Auto merge of #77171 - VFLashM:better_sso_structures, r=oli-obk

Better sso structures

This change greatly expands interface of MiniSet/MiniMap and renames them because they are no longer "Mini".

13 files changed, 902 insertions, 127 deletions

diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 90b0f254751..9958e5dd5e0 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -100,8 +100,7 @@ pub mod vec_linked_list;
 pub mod work_queue;
 pub use atomic_ref::AtomicRef;
 pub mod frozen;
-pub mod mini_map;
-pub mod mini_set;
+pub mod sso;
 pub mod tagged_ptr;
 pub mod temp_dir;
 pub mod unhash;
diff --git a/compiler/rustc_data_structures/src/mini_map.rs b/compiler/rustc_data_structures/src/mini_map.rs
deleted file mode 100644
index cd3e949d383..00000000000
--- a/compiler/rustc_data_structures/src/mini_map.rs
+++ /dev/null
@@ -1,61 +0,0 @@
-use crate::fx::FxHashMap;
-use arrayvec::ArrayVec;
-
-use std::hash::Hash;
-
-/// Small-storage-optimized implementation of a map
-/// made specifically for caching results.
-///
-/// Stores elements in a small array up to a certain length
-/// and switches to `HashMap` when that length is exceeded.
-pub enum MiniMap<K, V> {
-    Array(ArrayVec<[(K, V); 8]>),
-    Map(FxHashMap<K, V>),
-}
-
-impl<K: Eq + Hash, V> MiniMap<K, V> {
-    /// Creates an empty `MiniMap`.
-    pub fn new() -> Self {
-        MiniMap::Array(ArrayVec::new())
-    }
-
-    /// Inserts or updates value in the map.
-    pub fn insert(&mut self, key: K, value: V) {
-        match self {
-            MiniMap::Array(array) => {
-                for pair in array.iter_mut() {
-                    if pair.0 == key {
-                        pair.1 = value;
-                        return;
-                    }
-                }
-                if let Err(error) = array.try_push((key, value)) {
-                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
-                    let (key, value) = error.element();
-                    map.insert(key, value);
-                    *self = MiniMap::Map(map);
-                }
-            }
-            MiniMap::Map(map) => {
-                map.insert(key, value);
-            }
-        }
-    }
-
-    /// Return value by key if any.
-    pub fn get(&self, key: &K) -> Option<&V> {
-        match self {
-            MiniMap::Array(array) => {
-                for pair in array {
-                    if pair.0 == *key {
-                        return Some(&pair.1);
-                    }
-                }
-                return None;
-            }
-            MiniMap::Map(map) => {
-                return map.get(key);
-            }
-        }
-    }
-}
diff --git a/compiler/rustc_data_structures/src/mini_set.rs b/compiler/rustc_data_structures/src/mini_set.rs
deleted file mode 100644
index 9d45af723de..00000000000
--- a/compiler/rustc_data_structures/src/mini_set.rs
+++ /dev/null
@@ -1,41 +0,0 @@
-use crate::fx::FxHashSet;
-use arrayvec::ArrayVec;
-use std::hash::Hash;
-/// Small-storage-optimized implementation of a set.
-///
-/// Stores elements in a small array up to a certain length
-/// and switches to `HashSet` when that length is exceeded.
-pub enum MiniSet<T> {
-    Array(ArrayVec<[T; 8]>),
-    Set(FxHashSet<T>),
-}
-
-impl<T: Eq + Hash> MiniSet<T> {
-    /// Creates an empty `MiniSet`.
-    pub fn new() -> Self {
-        MiniSet::Array(ArrayVec::new())
-    }
-
-    /// Adds a value to the set.
-    ///
-    /// If the set did not have this value present, true is returned.
-    ///
-    /// If the set did have this value present, false is returned.
-    pub fn insert(&mut self, elem: T) -> bool {
-        match self {
-            MiniSet::Array(array) => {
-                if array.iter().any(|e| *e == elem) {
-                    false
-                } else {
-                    if let Err(error) = array.try_push(elem) {
-                        let mut set: FxHashSet<T> = array.drain(..).collect();
-                        set.insert(error.element());
-                        *self = MiniSet::Set(set);
-                    }
-                    true
-                }
-            }
-            MiniSet::Set(set) => set.insert(elem),
-        }
-    }
-}
diff --git a/compiler/rustc_data_structures/src/sso/either_iter.rs b/compiler/rustc_data_structures/src/sso/either_iter.rs
new file mode 100644
index 00000000000..af8ffcf4c13
--- /dev/null
+++ b/compiler/rustc_data_structures/src/sso/either_iter.rs
@@ -0,0 +1,75 @@
+use std::fmt;
+use std::iter::ExactSizeIterator;
+use std::iter::FusedIterator;
+use std::iter::Iterator;
+
+/// Iterator which may contain instance of
+/// one of two specific implementations.
+///
+/// Note: For most methods providing custom
+///       implementation may margianlly
+///       improve performance by avoiding
+///       doing Left/Right match on every step
+///       and doing it only once instead.
+#[derive(Clone)]
+pub enum EitherIter<L, R> {
+    Left(L),
+    Right(R),
+}
+
+impl<L, R> Iterator for EitherIter<L, R>
+where
+    L: Iterator,
+    R: Iterator<Item = L::Item>,
+{
+    type Item = L::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self {
+            EitherIter::Left(l) => l.next(),
