Adding debian version 1.65.0+dfsg1-2.debian/1.65.0+dfsg1-2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

11 files changed, 182 insertions, 343 deletions

diff --git a/compiler/rustc_data_structures/src/fingerprint.rs b/compiler/rustc_data_structures/src/fingerprint.rs
index 5ff2d18dd..a39178016 100644
--- a/compiler/rustc_data_structures/src/fingerprint.rs
+++ b/compiler/rustc_data_structures/src/fingerprint.rs
@@ -29,7 +29,7 @@ impl Fingerprint {
         // quality hash values, let's still combine the two values because the
         // Fingerprints in DefPathHash have the StableCrateId portion which is
         // the same for all DefPathHashes from the same crate. Combining the
-        // two halfs makes sure we get a good quality hash in such cases too.
+        // two halves makes sure we get a good quality hash in such cases too.
         self.0.wrapping_mul(3).wrapping_add(self.1)
     }
 
@@ -120,7 +120,7 @@ impl FingerprintHasher for crate::unhash::Unhasher {
         // quality hash values, let's still combine the two values because the
         // Fingerprints in DefPathHash have the StableCrateId portion which is
         // the same for all DefPathHashes from the same crate. Combining the
-        // two halfs makes sure we get a good quality hash in such cases too.
+        // two halves makes sure we get a good quality hash in such cases too.
         //
         // Since `Unhasher` is used only in the context of HashMaps, it is OK
         // to combine the two components in an order-independent way (which is
diff --git a/compiler/rustc_data_structures/src/fx.rs b/compiler/rustc_data_structures/src/fx.rs
index bbeb193db..0d0c51b68 100644
--- a/compiler/rustc_data_structures/src/fx.rs
+++ b/compiler/rustc_data_structures/src/fx.rs
@@ -2,13 +2,26 @@ use std::hash::BuildHasherDefault;
 
 pub use rustc_hash::{FxHashMap, FxHashSet, FxHasher};
 
+pub type StdEntry<'a, K, V> = std::collections::hash_map::Entry<'a, K, V>;
+
 pub type FxIndexMap<K, V> = indexmap::IndexMap<K, V, BuildHasherDefault<FxHasher>>;
 pub type FxIndexSet<V> = indexmap::IndexSet<V, BuildHasherDefault<FxHasher>>;
+pub type IndexEntry<'a, K, V> = indexmap::map::Entry<'a, K, V>;
 
 #[macro_export]
 macro_rules! define_id_collections {
-    ($map_name:ident, $set_name:ident, $key:ty) => {
+    ($map_name:ident, $set_name:ident, $entry_name:ident, $key:ty) => {
         pub type $map_name<T> = $crate::fx::FxHashMap<$key, T>;
         pub type $set_name = $crate::fx::FxHashSet<$key>;
+        pub type $entry_name<'a, T> = $crate::fx::StdEntry<'a, $key, T>;
+    };
+}
+
+#[macro_export]
+macro_rules! define_stable_id_collections {
+    ($map_name:ident, $set_name:ident, $entry_name:ident, $key:ty) => {
+        pub type $map_name<T> = $crate::fx::FxIndexMap<$key, T>;
+        pub type $set_name = $crate::fx::FxIndexSet<$key>;
+        pub type $entry_name<'a, T> = $crate::fx::IndexEntry<'a, $key, T>;
     };
 }
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 265f45b72..56f7823ef 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -13,7 +13,7 @@
 #![feature(cell_leak)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
-#![feature(let_else)]
+#![cfg_attr(bootstrap, feature(let_else))]
 #![feature(hash_raw_entry)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(maybe_uninit_uninit_array)]
@@ -28,6 +28,8 @@
 #![feature(vec_into_raw_parts)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate tracing;
@@ -73,7 +75,6 @@ pub mod profiling;
 pub mod sharded;
 pub mod stack;
 pub mod sync;
-pub mod thin_vec;
 pub mod tiny_list;
 pub mod transitive_relation;
 pub mod vec_linked_list;
diff --git a/compiler/rustc_data_structures/src/map_in_place.rs b/compiler/rustc_data_structures/src/map_in_place.rs
index 874de03d3..a0d4b7ade 100644
--- a/compiler/rustc_data_structures/src/map_in_place.rs
+++ b/compiler/rustc_data_structures/src/map_in_place.rs
@@ -1,5 +1,6 @@
 use smallvec::{Array, SmallVec};
 use std::ptr;
+use thin_vec::ThinVec;
 
 pub trait MapInPlace<T>: Sized {
     fn map_in_place<F>(&mut self, mut f: F)
@@ -15,94 +16,64 @@ pub trait MapInPlace<T>: Sized {
         I: IntoIterator<Item = T>;
 }
 
