1)** time (average).
+    fn get_full_mut2<Q: ?Sized>(&mut self, value: &Q) -> Option<(usize, &mut Self::Value)>
+    where
+        Q: Hash + Equivalent<Self::Value>;
+
+    /// Return mutable reference to the value at an index.
+    ///
+    /// Valid indices are *0 <= index < self.len()*
+    ///
+    /// Computes in **O(1)** time.
+    fn get_index_mut2(&mut self, index: usize) -> Option<&mut Self::Value>;
+
+    /// Scan through each value in the set and keep those where the
+    /// closure `keep` returns `true`.
+    ///
+    /// The values are visited in order, and remaining values keep their order.
+    ///
+    /// Computes in **O(n)** time (average).
+    fn retain2<F>(&mut self, keep: F)
+    where
+        F: FnMut(&mut Self::Value) -> bool;
+}
+
+/// Opt-in mutable access to [`IndexSet`] values.
+///
+/// See [`MutableValues`] for more information.
+impl<T, S> MutableValues for IndexSet<T, S>
+where
+    S: BuildHasher,
+{
+    type Value = T;
+
+    fn get_full_mut2<Q: ?Sized>(&mut self, value: &Q) -> Option<(usize, &mut T)>
+    where
+        Q: Hash + Equivalent<T>,
+    {
+        match self.map.get_full_mut2(value) {
+            Some((index, value, ())) => Some((index, value)),
+            None => None,
+        }
+    }
+
+    fn get_index_mut2(&mut self, index: usize) -> Option<&mut T> {
+        match self.map.get_index_mut2(index) {
+            Some((value, ())) => Some(value),
+            None => None,
+        }
+    }
+
+    fn retain2<F>(&mut self, mut keep: F)
+    where
+        F: FnMut(&mut T) -> bool,
+    {
+        self.map.retain2(move |value, ()| keep(value));
+    }
+}
+
+mod private {
+    pub trait Sealed {}
+
+    impl<T, S> Sealed for super::IndexSet<T, S> {}
+}