+            EitherIter::Right(r) => r.next(),
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self {
+            EitherIter::Left(l) => l.size_hint(),
+            EitherIter::Right(r) => r.size_hint(),
+        }
+    }
+}
+
+impl<L, R> ExactSizeIterator for EitherIter<L, R>
+where
+    L: ExactSizeIterator,
+    R: ExactSizeIterator,
+    EitherIter<L, R>: Iterator,
+{
+    fn len(&self) -> usize {
+        match self {
+            EitherIter::Left(l) => l.len(),
+            EitherIter::Right(r) => r.len(),
+        }
+    }
+}
+
+impl<L, R> FusedIterator for EitherIter<L, R>
+where
+    L: FusedIterator,
+    R: FusedIterator,
+    EitherIter<L, R>: Iterator,
+{
+}
+
+impl<L, R> fmt::Debug for EitherIter<L, R>
+where
+    L: fmt::Debug,
+    R: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            EitherIter::Left(l) => l.fmt(f),
+            EitherIter::Right(r) => r.fmt(f),
+        }
+    }
+}
diff --git a/compiler/rustc_data_structures/src/sso/map.rs b/compiler/rustc_data_structures/src/sso/map.rs
new file mode 100644
index 00000000000..fa510e58314
--- /dev/null
+++ b/compiler/rustc_data_structures/src/sso/map.rs
@@ -0,0 +1,560 @@
+use super::either_iter::EitherIter;
+use crate::fx::FxHashMap;
+use arrayvec::ArrayVec;
+use std::fmt;
+use std::hash::Hash;
+use std::iter::FromIterator;
+use std::ops::Index;
+
+// For pointer-sized arguments arrays
+// are faster than set/map for up to 64
+// arguments.
+//
+// On the other hand such a big array
+// hurts cache performance, makes passing
+// sso structures around very expensive.
+//
+// Biggest performance benefit is gained
+// for reasonably small arrays that stay
+// small in vast majority of cases.
+//
+// '8' is choosen as a sane default, to be
+// reevaluated later.
+//
+// Note: As of now ArrayVec design prevents
+//       us from making it user-customizable.
+const SSO_ARRAY_SIZE: usize = 8;
+
+/// Small-storage-optimized implementation of a map.
+///
+/// Stores elements in a small array up to a certain length
+/// and switches to `HashMap` when that length is exceeded.
+//
+// FIXME: Implements subset of HashMap API.
+//
+// Missing HashMap API:
+//   all hasher-related
+//   try_reserve (unstable)
+//   shrink_to (unstable)
+//   drain_filter (unstable)
+//   into_keys/into_values (unstable)
+//   all raw_entry-related
+//   PartialEq/Eq (requires sorting the array)
+//   Entry::or_insert_with_key (unstable)
+//   Vacant/Occupied entries and related
+//
+// FIXME: In HashMap most methods accepting key reference
+// accept reference to generic `Q` where `K: Borrow<Q>`.
+//
+// However, using this approach in `HashMap::get` apparently
+// breaks inlining and noticeably reduces performance.
+//
+// Performance *should* be the same given that borrow is
+// a NOP in most cases, but in practice that's not the case.
+//
+// Further investigation is required.
+//
+// Affected methods:
+//   SsoHashMap::get
+//   SsoHashMap::get_mut
+//   SsoHashMap::get_entry
+//   SsoHashMap::get_key_value
+//   SsoHashMap::contains_key
+//   SsoHashMap::remove
+//   SsoHashMap::remove_entry
+//   Index::index
+//   SsoHashSet::take
+//   SsoHashSet::get
+//   SsoHashSet::remove
+//   SsoHashSet::contains
+
+#[derive(Clone)]
+pub enum SsoHashMap<K, V> {
+    Array(ArrayVec<[(K, V); SSO_ARRAY_SIZE]>),
+    Map(FxHashMap<K, V>),
+}
+
+impl<K, V> SsoHashMap<K, V> {
+    /// Creates an empty `SsoHashMap`.
+    #[inline]
+    pub fn new() -> Self {
+        SsoHashMap::Array(ArrayVec::new())
+    }
+
+    /// Creates an empty `SsoHashMap` with the specified capacity.
+    pub fn with_capacity(cap: usize) -> Self {
+        if cap <= SSO_ARRAY_SIZE {
+            Self::new()
+        } else {
+            SsoHashMap::Map(FxHashMap::with_capacity_and_hasher(cap, Default::default()))
+        }
+    }
+
+    /// Clears the map, removing all key-value pairs. Keeps the allocated memory
+    /// for reuse.
+    pub fn clear(&mut self) {
+        match self {
+            SsoHashMap::Array(array) => array.clear(),
+            SsoHashMap::Map(map) => map.clear(),
+        }
+    }
+
+    /// Returns the number of elements the map can hold without reallocating.
+    pub fn capacity(&self) -> usize {
+        match self {
+            SsoHashMap::Array(_) => SSO_ARRAY_SIZE,
+            SsoHashMap::Map(map) => map.capacity(),
+        }
+    }
+
+    /// Returns the number of elements in the map.
+    pub fn len(&self) -> usize {
+        match self {
+            SsoHashMap::Array(array) => array.len(),
+            SsoHashMap::Map(map) => map.len(),
+        }
+    }
+
+    /// Returns `true` if the map contains no elements.
+    pub fn is_empty(&self) -> bool {
+        match self {
+            SsoHashMap::Array(array) => array.is_empty(),
+            SsoHashMap::Map(map) => map.is_empty(),
+        }
+    }
+
+    /// An iterator visiting all key-value pairs in arbitrary order.
+    /// The iterator element type is `(&'a K, &'a V)`.
+    #[inline]
+    pub fn iter(&self) -> <&Self as IntoIterator>::IntoIter {
+        self.into_iter()
+    }
+
+    /// An iterator visiting all key-value pairs in arbitrary order,
+    /// with mutable references to the values.