-impl<T> MapInPlace<T> for Vec<T> {
-    fn flat_map_in_place<F, I>(&mut self, mut f: F)
-    where
-        F: FnMut(T) -> I,
-        I: IntoIterator<Item = T>,
-    {
-        let mut read_i = 0;
-        let mut write_i = 0;
-        unsafe {
-            let mut old_len = self.len();
-            self.set_len(0); // make sure we just leak elements in case of panic
+// The implementation of this method is syntactically identical for all the
+// different vector types.
+macro_rules! flat_map_in_place {
+    () => {
+        fn flat_map_in_place<F, I>(&mut self, mut f: F)
+        where
+            F: FnMut(T) -> I,
+            I: IntoIterator<Item = T>,
+        {
+            let mut read_i = 0;
+            let mut write_i = 0;
+            unsafe {
+                let mut old_len = self.len();
+                self.set_len(0); // make sure we just leak elements in case of panic
 
-            while read_i < old_len {
-                // move the read_i'th item out of the vector and map it
-                // to an iterator
-                let e = ptr::read(self.as_ptr().add(read_i));
-                let iter = f(e).into_iter();
-                read_i += 1;
+                while read_i < old_len {
+                    // move the read_i'th item out of the vector and map it
+                    // to an iterator
+                    let e = ptr::read(self.as_ptr().add(read_i));
+                    let iter = f(e).into_iter();
+                    read_i += 1;
 
-                for e in iter {
-                    if write_i < read_i {
-                        ptr::write(self.as_mut_ptr().add(write_i), e);
-                        write_i += 1;
-                    } else {
-                        // If this is reached we ran out of space
-                        // in the middle of the vector.
-                        // However, the vector is in a valid state here,
-                        // so we just do a somewhat inefficient insert.
-                        self.set_len(old_len);
-                        self.insert(write_i, e);
+                    for e in iter {
+                        if write_i < read_i {
+                            ptr::write(self.as_mut_ptr().add(write_i), e);
+                            write_i += 1;
+                        } else {
+                            // If this is reached we ran out of space
+                            // in the middle of the vector.
+                            // However, the vector is in a valid state here,
+                            // so we just do a somewhat inefficient insert.
+                            self.set_len(old_len);
+                            self.insert(write_i, e);
 
-                        old_len = self.len();
-                        self.set_len(0);
+                            old_len = self.len();
+                            self.set_len(0);
 
-                        read_i += 1;
-                        write_i += 1;
+                            read_i += 1;
+                            write_i += 1;
+                        }
                     }
                 }
-            }
 
-            // write_i tracks the number of actually written new items.
-            self.set_len(write_i);
+                // write_i tracks the number of actually written new items.
+                self.set_len(write_i);
+            }
         }
-    }
+    };
 }
 
-impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
-    fn flat_map_in_place<F, I>(&mut self, mut f: F)
-    where
-        F: FnMut(T) -> I,
-        I: IntoIterator<Item = T>,
-    {
-        let mut read_i = 0;
-        let mut write_i = 0;
-        unsafe {
-            let mut old_len = self.len();
-            self.set_len(0); // make sure we just leak elements in case of panic
-
-            while read_i < old_len {
-                // move the read_i'th item out of the vector and map it
-                // to an iterator
-                let e = ptr::read(self.as_ptr().add(read_i));
-                let iter = f(e).into_iter();
-                read_i += 1;
-
-                for e in iter {
-                    if write_i < read_i {
-                        ptr::write(self.as_mut_ptr().add(write_i), e);
-                        write_i += 1;
-                    } else {
-                        // If this is reached we ran out of space
-                        // in the middle of the vector.
-                        // However, the vector is in a valid state here,
-                        // so we just do a somewhat inefficient insert.
-                        self.set_len(old_len);
-                        self.insert(write_i, e);
-
-                        old_len = self.len();
-                        self.set_len(0);
+impl<T> MapInPlace<T> for Vec<T> {
+    flat_map_in_place!();
+}
 
