Auto merge of #33940 - seanmonstar:siphash-1-3, r=alexcrichton

hashmap: use siphash-1-3 as default hasher

Also exposes `SipHash13` and `SipHash24` in `core::hash::sip`, for those that want to differentiate.

For motivation, see this quote from the original issue:

> we proposed SipHash-2-4 as a (strong) PRF/MAC, and so far no attack whatsoever has been found,
although many competent people tried to break it. However, fewer rounds may be sufficient and I would
be very surprised if SipHash-1-3 introduced weaknesses for hash tables.

This keeps a type alias of `SipHasher` to `SipHash24`, and since the internal default hasher of HashMap is specified as "not specified", changing it should not be a breaking change.

Closes #29754

2 files changed, 25 insertions, 4 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index 536f168e401..60d7e01d988 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -14,7 +14,7 @@ use self::VacantEntryState::*;
 use borrow::Borrow;
 use cmp::max;
 use fmt::{self, Debug};
-use hash::{Hash, SipHasher, BuildHasher};
+use hash::{Hash, Hasher, BuildHasher, SipHasher13};
 use iter::FromIterator;
 use mem::{self, replace};
 use ops::{Deref, Index};
@@ -1711,10 +1711,30 @@ impl RandomState {
 
 #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
 impl BuildHasher for RandomState {
-    type Hasher = SipHasher;
+    type Hasher = DefaultHasher;
     #[inline]
-    fn build_hasher(&self) -> SipHasher {
-        SipHasher::new_with_keys(self.k0, self.k1)
+    fn build_hasher(&self) -> DefaultHasher {
+        DefaultHasher(SipHasher13::new_with_keys(self.k0, self.k1))
+    }
+}
+
+/// The default `Hasher` used by `RandomState`.
+///
+/// The internal algorithm is not specified, and so it and its hashes should
+/// not be relied upon over releases.
+#[unstable(feature = "hashmap_default_hasher", issue = "0")]
+pub struct DefaultHasher(SipHasher13);
+
+#[unstable(feature = "hashmap_default_hasher", issue = "0")]
+impl Hasher for DefaultHasher {
+    #[inline]
+    fn write(&mut self, msg: &[u8]) {
+        self.0.write(msg)
+    }
+
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.0.finish()
     }
 }
 
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index 135ea8a5e7c..a396c7be09a 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -255,6 +255,7 @@
 #![feature(reflect_marker)]
 #![feature(rustc_attrs)]
 #![feature(shared)]
+#![feature(sip_hash_13)]
 #![feature(slice_bytes)]
 #![feature(slice_concat_ext)]
 #![feature(slice_patterns)]