+    /// The iterator element type is `(&'a K, &'a mut V)`.
+    #[inline]
+    pub fn iter_mut(&mut self) -> impl Iterator<Item = (&'_ K, &'_ mut V)> {
+        self.into_iter()
+    }
+
+    /// An iterator visiting all keys in arbitrary order.
+    /// The iterator element type is `&'a K`.
+    pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.iter().map(|(k, _v)| k)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.keys()),
+        }
+    }
+
+    /// An iterator visiting all values in arbitrary order.
+    /// The iterator element type is `&'a V`.
+    pub fn values(&self) -> impl Iterator<Item = &'_ V> {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.iter().map(|(_k, v)| v)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.values()),
+        }
+    }
+
+    /// An iterator visiting all values mutably in arbitrary order.
+    /// The iterator element type is `&'a mut V`.
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.iter_mut().map(|(_k, v)| v)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.values_mut()),
+        }
+    }
+
+    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
+    /// allocated memory for reuse.
+    pub fn drain(&mut self) -> impl Iterator<Item = (K, V)> + '_ {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.drain(..)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.drain()),
+        }
+    }
+}
+
+impl<K: Eq + Hash, V> SsoHashMap<K, V> {
+    /// Changes underlying storage from array to hashmap
+    /// if array is full.
+    fn migrate_if_full(&mut self) {
+        if let SsoHashMap::Array(array) = self {
+            if array.is_full() {
+                *self = SsoHashMap::Map(array.drain(..).collect());
+            }
+        }
+    }
+
+    /// Reserves capacity for at least `additional` more elements to be inserted
+    /// in the `SsoHashMap`. The collection may reserve more space to avoid
+    /// frequent reallocations.
+    pub fn reserve(&mut self, additional: usize) {
+        match self {
+            SsoHashMap::Array(array) => {
+                if SSO_ARRAY_SIZE < (array.len() + additional) {
+                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
+                    map.reserve(additional);
+                    *self = SsoHashMap::Map(map);
+                }
+            }
+            SsoHashMap::Map(map) => map.reserve(additional),
+        }
+    }
+
+    /// Shrinks the capacity of the map as much as possible. It will drop
+    /// down as much as possible while maintaining the internal rules
+    /// and possibly leaving some space in accordance with the resize policy.
+    pub fn shrink_to_fit(&mut self) {
+        if let SsoHashMap::Map(map) = self {
+            if map.len() <= SSO_ARRAY_SIZE {
+                *self = SsoHashMap::Array(map.drain().collect());
+            } else {
+                map.shrink_to_fit();
+            }
+        }
+    }
+
+    /// Retains only the elements specified by the predicate.
+    pub fn retain<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&K, &mut V) -> bool,
+    {
+        match self {
+            SsoHashMap::Array(array) => array.retain(|(k, v)| f(k, v)),
+            SsoHashMap::Map(map) => map.retain(f),
+        }
+    }
+
+    /// Inserts a key-value pair into the map.
+    ///
+    /// If the map did not have this key present, [`None`] is returned.
+    ///
+    /// If the map did have this key present, the value is updated, and the old
+    /// value is returned. The key is not updated, though; this matters for
+    /// types that can be `==` without being identical. See the [module-level
+    /// documentation] for more.
+    pub fn insert(&mut self, key: K, value: V) -> Option<V> {
+        match self {
+            SsoHashMap::Array(array) => {
+                for (k, v) in array.iter_mut() {
+                    if *k == key {
+                        let old_value = std::mem::replace(v, value);
+                        return Some(old_value);
+                    }
+                }
+                if let Err(error) = array.try_push((key, value)) {
+                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
+                    let (key, value) = error.element();
+                    map.insert(key, value);
+                    *self = SsoHashMap::Map(map);
+                }
+                None
+            }
+            SsoHashMap::Map(map) => map.insert(key, value),
+        }
+    }
+
+    /// Removes a key from the map, returning the value at the key if the key
+    /// was previously in the map.
+    pub fn remove(&mut self, key: &K) -> Option<V> {
+        match self {
+            SsoHashMap::Array(array) => {
+                if let Some(index) = array.iter().position(|(k, _v)| k == key) {
+                    Some(array.swap_remove(index).1)
+                } else {
+                    None
+                }
+            }
+            SsoHashMap::Map(map) => map.remove(key),
+        }
+    }
+
+    /// Removes a key from the map, returning the stored key and value if the
+    /// key was previously in the map.
+    pub fn remove_entry(&mut self, key: &K) -> Option<(K, V)> {
+        match self {
+            SsoHashMap::Array(array) => {