-                        read_i += 1;
-                        write_i += 1;
-                    }
-                }
-            }
+impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
+    flat_map_in_place!();
+}
 
-            // write_i tracks the number of actually written new items.
-            self.set_len(write_i);
-        }
-    }
+impl<T> MapInPlace<T> for ThinVec<T> {
+    flat_map_in_place!();
 }
diff --git a/compiler/rustc_data_structures/src/obligation_forest/mod.rs b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
index 07a96dd7d..e351b650a 100644
--- a/compiler/rustc_data_structures/src/obligation_forest/mod.rs
+++ b/compiler/rustc_data_structures/src/obligation_forest/mod.rs
@@ -117,6 +117,10 @@ pub trait ObligationProcessor {
 }
 
 /// The result type used by `process_obligation`.
+// `repr(C)` to inhibit the niche filling optimization. Otherwise, the `match` appearing
+// in `process_obligations` is significantly slower, which can substantially affect
+// benchmarks like `rustc-perf`'s inflate and keccak.
+#[repr(C)]
 #[derive(Debug)]
 pub enum ProcessResult<O, E> {
     Unchanged,
diff --git a/compiler/rustc_data_structures/src/sorted_map.rs b/compiler/rustc_data_structures/src/sorted_map.rs
index 9efea1228..937cb6715 100644
--- a/compiler/rustc_data_structures/src/sorted_map.rs
+++ b/compiler/rustc_data_structures/src/sorted_map.rs
@@ -164,7 +164,7 @@ impl<K: Ord, V> SortedMap<K, V> {
     /// It is up to the caller to make sure that the elements are sorted by key
     /// and that there are no duplicates.
     #[inline]
-    pub fn insert_presorted(&mut self, mut elements: Vec<(K, V)>) {
+    pub fn insert_presorted(&mut self, elements: Vec<(K, V)>) {
         if elements.is_empty() {
             return;
         }
@@ -173,28 +173,28 @@ impl<K: Ord, V> SortedMap<K, V> {
 
         let start_index = self.lookup_index_for(&elements[0].0);
 
-        let drain = match start_index {
+        let elements = match start_index {
             Ok(index) => {
-                let mut drain = elements.drain(..);
-                self.data[index] = drain.next().unwrap();
-                drain
+                let mut elements = elements.into_iter();
+                self.data[index] = elements.next().unwrap();
+                elements
             }
             Err(index) => {
                 if index == self.data.len() || elements.last().unwrap().0 < self.data[index].0 {
                     // We can copy the whole range without having to mix with
                     // existing elements.
-                    self.data.splice(index..index, elements.drain(..));
+                    self.data.splice(index..index, elements.into_iter());
                     return;
                 }
 
-                let mut drain = elements.drain(..);
-                self.data.insert(index, drain.next().unwrap());
-                drain
+                let mut elements = elements.into_iter();
+                self.data.insert(index, elements.next().unwrap());
+                elements
             }
         };
 
         // Insert the rest
-        for (k, v) in drain {
+        for (k, v) in elements {
             self.insert(k, v);
         }
     }
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 52952a793..9c0fb8265 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -48,7 +48,7 @@ cfg_if! {
         /// the native atomic types.
         /// You should use this type through the `AtomicU64`, `AtomicUsize`, etc, type aliases
         /// as it's not intended to be used separately.
-        #[derive(Debug)]
+        #[derive(Debug, Default)]
         pub struct Atomic<T: Copy>(Cell<T>);
 
         impl<T: Copy> Atomic<T> {
@@ -56,9 +56,7 @@ cfg_if! {
             pub fn new(v: T) -> Self {
                 Atomic(Cell::new(v))
             }
-        }
 
