Add sanity checks with alternate hashers

tests/hasher.rs

@@ -0,0 +1,65 @@
+//! Sanity check that alternate hashers work correctly.
+
+#![cfg(not(miri))] // FIXME: takes too long
+
+use hashbrown::HashSet;
+use std::hash::{BuildHasher, BuildHasherDefault, Hasher};
+
+fn check<S: BuildHasher + Default>() {
+    let range = 0..1_000;
+
+    let mut set = HashSet::<i32, S>::default();
+    set.extend(range.clone());
+
+    assert!(!set.contains(&i32::min_value()));
+    assert!(!set.contains(&(range.start - 1)));
+    for i in range.clone() {
+        assert!(set.contains(&i));
+    }
+    assert!(!set.contains(&range.end));
+    assert!(!set.contains(&i32::max_value()));
+}
+
+/// Use hashbrown's default hasher.
+#[test]
+fn default() {
+    check::<hashbrown::hash_map::DefaultHashBuilder>();
+}
+
+/// Use std's default hasher.
+#[test]
+fn random_state() {
+    check::<std::collections::hash_map::RandomState>();
+}
+
+/// Use a constant 0 hash.
+#[test]
+fn zero() {
+    #[derive(Default)]
+    struct ZeroHasher;
+
+    impl Hasher for ZeroHasher {
+        fn finish(&self) -> u64 {
+            0
+        }
+        fn write(&mut self, _: &[u8]) {}
+    }
+
+    check::<BuildHasherDefault<ZeroHasher>>();
+}
+
+/// Use a constant maximum hash.
+#[test]
+fn max() {
+    #[derive(Default)]
+    struct MaxHasher;
+
+    impl Hasher for MaxHasher {
+        fn finish(&self) -> u64 {
+            u64::max_value()
+        }
+        fn write(&mut self, _: &[u8]) {}
+    }
+
+    check::<BuildHasherDefault<MaxHasher>>();
+}