+                if let Some(index) = array.iter().position(|(k, _v)| k == key) {
+                    Some(array.swap_remove(index))
+                } else {
+                    None
+                }
+            }
+            SsoHashMap::Map(map) => map.remove_entry(key),
+        }
+    }
+
+    /// Returns a reference to the value corresponding to the key.
+    pub fn get(&self, key: &K) -> Option<&V> {
+        match self {
+            SsoHashMap::Array(array) => {
+                for (k, v) in array {
+                    if k == key {
+                        return Some(v);
+                    }
+                }
+                None
+            }
+            SsoHashMap::Map(map) => map.get(key),
+        }
+    }
+
+    /// Returns a mutable reference to the value corresponding to the key.
+    pub fn get_mut(&mut self, key: &K) -> Option<&mut V> {
+        match self {
+            SsoHashMap::Array(array) => {
+                for (k, v) in array {
+                    if k == key {
+                        return Some(v);
+                    }
+                }
+                None
+            }
+            SsoHashMap::Map(map) => map.get_mut(key),
+        }
+    }
+
+    /// Returns the key-value pair corresponding to the supplied key.
+    pub fn get_key_value(&self, key: &K) -> Option<(&K, &V)> {
+        match self {
+            SsoHashMap::Array(array) => {
+                for (k, v) in array {
+                    if k == key {
+                        return Some((k, v));
+                    }
+                }
+                None
+            }
+            SsoHashMap::Map(map) => map.get_key_value(key),
+        }
+    }
+
+    /// Returns `true` if the map contains a value for the specified key.
+    pub fn contains_key(&self, key: &K) -> bool {
+        match self {
+            SsoHashMap::Array(array) => array.iter().any(|(k, _v)| k == key),
+            SsoHashMap::Map(map) => map.contains_key(key),
+        }
+    }
+
+    /// Gets the given key's corresponding entry in the map for in-place manipulation.
+    #[inline]
+    pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
+        Entry { ssomap: self, key }
+    }
+}
+
+impl<K, V> Default for SsoHashMap<K, V> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K: Eq + Hash, V> FromIterator<(K, V)> for SsoHashMap<K, V> {
+    fn from_iter<I: IntoIterator<Item = (K, V)>>(iter: I) -> SsoHashMap<K, V> {
+        let mut map: SsoHashMap<K, V> = Default::default();
+        map.extend(iter);
+        map
+    }
+}
+
+impl<K: Eq + Hash, V> Extend<(K, V)> for SsoHashMap<K, V> {
+    fn extend<I>(&mut self, iter: I)
+    where
+        I: IntoIterator<Item = (K, V)>,
+    {
+        for (key, value) in iter.into_iter() {
+            self.insert(key, value);
+        }
+    }
+
+    #[inline]
+    fn extend_one(&mut self, (k, v): (K, V)) {
+        self.insert(k, v);
+    }
+
+    fn extend_reserve(&mut self, additional: usize) {
+        match self {
+            SsoHashMap::Array(array) => {
+                if SSO_ARRAY_SIZE < (array.len() + additional) {
+                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
+                    map.extend_reserve(additional);
+                    *self = SsoHashMap::Map(map);
+                }
+            }
+            SsoHashMap::Map(map) => map.extend_reserve(additional),
+        }
+    }
+}
+
+impl<'a, K, V> Extend<(&'a K, &'a V)> for SsoHashMap<K, V>
+where
+    K: Eq + Hash + Copy,
+    V: Copy,
+{
+    fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T) {
+        self.extend(iter.into_iter().map(|(k, v)| (k.clone(), v.clone())))
+    }
+
+    #[inline]
+    fn extend_one(&mut self, (&k, &v): (&'a K, &'a V)) {
+        self.insert(k, v);
+    }
+
+    #[inline]
+    fn extend_reserve(&mut self, additional: usize) {
+        Extend::<(K, V)>::extend_reserve(self, additional)
+    }
+}
+
+impl<K, V> IntoIterator for SsoHashMap<K, V> {
+    type IntoIter = EitherIter<
+        <ArrayVec<[(K, V); 8]> as IntoIterator>::IntoIter,
+        <FxHashMap<K, V> as IntoIterator>::IntoIter,
+    >;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    fn into_iter(self) -> Self::IntoIter {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter()),
+            SsoHashMap::Map(map) => EitherIter::Right(map.into_iter()),
+        }
+    }
+}
+
+/// adapts Item of array reference iterator to Item of hashmap reference iterator.
+#[inline(always)]
+fn adapt_array_ref_it<K, V>(pair: &'a (K, V)) -> (&'a K, &'a V) {
+    let (a, b) = pair;
+    (a, b)
+}
+
+/// adapts Item of array mut reference iterator to Item of hashmap mut reference iterator.
+#[inline(always)]
+fn adapt_array_mut_it<K, V>(pair: &'a mut (K, V)) -> (&'a K, &'a mut V) {
+    let (a, b) = pair;
+    (a, b)
+}
+
+impl<'a, K, V> IntoIterator for &'a SsoHashMap<K, V> {
+    type IntoIter = EitherIter<
+        std::iter::Map<
+            <&'a ArrayVec<[(K, V); 8]> as IntoIterator>::IntoIter,
+            fn(&'a (K, V)) -> (&'a K, &'a V),
+        >,
+        <&'a FxHashMap<K, V> as IntoIterator>::IntoIter,