-        impl<T: Copy> Atomic<T> {
             #[inline]
             pub fn into_inner(self) -> T {
                 self.0.into_inner()
diff --git a/compiler/rustc_data_structures/src/thin_vec.rs b/compiler/rustc_data_structures/src/thin_vec.rs
deleted file mode 100644
index 716259142..000000000
--- a/compiler/rustc_data_structures/src/thin_vec.rs
+++ /dev/null
@@ -1,135 +0,0 @@
-use crate::stable_hasher::{HashStable, StableHasher};
-
-use std::iter::FromIterator;
-
-/// A vector type optimized for cases where this size is usually 0 (cf. `SmallVec`).
-/// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
-/// which uses only a single (null) pointer.
-#[derive(Clone, Encodable, Decodable, Debug, Hash, Eq, PartialEq)]
-pub struct ThinVec<T>(Option<Box<Vec<T>>>);
-
-impl<T> ThinVec<T> {
-    pub fn new() -> Self {
-        ThinVec(None)
-    }
-
-    pub fn iter(&self) -> std::slice::Iter<'_, T> {
-        self.into_iter()
-    }
-
-    pub fn iter_mut(&mut self) -> std::slice::IterMut<'_, T> {
-        self.into_iter()
-    }
-
-    pub fn push(&mut self, item: T) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.push(item),
-            ThinVec(None) => *self = vec![item].into(),
-        }
-    }
-}
-
-impl<T> From<Vec<T>> for ThinVec<T> {
-    fn from(vec: Vec<T>) -> Self {
-        if vec.is_empty() { ThinVec(None) } else { ThinVec(Some(Box::new(vec))) }
-    }
-}
-
-impl<T> Into<Vec<T>> for ThinVec<T> {
-    fn into(self) -> Vec<T> {
-        match self {
-            ThinVec(None) => Vec::new(),
-            ThinVec(Some(vec)) => *vec,
-        }
-    }
-}
-
-impl<T> ::std::ops::Deref for ThinVec<T> {
-    type Target = [T];
-    fn deref(&self) -> &[T] {
-        match *self {
-            ThinVec(None) => &[],
-            ThinVec(Some(ref vec)) => vec,
-        }
-    }
-}
-
-impl<T> ::std::ops::DerefMut for ThinVec<T> {
-    fn deref_mut(&mut self) -> &mut [T] {
-        match *self {
-            ThinVec(None) => &mut [],
-            ThinVec(Some(ref mut vec)) => vec,
-        }
-    }
-}
-
-impl<T> FromIterator<T> for ThinVec<T> {
-    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
-        // `Vec::from_iter()` should not allocate if the iterator is empty.
-        let vec: Vec<_> = iter.into_iter().collect();
-        if vec.is_empty() { ThinVec(None) } else { ThinVec(Some(Box::new(vec))) }
-    }
-}
-
-impl<T> IntoIterator for ThinVec<T> {
-    type Item = T;
-    type IntoIter = std::vec::IntoIter<T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        // This is still performant because `Vec::new()` does not allocate.
-        self.0.map_or_else(Vec::new, |ptr| *ptr).into_iter()
-    }
-}
-
-impl<'a, T> IntoIterator for &'a ThinVec<T> {
-    type Item = &'a T;
-    type IntoIter = std::slice::Iter<'a, T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.as_ref().iter()
-    }
-}
-
-impl<'a, T> IntoIterator for &'a mut ThinVec<T> {
-    type Item = &'a mut T;
-    type IntoIter = std::slice::IterMut<'a, T>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.as_mut().iter_mut()
-    }
-}
-
-impl<T> Extend<T> for ThinVec<T> {
-    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.extend(iter),
