Add a way to seed FxHasher with random seeds

This commit adds an optional dependency on `rand` (behind a `rand`)
feature that allows seeding `FxHasher` with random seeds.

This is done via `FxRandomState` that implemented similar to
`std::collections::hash_map::RandomState`.

`FxHashMapRnd` and `FxHashSetRnd` are also introduced as type aliases
to `HashMap` and `HashSet` with `S = FxRandomState`.

Author: WaffleLapkin

Cargo.toml

@@ -9,5 +9,9 @@ keywords = ["hash", "fxhash", "rustc"]
 repository = "https://github.com/rust-lang-nursery/rustc-hash"
 
 [features]
-std = []
 default = ["std"]
+std = []
+rand = ["dep:rand", "std"]
+
+[dependencies]
+rand = { version = "0.8", optional = true }

src/lib.rs

@@ -28,6 +28,9 @@
 #[cfg(feature = "std")]
 extern crate std;
 
+#[cfg(feature = "rand")]
+extern crate rand;
+
 use core::convert::TryInto;
 use core::default::Default;
 #[cfg(feature = "std")]
@@ -46,6 +49,14 @@ pub type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
 #[cfg(feature = "std")]
 pub type FxHashSet<V> = HashSet<V, BuildHasherDefault<FxHasher>>;
 
+/// Type alias for a hashmap using the `fx` hash algorithm with [`FxRandomState`].
+#[cfg(feature = "rand")]
+pub type FxHashMapRnd<K, V> = HashMap<K, V, FxRandomState>;
+
+/// Type alias for a hashmap using the `fx` hash algorithm with [`FxRandomState`].
+#[cfg(feature = "rand")]
+pub type FxHashSetRnd<V> = HashSet<V, FxRandomState>;
+
 /// A speedy hash algorithm for use within rustc. The hashmap in liballoc
 /// by default uses SipHash which isn't quite as speedy as we want. In the
 /// compiler we're not really worried about DOS attempts, so we use a fast
@@ -61,6 +72,16 @@ pub struct FxHasher {
     hash: usize,
 }
 
+/// `FxRandomState` is an alternative state for `HashMap` types.
+///
+/// A particular instance `FxRandomState` will create the same instances of
+/// [`Hasher`], but the hashers created by two different `FxRandomState`
+/// instances are unlikely to produce the same result for the same values.
+#[cfg(feature = "rand")]
+pub struct FxRandomState {
+    seed: usize,
+}
+
 #[cfg(target_pointer_width = "32")]
 const K: usize = 0x9e3779b9;
 #[cfg(target_pointer_width = "64")]
@@ -153,3 +174,38 @@ impl Hasher for FxHasher {
         self.hash as u64
     }
 }
+
+#[cfg(feature = "rand")]
+impl FxRandomState {
+    // Constructs a new `FxRandomState` that is initialized with random seed.
+    pub fn new() -> FxRandomState {
+        use rand::Rng;
+        use std::{cell::Cell, thread_local};
+
+        // This mirrors what `std::collections::hash_map::RandomState` does.
+        //
+        // Basically
+        // 1. Cache result of the rng in a thread local, so repeatedly
+        //    creating maps is cheaper
+        // 2. Change the cached result on every creation, so maps created
+        //    on the same thread don't have the same iteration order
+        thread_local!(static SEED: Cell<usize> = {
+            Cell::new(rand::thread_rng().gen())
+        });
+
+        SEED.with(|seed| {
+            let s = seed.get();
+            seed.set(s.wrapping_add(1));
+            FxRandomState { seed: s }
+        })
+    }
+}
+
+#[cfg(feature = "rand")]
+impl core::hash::BuildHasher for FxRandomState {
+    type Hasher = FxHasher;
+
+    fn build_hasher(&self) -> Self::Hasher {
+        FxHasher { hash: self.seed }
+    }
+}