+    >;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    fn into_iter(self) -> Self::IntoIter {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter().map(adapt_array_ref_it)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.into_iter()),
+        }
+    }
+}
+
+impl<'a, K, V> IntoIterator for &'a mut SsoHashMap<K, V> {
+    type IntoIter = EitherIter<
+        std::iter::Map<
+            <&'a mut ArrayVec<[(K, V); 8]> as IntoIterator>::IntoIter,
+            fn(&'a mut (K, V)) -> (&'a K, &'a mut V),
+        >,
+        <&'a mut FxHashMap<K, V> as IntoIterator>::IntoIter,
+    >;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    fn into_iter(self) -> Self::IntoIter {
+        match self {
+            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter().map(adapt_array_mut_it)),
+            SsoHashMap::Map(map) => EitherIter::Right(map.into_iter()),
+        }
+    }
+}
+
+impl<K, V> fmt::Debug for SsoHashMap<K, V>
+where
+    K: fmt::Debug,
+    V: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_map().entries(self.iter()).finish()
+    }
+}
+
+impl<'a, K, V> Index<&'a K> for SsoHashMap<K, V>
+where
+    K: Eq + Hash,
+{
+    type Output = V;
+
+    #[inline]
+    fn index(&self, key: &K) -> &V {
+        self.get(key).expect("no entry found for key")
+    }
+}
+
+/// A view into a single entry in a map.
+pub struct Entry<'a, K, V> {
+    ssomap: &'a mut SsoHashMap<K, V>,
+    key: K,
+}
+
+impl<'a, K: Eq + Hash, V> Entry<'a, K, V> {
+    /// Provides in-place mutable access to an occupied entry before any
+    /// potential inserts into the map.
+    pub fn and_modify<F>(self, f: F) -> Self
+    where
+        F: FnOnce(&mut V),
+    {
+        if let Some(value) = self.ssomap.get_mut(&self.key) {
+            f(value);
+        }
+        self
+    }
+
+    /// Ensures a value is in the entry by inserting the default if empty, and returns
+    /// a mutable reference to the value in the entry.
+    #[inline]
+    pub fn or_insert(self, value: V) -> &'a mut V {
+        self.or_insert_with(|| value)
+    }
+
+    /// Ensures a value is in the entry by inserting the result of the default function if empty,
+    /// and returns a mutable reference to the value in the entry.
+    pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
+        self.ssomap.migrate_if_full();
+        match self.ssomap {
+            SsoHashMap::Array(array) => {
+                let key_ref = &self.key;
+                let found_index = array.iter().position(|(k, _v)| k == key_ref);
+                let index = if let Some(index) = found_index {
+                    index
+                } else {
+                    let index = array.len();
+                    array.try_push((self.key, default())).unwrap();
+                    index
+                };
+                &mut array[index].1
+            }
+            SsoHashMap::Map(map) => map.entry(self.key).or_insert_with(default),
+        }
+    }
+
+    /// Returns a reference to this entry's key.
+    #[inline]
+    pub fn key(&self) -> &K {
+        &self.key
+    }
+}
+
+impl<'a, K: Eq + Hash, V: Default> Entry<'a, K, V> {
+    /// Ensures a value is in the entry by inserting the default value if empty,
+    /// and returns a mutable reference to the value in the entry.
+    #[inline]
+    pub fn or_default(self) -> &'a mut V {
+        self.or_insert_with(Default::default)
+    }
+}
diff --git a/compiler/rustc_data_structures/src/sso/mod.rs b/compiler/rustc_data_structures/src/sso/mod.rs
new file mode 100644
index 00000000000..dd21bc8e696
--- /dev/null
+++ b/compiler/rustc_data_structures/src/sso/mod.rs
@@ -0,0 +1,6 @@
+mod either_iter;
+mod map;
+mod set;
+
+pub use map::SsoHashMap;
+pub use set::SsoHashSet;
diff --git a/compiler/rustc_data_structures/src/sso/set.rs b/compiler/rustc_data_structures/src/sso/set.rs
new file mode 100644
index 00000000000..23cff0206c5
--- /dev/null
+++ b/compiler/rustc_data_structures/src/sso/set.rs
@@ -0,0 +1,237 @@
+use std::fmt;
+use std::hash::Hash;
+use std::iter::FromIterator;
+
+use super::map::SsoHashMap;
+
+/// Small-storage-optimized implementation of a set.
+///
+/// Stores elements in a small array up to a certain length
+/// and switches to `HashSet` when that length is exceeded.
+//
+// FIXME: Implements subset of HashSet API.
+//
+// Missing HashSet API:
+//   all hasher-related
+//   try_reserve (unstable)
+//   shrink_to (unstable)
+//   drain_filter (unstable)
+//   replace
+//   get_or_insert/get_or_insert_owned/get_or_insert_with (unstable)
+//   difference/symmetric_difference/intersection/union
+//   is_disjoint/is_subset/is_superset
+//   PartialEq/Eq (requires SsoHashMap implementation)
+//   BitOr/BitAnd/BitXor/Sub
+#[derive(Clone)]
+pub struct SsoHashSet<T> {
+    map: SsoHashMap<T, ()>,
+}
+
+/// Adapter function used ot return
+/// result if SsoHashMap functions into
+/// result SsoHashSet should return.
+#[inline(always)]
+fn entry_to_key<K, V>((k, _v): (K, V)) -> K {
+    k
+}
+
+impl<T> SsoHashSet<T> {
+    /// Creates an empty `SsoHashSet`.
+    #[inline]
+    pub fn new() -> Self {
+        Self { map: SsoHashMap::new() }
+    }
+
+    /// Creates an empty `SsoHashSet` with the specified capacity.
+    #[inline]
+    pub fn with_capacity(cap: usize) -> Self {