-            ThinVec(None) => *self = iter.into_iter().collect::<Vec<_>>().into(),
-        }
-    }
-
-    fn extend_one(&mut self, item: T) {
-        self.push(item)
-    }
-
-    fn extend_reserve(&mut self, additional: usize) {
-        match *self {
-            ThinVec(Some(ref mut vec)) => vec.reserve(additional),
-            ThinVec(None) => *self = Vec::with_capacity(additional).into(),
-        }
-    }
-}
-
-impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
-    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
-        (**self).hash_stable(hcx, hasher)
-    }
-}
-
-impl<T> Default for ThinVec<T> {
-    fn default() -> Self {
-        Self(None)
-    }
-}
-
-#[cfg(test)]
-mod tests;
diff --git a/compiler/rustc_data_structures/src/thin_vec/tests.rs b/compiler/rustc_data_structures/src/thin_vec/tests.rs
deleted file mode 100644
index 0221b9912..000000000
--- a/compiler/rustc_data_structures/src/thin_vec/tests.rs
+++ /dev/null
@@ -1,42 +0,0 @@
-use super::*;
-
-impl<T> ThinVec<T> {
-    fn into_vec(self) -> Vec<T> {
-        self.into()
-    }
-}
-
-#[test]
-fn test_from_iterator() {
-    assert_eq!(std::iter::empty().collect::<ThinVec<String>>().into_vec(), Vec::<String>::new());
-    assert_eq!(std::iter::once(42).collect::<ThinVec<_>>().into_vec(), vec![42]);
-    assert_eq!([1, 2].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2]);
-    assert_eq!([1, 2, 3].into_iter().collect::<ThinVec<_>>().into_vec(), vec![1, 2, 3]);
-}
-
-#[test]
-fn test_into_iterator_owned() {
-    assert_eq!(ThinVec::new().into_iter().collect::<Vec<String>>(), Vec::<String>::new());
-    assert_eq!(ThinVec::from(vec![1]).into_iter().collect::<Vec<_>>(), vec![1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).into_iter().collect::<Vec<_>>(), vec![1, 2]);
-    assert_eq!(ThinVec::from(vec![1, 2, 3]).into_iter().collect::<Vec<_>>(), vec![1, 2, 3]);
-}
-
-#[test]
-fn test_into_iterator_ref() {
-    assert_eq!(ThinVec::new().iter().collect::<Vec<&String>>(), Vec::<&String>::new());
-    assert_eq!(ThinVec::from(vec![1]).iter().collect::<Vec<_>>(), vec![&1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).iter().collect::<Vec<_>>(), vec![&1, &2]);
-    assert_eq!(ThinVec::from(vec![1, 2, 3]).iter().collect::<Vec<_>>(), vec![&1, &2, &3]);
-}
-
-#[test]
-fn test_into_iterator_ref_mut() {
-    assert_eq!(ThinVec::new().iter_mut().collect::<Vec<&mut String>>(), Vec::<&mut String>::new());
-    assert_eq!(ThinVec::from(vec![1]).iter_mut().collect::<Vec<_>>(), vec![&mut 1]);
-    assert_eq!(ThinVec::from(vec![1, 2]).iter_mut().collect::<Vec<_>>(), vec![&mut 1, &mut 2]);
-    assert_eq!(
-        ThinVec::from(vec![1, 2, 3]).iter_mut().collect::<Vec<_>>(),
-        vec![&mut 1, &mut 2, &mut 3],
-    );
-}
diff --git a/compiler/rustc_data_structures/src/transitive_relation.rs b/compiler/rustc_data_structures/src/transitive_relation.rs
index 0ff64969b..f016c391f 100644
--- a/compiler/rustc_data_structures/src/transitive_relation.rs
+++ b/compiler/rustc_data_structures/src/transitive_relation.rs
@@ -1,45 +1,57 @@
+use crate::frozen::Frozen;
 use crate::fx::FxIndexSet;
-use crate::sync::Lock;
 use rustc_index::bit_set::BitMatrix;
 use std::fmt::Debug;
 use std::hash::Hash;
 use std::mem;
+use std::ops::Deref;
 
