In BTreeMap::eq, do not compare the elements if the sizes are different.

library/alloc/src/collections/btree/map.rs

@@ -2416,7 +2416,7 @@ impl<K, V> Default for BTreeMap<K, V> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: PartialEq, V: PartialEq, A: Allocator + Clone> PartialEq for BTreeMap<K, V, A> {
     fn eq(&self, other: &BTreeMap<K, V, A>) -> bool {
-        self.iter().eq(other)
+        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a == b)
     }
 }
 

library/alloctests/tests/collections/eq_diff_len.rs

@@ -0,0 +1,96 @@
+//! Regression tests which fail if some collections' `PartialEq::eq` impls compare
+//! elements when the collections have different sizes.
+//! This behavior is not guaranteed either way, so regressing these tests is fine
+//! if it is done on purpose.
+use std::cmp::Ordering;
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, LinkedList};
+
+/// This intentionally has a panicking `PartialEq` impl, to test that various
+/// collections' `PartialEq` impls don't actually compare elements if their sizes
+/// are unequal.
+///
+/// This is not advisable in normal code.
+#[derive(Debug, Clone, Copy, Hash)]
+struct Evil;
+
+impl PartialEq for Evil {
+    fn eq(&self, _: &Self) -> bool {
+        panic!("Evil::eq is evil");
+    }
+}
+impl Eq for Evil {}
+
+impl PartialOrd for Evil {
+    fn partial_cmp(&self, _: &Self) -> Option<Ordering> {
+        Some(Ordering::Equal)
+    }
+}
+
+impl Ord for Evil {
+    fn cmp(&self, _: &Self) -> Ordering {
+        // Constructing a `BTreeSet`/`BTreeMap` uses `cmp` on the elements,
+        // but comparing it with with `==` uses `eq` on the elements,
+        // so Evil::cmp doesn't need to be evil.
+        Ordering::Equal
+    }
+}
+
+// check Evil works
+#[test]
+#[should_panic = "Evil::eq is evil"]
+fn evil_eq_works() {
+    let v1 = vec![Evil];
+    let v2 = vec![Evil];
+
+    _ = v1 == v2;
+}
+
+// check various containers don't compare if their sizes are different
+
+#[test]
+fn vec_evil_eq() {
+    let v1 = vec![Evil];
+    let v2 = vec![Evil; 2];
+
+    assert_eq!(false, v1 == v2);
+}
+
+#[test]
+fn hashset_evil_eq() {
+    let s1 = HashSet::from([(0, Evil)]);
+    let s2 = HashSet::from([(0, Evil), (1, Evil)]);
+
+    assert_eq!(false, s1 == s2);
+}
+
+#[test]
+fn hashmap_evil_eq() {
+    let m1 = HashMap::from([(0, Evil)]);
+    let m2 = HashMap::from([(0, Evil), (1, Evil)]);
+
+    assert_eq!(false, m1 == m2);
+}
+
+#[test]
+fn btreeset_evil_eq() {
+    let s1 = BTreeSet::from([(0, Evil)]);
+    let s2 = BTreeSet::from([(0, Evil), (1, Evil)]);
+
+    assert_eq!(false, s1 == s2);
+}
+
+#[test]
+fn btreemap_evil_eq() {
+    let m1 = BTreeMap::from([(0, Evil)]);
+    let m2 = BTreeMap::from([(0, Evil), (1, Evil)]);
+
+    assert_eq!(false, m1 == m2);
+}
+
+#[test]
+fn linkedlist_evil_eq() {
+    let m1 = LinkedList::from([Evil]);
+    let m2 = LinkedList::from([Evil; 2]);
+
+    assert_eq!(false, m1 == m2);
+}

library/alloctests/tests/collections/mod.rs

@@ -1 +1,2 @@
 mod binary_heap;
+mod eq_diff_len;