+        Self { map: SsoHashMap::with_capacity(cap) }
+    }
+
+    /// Clears the set, removing all values.
+    #[inline]
+    pub fn clear(&mut self) {
+        self.map.clear()
+    }
+
+    /// Returns the number of elements the set can hold without reallocating.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.map.capacity()
+    }
+
+    /// Returns the number of elements in the set.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.map.len()
+    }
+
+    /// Returns `true` if the set contains no elements.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.map.is_empty()
+    }
+
+    /// An iterator visiting all elements in arbitrary order.
+    /// The iterator element type is `&'a T`.
+    #[inline]
+    pub fn iter(&'a self) -> impl Iterator<Item = &'a T> {
+        self.into_iter()
+    }
+
+    /// Clears the set, returning all elements in an iterator.
+    #[inline]
+    pub fn drain(&mut self) -> impl Iterator<Item = T> + '_ {
+        self.map.drain().map(entry_to_key)
+    }
+}
+
+impl<T: Eq + Hash> SsoHashSet<T> {
+    /// Reserves capacity for at least `additional` more elements to be inserted
+    /// in the `SsoHashSet`. The collection may reserve more space to avoid
+    /// frequent reallocations.
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        self.map.reserve(additional)
+    }
+
+    /// Shrinks the capacity of the set as much as possible. It will drop
+    /// down as much as possible while maintaining the internal rules
+    /// and possibly leaving some space in accordance with the resize policy.
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.map.shrink_to_fit()
+    }
+
+    /// Retains only the elements specified by the predicate.
+    #[inline]
+    pub fn retain<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&T) -> bool,
+    {
+        self.map.retain(|k, _v| f(k))
+    }
+
+    /// Removes and returns the value in the set, if any, that is equal to the given one.
+    #[inline]
+    pub fn take(&mut self, value: &T) -> Option<T> {
+        self.map.remove_entry(value).map(entry_to_key)
+    }
+
+    /// Returns a reference to the value in the set, if any, that is equal to the given value.
+    #[inline]
+    pub fn get(&self, value: &T) -> Option<&T> {
+        self.map.get_key_value(value).map(entry_to_key)
+    }
+
+    /// Adds a value to the set.
+    ///
+    /// If the set did not have this value present, `true` is returned.
+    ///
+    /// If the set did have this value present, `false` is returned.
+    #[inline]
+    pub fn insert(&mut self, elem: T) -> bool {
+        self.map.insert(elem, ()).is_none()
+    }
+
+    /// Removes a value from the set. Returns whether the value was
+    /// present in the set.
+    #[inline]
+    pub fn remove(&mut self, value: &T) -> bool {
+        self.map.remove(value).is_some()
+    }
+
+    /// Returns `true` if the set contains a value.
+    #[inline]
+    pub fn contains(&self, value: &T) -> bool {
+        self.map.contains_key(value)
+    }
+}
+
+impl<T: Eq + Hash> FromIterator<T> for SsoHashSet<T> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> SsoHashSet<T> {
+        let mut set: SsoHashSet<T> = Default::default();
+        set.extend(iter);
+        set
+    }
+}
+
+impl<T> Default for SsoHashSet<T> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T: Eq + Hash> Extend<T> for SsoHashSet<T> {
+    fn extend<I>(&mut self, iter: I)
+    where
+        I: IntoIterator<Item = T>,
+    {
+        for val in iter.into_iter() {
+            self.insert(val);
+        }
+    }
+
+    #[inline]
+    fn extend_one(&mut self, item: T) {
+        self.insert(item);
+    }
+
+    #[inline]
+    fn extend_reserve(&mut self, additional: usize) {
+        self.map.extend_reserve(additional)
+    }
+}
+
+impl<'a, T> Extend<&'a T> for SsoHashSet<T>
+where
+    T: 'a + Eq + Hash + Copy,
+{
+    #[inline]
+    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
+        self.extend(iter.into_iter().cloned());
+    }
+
+    #[inline]
+    fn extend_one(&mut self, &item: &'a T) {
+        self.insert(item);
+    }
+
+    #[inline]
+    fn extend_reserve(&mut self, additional: usize) {
+        Extend::<T>::extend_reserve(self, additional)
+    }
+}
+
+impl<T> IntoIterator for SsoHashSet<T> {
+    type IntoIter = std::iter::Map<<SsoHashMap<T, ()> as IntoIterator>::IntoIter, fn((T, ())) -> T>;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        self.map.into_iter().map(entry_to_key)
+    }
+}
+
+impl<'a, T> IntoIterator for &'a SsoHashSet<T> {
+    type IntoIter = std::iter::Map<
+        <&'a SsoHashMap<T, ()> as IntoIterator>::IntoIter,
+        fn((&'a T, &'a ())) -> &'a T,
+    >;
+    type Item = <Self::IntoIter as Iterator>::Item;
+
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        self.map.iter().map(entry_to_key)
+    }
+}
+
+impl<T> fmt::Debug for SsoHashSet<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_set().entries(self.iter()).finish()
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index a540face4f2..6a1715ef818 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -35,7 +35,7 @@ use super::{InferCtxt, MiscVariable, TypeTrace};
 use crate::traits::{Obligation, PredicateObligations};
 