 #[cfg(test)]
 mod tests;
 
 #[derive(Clone, Debug)]
-pub struct TransitiveRelation<T> {
+pub struct TransitiveRelationBuilder<T> {
     // List of elements. This is used to map from a T to a usize.
     elements: FxIndexSet<T>,
 
     // List of base edges in the graph. Require to compute transitive
     // closure.
     edges: Vec<Edge>,
+}
+
+#[derive(Debug)]
+pub struct TransitiveRelation<T> {
+    // Frozen transitive relation elements and edges.
+    builder: Frozen<TransitiveRelationBuilder<T>>,
 
-    // This is a cached transitive closure derived from the edges.
-    // Currently, we build it lazily and just throw out any existing
-    // copy whenever a new edge is added. (The Lock is to permit
-    // the lazy computation.) This is kind of silly, except for the
-    // fact its size is tied to `self.elements.len()`, so I wanted to
-    // wait before building it up to avoid reallocating as new edges
-    // are added with new elements. Perhaps better would be to ask the
-    // user for a batch of edges to minimize this effect, but I
-    // already wrote the code this way. :P -nmatsakis
-    closure: Lock<Option<BitMatrix<usize, usize>>>,
+    // Cached transitive closure derived from the edges.
+    closure: Frozen<BitMatrix<usize, usize>>,
 }
 
-// HACK(eddyb) manual impl avoids `Default` bound on `T`.
-impl<T: Eq + Hash> Default for TransitiveRelation<T> {
-    fn default() -> Self {
+impl<T> Deref for TransitiveRelation<T> {
+    type Target = Frozen<TransitiveRelationBuilder<T>>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.builder
+    }
+}
+
+impl<T: Clone> Clone for TransitiveRelation<T> {
+    fn clone(&self) -> Self {
         TransitiveRelation {
-            elements: Default::default(),
-            edges: Default::default(),
-            closure: Default::default(),
+            builder: Frozen::freeze(self.builder.deref().clone()),
+            closure: Frozen::freeze(self.closure.deref().clone()),
         }
     }
 }
 
+// HACK(eddyb) manual impl avoids `Default` bound on `T`.
+impl<T: Eq + Hash> Default for TransitiveRelationBuilder<T> {
+    fn default() -> Self {
+        TransitiveRelationBuilder { elements: Default::default(), edges: Default::default() }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Debug)]
 struct Index(usize);
 
@@ -49,7 +61,7 @@ struct Edge {
     target: Index,
 }
 
-impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
+impl<T: Eq + Hash + Copy> TransitiveRelationBuilder<T> {
     pub fn is_empty(&self) -> bool {
         self.edges.is_empty()
     }
@@ -63,23 +75,19 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
     }
 
     fn add_index(&mut self, a: T) -> Index {
-        let (index, added) = self.elements.insert_full(a);
-        if added {
-            // if we changed the dimensions, clear the cache
-            *self.closure.get_mut() = None;
-        }
+        let (index, _added) = self.elements.insert_full(a);
         Index(index)
     }
 
     /// Applies the (partial) function to each edge and returns a new
-    /// relation. If `f` returns `None` for any end-point, returns
-    /// `None`.
-    pub fn maybe_map<F, U>(&self, mut f: F) -> Option<TransitiveRelation<U>>
+    /// relation builder. If `f` returns `None` for any end-point,
+    /// returns `None`.
+    pub fn maybe_map<F, U>(&self, mut f: F) -> Option<TransitiveRelationBuilder<U>>
     where
         F: FnMut(T) -> Option<U>,
         U: Clone + Debug + Eq + Hash + Copy,
     {
-        let mut result = TransitiveRelation::default();
+        let mut result = TransitiveRelationBuilder::default();
         for edge in &self.edges {
             result.add(f(self.elements[edge.source.0])?, f(self.elements[edge.target.0])?);
         }
@@ -93,10 +101,38 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
         let edge = Edge { source: a, target: b };
         if !self.edges.contains(&edge) {
             self.edges.push(edge);
+        }
+    }
+
+    /// Compute the transitive closure derived from the edges, and converted to
+    /// the final result. After this, all elements will be immutable to maintain
+    /// the correctness of the result.
+    pub fn freeze(self) -> TransitiveRelation<T> {
+        let mut matrix = BitMatrix::new(self.elements.len(), self.elements.len());
+        let mut changed = true;
+        while changed {
+            changed = false;
+            for edge in &self.edges {
+                // add an edge from S -> T
+                changed |= matrix.insert(edge.source.0, edge.target.0);
 
-            // added an edge, clear the cache
-            *self.closure.get_mut() = None;
+                // add all outgoing edges from T into S
+                changed |= matrix.union_rows(edge.target.0, edge.source.0);
+            }
         }
+        TransitiveRelation { builder: Frozen::freeze(self), closure: Frozen::freeze(matrix) }
+    }
+}
+
+impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
+    /// Applies the (partial) function to each edge and returns a new
+    /// relation including transitive closures.
+    pub fn maybe_map<F, U>(&self, f: F) -> Option<TransitiveRelation<U>>
+    where
+        F: FnMut(T) -> Option<U>,
+        U: Clone + Debug + Eq + Hash + Copy,
+    {
+        Some(self.builder.maybe_map(f)?.freeze())
     }
 
     /// Checks whether `a < target` (transitively)
@@ -322,30 +358,7 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
     where
         OP: FnOnce(&BitMatrix<usize, usize>) -> R,
     {
-        let mut closure_cell = self.closure.borrow_mut();
-        let mut closure = closure_cell.take();
-        if closure.is_none() {
-            closure = Some(self.compute_closure());
-        }
-        let result = op(closure.as_ref().unwrap());
-        *closure_cell = closure;
-        result
-    }
-
-    fn compute_closure(&self) -> BitMatrix<usize, usize> {
-        let mut matrix = BitMatrix::new(self.elements.len(), self.elements.len());
-        let mut changed = true;
-        while changed {
-            changed = false;
-            for edge in &self.edges {
-                // add an edge from S -> T
-                changed |= matrix.insert(edge.source.0, edge.target.0);
-
-                // add all outgoing edges from T into S
-                changed |= matrix.union_rows(edge.target.0, edge.source.0);
-            }
-        }
-        matrix
+        op(&self.closure)
     }
 
     /// Lists all the base edges in the graph: the initial _non-transitive_ set of element
diff --git a/compiler/rustc_data_structures/src/transitive_relation/tests.rs b/compiler/rustc_data_structures/src/transitive_relation/tests.rs
index e1f4c7ee0..e756c546e 100644
--- a/compiler/rustc_data_structures/src/transitive_relation/tests.rs
+++ b/compiler/rustc_data_structures/src/transitive_relation/tests.rs
@@ -10,9 +10,10 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
 
 #[test]
 fn test_one_step() {
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "b");
     relation.add("a", "c");
+    let relation = relation.freeze();
     assert!(relation.contains("a", "c"));
     assert!(relation.contains("a", "b"));
     assert!(!relation.contains("b", "a"));
@@ -21,7 +22,7 @@ fn test_one_step() {
 
 #[test]
 fn test_many_steps() {
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "b");
     relation.add("a", "c");
     relation.add("a", "f");
@@ -31,6 +32,7 @@ fn test_many_steps() {
     relation.add("b", "e");
 
     relation.add("e", "g");
+    let relation = relation.freeze();
 
     assert!(relation.contains("a", "b"));
     assert!(relation.contains("a", "c"));
@@ -51,9 +53,10 @@ fn mubs_triangle() {
     //      ^
     //      |
     //      b
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "tcx");
     relation.add("b", "tcx");
+    let relation = relation.freeze();
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["tcx"]);
     assert_eq!(relation.parents("a"), vec!["tcx"]);
     assert_eq!(relation.parents("b"), vec!["tcx"]);
@@ -72,7 +75,7 @@ fn mubs_best_choice1() {
     // need the second pare down call to get the right result (after
     // intersection, we have [1, 2], but 2 -> 1).
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("0", "1");
     relation.add("0", "2");
 
@@ -80,6 +83,7 @@ fn mubs_best_choice1() {
 
     relation.add("3", "1");
     relation.add("3", "2");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("0", "3"), vec!["2"]);
     assert_eq!(relation.parents("0"), vec!["2"]);
@@ -99,7 +103,7 @@ fn mubs_best_choice2() {
     // Like the preceding test, but in this case intersection is [2,
     // 1], and hence we rely on the first pare down call.
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("0", "1");
     relation.add("0", "2");
 
@@ -107,6 +111,7 @@ fn mubs_best_choice2() {
 
     relation.add("3", "1");
     relation.add("3", "2");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("0", "3"), vec!["1"]);
     assert_eq!(relation.parents("0"), vec!["1"]);
@@ -118,12 +123,13 @@ fn mubs_best_choice2() {
 fn mubs_no_best_choice() {
     // in this case, the intersection yields [1, 2], and the "pare
     // down" calls find nothing to remove.
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("0", "1");
     relation.add("0", "2");
 
     relation.add("3", "1");
     relation.add("3", "2");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("0", "3"), vec!["1", "2"]);
     assert_eq!(relation.parents("0"), vec!["1", "2"]);
@@ -135,7 +141,7 @@ fn mubs_best_choice_scc() {
     // in this case, 1 and 2 form a cycle; we pick arbitrarily (but
     // consistently).
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("0", "1");
     relation.add("0", "2");
 