 use rustc_ast as ast;
-use rustc_data_structures::mini_map::MiniMap;
+use rustc_data_structures::sso::SsoHashMap;
 use rustc_hir::def_id::DefId;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::TypeError;
@@ -429,7 +429,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             needs_wf: false,
             root_ty: ty,
             param_env: self.param_env,
-            cache: MiniMap::new(),
+            cache: SsoHashMap::new(),
         };
 
         let ty = match generalize.relate(ty, ty) {
@@ -490,7 +490,7 @@ struct Generalizer<'cx, 'tcx> {
 
     param_env: ty::ParamEnv<'tcx>,
 
-    cache: MiniMap<Ty<'tcx>, RelateResult<'tcx, Ty<'tcx>>>,
+    cache: SsoHashMap<Ty<'tcx>, RelateResult<'tcx, Ty<'tcx>>>,
 }
 
 /// Result from a generalization operation. This includes
diff --git a/compiler/rustc_infer/src/infer/outlives/verify.rs b/compiler/rustc_infer/src/infer/outlives/verify.rs
index 21b0836563f..07924298c24 100644
--- a/compiler/rustc_infer/src/infer/outlives/verify.rs
+++ b/compiler/rustc_infer/src/infer/outlives/verify.rs
@@ -1,7 +1,7 @@
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::{GenericKind, VerifyBound};
 use rustc_data_structures::captures::Captures;
-use rustc_data_structures::mini_set::MiniSet;
+use rustc_data_structures::sso::SsoHashSet;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -32,7 +32,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     /// Returns a "verify bound" that encodes what we know about
     /// `generic` and the regions it outlives.
     pub fn generic_bound(&self, generic: GenericKind<'tcx>) -> VerifyBound<'tcx> {
-        let mut visited = MiniSet::new();
+        let mut visited = SsoHashSet::new();
         match generic {
             GenericKind::Param(param_ty) => self.param_bound(param_ty),
             GenericKind::Projection(projection_ty) => {
@@ -44,7 +44,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     fn type_bound(
         &self,
         ty: Ty<'tcx>,
-        visited: &mut MiniSet<GenericArg<'tcx>>,
+        visited: &mut SsoHashSet<GenericArg<'tcx>>,
     ) -> VerifyBound<'tcx> {
         match *ty.kind() {
             ty::Param(p) => self.param_bound(p),
@@ -148,7 +148,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     pub fn projection_bound(
         &self,
         projection_ty: ty::ProjectionTy<'tcx>,
-        visited: &mut MiniSet<GenericArg<'tcx>>,
+        visited: &mut SsoHashSet<GenericArg<'tcx>>,
     ) -> VerifyBound<'tcx> {
         debug!("projection_bound(projection_ty={:?})", projection_ty);
 
@@ -186,7 +186,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     fn recursive_bound(
         &self,
         parent: GenericArg<'tcx>,
-        visited: &mut MiniSet<GenericArg<'tcx>>,
+        visited: &mut SsoHashSet<GenericArg<'tcx>>,
     ) -> VerifyBound<'tcx> {
         let mut bounds = parent
             .walk_shallow(visited)
diff --git a/compiler/rustc_middle/src/ty/outlives.rs b/compiler/rustc_middle/src/ty/outlives.rs
index ca992d36e95..4c20141bbe6 100644
--- a/compiler/rustc_middle/src/ty/outlives.rs
+++ b/compiler/rustc_middle/src/ty/outlives.rs
@@ -4,7 +4,7 @@
 
 use crate::ty::subst::{GenericArg, GenericArgKind};
 use crate::ty::{self, Ty, TyCtxt, TypeFoldable};
-use rustc_data_structures::mini_set::MiniSet;
+use rustc_data_structures::sso::SsoHashSet;
 use smallvec::SmallVec;
 