@@ -144,6 +150,7 @@ fn mubs_best_choice_scc() {
 
     relation.add("3", "1");
     relation.add("3", "2");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("0", "3"), vec!["1"]);
     assert_eq!(relation.parents("0"), vec!["1"]);
@@ -157,13 +164,14 @@ fn pdub_crisscross() {
     //   /\       |
     // b -> b1 ---+
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "a1");
     relation.add("a", "b1");
     relation.add("b", "a1");
     relation.add("b", "b1");
     relation.add("a1", "x");
     relation.add("b1", "x");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["a1", "b1"]);
     assert_eq!(relation.postdom_upper_bound("a", "b"), Some("x"));
@@ -179,7 +187,7 @@ fn pdub_crisscross_more() {
     //   /\    /\             |
     // b -> b1 -> b2 ---------+
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "a1");
     relation.add("a", "b1");
     relation.add("b", "a1");
@@ -194,6 +202,7 @@ fn pdub_crisscross_more() {
 
     relation.add("a3", "x");
     relation.add("b2", "x");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["a1", "b1"]);
     assert_eq!(relation.minimal_upper_bounds("a1", "b1"), vec!["a2", "b2"]);
@@ -210,11 +219,12 @@ fn pdub_lub() {
     //            |
     // b -> b1 ---+
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "a1");
     relation.add("b", "b1");
     relation.add("a1", "x");
     relation.add("b1", "x");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["x"]);
     assert_eq!(relation.postdom_upper_bound("a", "b"), Some("x"));
@@ -233,10 +243,11 @@ fn mubs_intermediate_node_on_one_side_only() {
     //           b
 
     // "digraph { a -> c -> d; b -> d; }",
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "c");
     relation.add("c", "d");
     relation.add("b", "d");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["d"]);
 }
@@ -252,12 +263,13 @@ fn mubs_scc_1() {
     //           b
 
     // "digraph { a -> c -> d; d -> c; a -> d; b -> d; }",
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "c");
     relation.add("c", "d");
     relation.add("d", "c");
     relation.add("a", "d");
     relation.add("b", "d");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["c"]);
 }
@@ -272,12 +284,13 @@ fn mubs_scc_2() {
     //      +--- b
 
     // "digraph { a -> c -> d; d -> c; b -> d; b -> c; }",
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "c");
     relation.add("c", "d");
     relation.add("d", "c");
     relation.add("b", "d");
     relation.add("b", "c");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["c"]);
 }
@@ -292,13 +305,14 @@ fn mubs_scc_3() {
     //           b ---+
 
     // "digraph { a -> c -> d -> e -> c; b -> d; b -> e; }",
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "c");
     relation.add("c", "d");
     relation.add("d", "e");
     relation.add("e", "c");
     relation.add("b", "d");
     relation.add("b", "e");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["c"]);
 }
@@ -314,13 +328,14 @@ fn mubs_scc_4() {
     //           b ---+
 
     // "digraph { a -> c -> d -> e -> c; a -> d; b -> e; }"
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     relation.add("a", "c");
     relation.add("c", "d");
     relation.add("d", "e");
     relation.add("e", "c");
     relation.add("a", "d");
     relation.add("b", "e");
+    let relation = relation.freeze();
 
     assert_eq!(relation.minimal_upper_bounds("a", "b"), vec!["c"]);
 }
@@ -352,10 +367,11 @@ fn parent() {
         (1, /*->*/ 3),
     ];
 
-    let mut relation = TransitiveRelation::default();
+    let mut relation = TransitiveRelationBuilder::default();
     for (a, b) in pairs {
         relation.add(a, b);
     }
+    let relation = relation.freeze();
 
     let p = relation.postdom_parent(3);
     assert_eq!(p, Some(0));