 #[derive(Debug)]
@@ -51,7 +51,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Push onto `out` all the things that must outlive `'a` for the condition
     /// `ty0: 'a` to hold. Note that `ty0` must be a **fully resolved type**.
     pub fn push_outlives_components(self, ty0: Ty<'tcx>, out: &mut SmallVec<[Component<'tcx>; 4]>) {
-        let mut visited = MiniSet::new();
+        let mut visited = SsoHashSet::new();
         compute_components(self, ty0, out, &mut visited);
         debug!("components({:?}) = {:?}", ty0, out);
     }
@@ -61,7 +61,7 @@ fn compute_components(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>,
     out: &mut SmallVec<[Component<'tcx>; 4]>,
-    visited: &mut MiniSet<GenericArg<'tcx>>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
 ) {
     // Descend through the types, looking for the various "base"
     // components and collecting them into `out`. This is not written
@@ -142,7 +142,7 @@ fn compute_components(
                     // OutlivesProjectionComponents.  Continue walking
                     // through and constrain Pi.
                     let mut subcomponents = smallvec![];
-                    let mut subvisited = MiniSet::new();
+                    let mut subvisited = SsoHashSet::new();
                     compute_components_recursive(tcx, ty.into(), &mut subcomponents, &mut subvisited);
                     out.push(Component::EscapingProjection(subcomponents.into_iter().collect()));
                 }
@@ -194,7 +194,7 @@ fn compute_components_recursive(
     tcx: TyCtxt<'tcx>,
     parent: GenericArg<'tcx>,
     out: &mut SmallVec<[Component<'tcx>; 4]>,
-    visited: &mut MiniSet<GenericArg<'tcx>>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
 ) {
     for child in parent.walk_shallow(visited) {
         match child.unpack() {
diff --git a/compiler/rustc_middle/src/ty/print/mod.rs b/compiler/rustc_middle/src/ty/print/mod.rs
index 225ea2399fb..2e00be2395b 100644
--- a/compiler/rustc_middle/src/ty/print/mod.rs
+++ b/compiler/rustc_middle/src/ty/print/mod.rs
@@ -2,7 +2,7 @@ use crate::ty::subst::{GenericArg, Subst};
 use crate::ty::{self, DefIdTree, Ty, TyCtxt};
 
 use rustc_data_structures::fx::FxHashSet;
-use rustc_data_structures::mini_set::MiniSet;
+use rustc_data_structures::sso::SsoHashSet;
 use rustc_hir::def_id::{CrateNum, DefId};
 use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
 
@@ -269,7 +269,7 @@ pub trait Printer<'tcx>: Sized {
 /// deeply nested tuples that have no DefId.
 fn characteristic_def_id_of_type_cached<'a>(
     ty: Ty<'a>,
-    visited: &mut MiniSet<Ty<'a>>,
+    visited: &mut SsoHashSet<Ty<'a>>,
 ) -> Option<DefId> {
     match *ty.kind() {
         ty::Adt(adt_def, _) => Some(adt_def.did),
@@ -316,7 +316,7 @@ fn characteristic_def_id_of_type_cached<'a>(
     }
 }
 pub fn characteristic_def_id_of_type(ty: Ty<'_>) -> Option<DefId> {
-    characteristic_def_id_of_type_cached(ty, &mut MiniSet::new())
+    characteristic_def_id_of_type_cached(ty, &mut SsoHashSet::new())
 }
 
 impl<'tcx, P: Printer<'tcx>> Print<'tcx, P> for ty::RegionKind {
diff --git a/compiler/rustc_middle/src/ty/walk.rs b/compiler/rustc_middle/src/ty/walk.rs
index 80ade7dda4c..357a0dd65c4 100644
--- a/compiler/rustc_middle/src/ty/walk.rs
+++ b/compiler/rustc_middle/src/ty/walk.rs
@@ -3,7 +3,7 @@
 
 use crate::ty;
 use crate::ty::subst::{GenericArg, GenericArgKind};
-use rustc_data_structures::mini_set::MiniSet;
+use rustc_data_structures::sso::SsoHashSet;
 use smallvec::{self, SmallVec};
 
 // The TypeWalker's stack is hot enough that it's worth going to some effort to
@@ -13,7 +13,7 @@ type TypeWalkerStack<'tcx> = SmallVec<[GenericArg<'tcx>; 8]>;
 pub struct TypeWalker<'tcx> {
     stack: TypeWalkerStack<'tcx>,
     last_subtree: usize,
-    visited: MiniSet<GenericArg<'tcx>>,
+    visited: SsoHashSet<GenericArg<'tcx>>,
 }
 
 /// An iterator for walking the type tree.
@@ -26,7 +26,7 @@ pub struct TypeWalker<'tcx> {
 /// skips any types that are already there.
 impl<'tcx> TypeWalker<'tcx> {
     pub fn new(root: GenericArg<'tcx>) -> Self {
-        Self { stack: smallvec![root], last_subtree: 1, visited: MiniSet::new() }
+        Self { stack: smallvec![root], last_subtree: 1, visited: SsoHashSet::new() }
     }
 
     /// Skips the subtree corresponding to the last type
@@ -87,7 +87,7 @@ impl GenericArg<'tcx> {
     /// and skips any types that are already there.
     pub fn walk_shallow(
         self,
-        visited: &mut MiniSet<GenericArg<'tcx>>,
+        visited: &mut SsoHashSet<GenericArg<'tcx>>,
     ) -> impl Iterator<Item = GenericArg<'tcx>> {
         let mut stack = SmallVec::new();
         push_inner(&mut stack, self);
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index 3dcebbcc244..bdbf45f78a2 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -7,7 +7,7 @@ use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::{InferCtxt, InferOk};
 use crate::traits::error_reporting::InferCtxtExt;
 use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
-use rustc_data_structures::mini_map::MiniMap;
+use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
@@ -58,7 +58,7 @@ impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
             param_env: self.param_env,
             obligations: vec![],
             error: false,
-            cache: MiniMap::new(),
+            cache: SsoHashMap::new(),
             anon_depth: 0,
         };
 
@@ -87,7 +87,7 @@ struct QueryNormalizer<'cx, 'tcx> {
     cause: &'cx ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     obligations: Vec<PredicateObligation<'tcx>>,
-    cache: MiniMap<Ty<'tcx>, Ty<'tcx>>,
+    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
     error: bool,
     anon_depth: usize,
 }