Replace FxHash with AHash as the default hasher

Author: Amanieu

Cargo.toml

@@ -13,6 +13,9 @@ edition = "2018"
 build = "build.rs"
 
 [dependencies]
+# For the default hasher
+ahash = { version = "0.2", optional = true }
+
 # For external trait impls
 rayon = { version = "1.0", optional = true }
 serde = { version = "1.0.25", default-features = false, optional = true }
@@ -34,7 +37,7 @@ serde_test = "1.0"
 doc-comment = "0.3.1"
 
 [features]
-default = []
+default = ["ahash"]
 nightly = []
 rustc-internal-api = []
 rustc-dep-of-std = ["nightly", "core", "compiler_builtins", "alloc", "rustc-internal-api"]

src/fx.rs

@@ -44,7 +44,6 @@ doc_comment::doctest!("../README.md");
 mod macros;
 
 mod external_trait_impls;
-mod fx;
 mod map;
 mod raw;
 #[cfg(feature = "rustc-internal-api")]

src/lib.rs

@@ -8,7 +8,13 @@ use core::marker::PhantomData;
 use core::mem;
 use core::ops::Index;
 
-pub use crate::fx::FxHashBuilder as DefaultHashBuilder;
+/// Default hasher for `HashMap`.
+#[cfg(feature = "ahash")]
+pub type DefaultHashBuilder = ahash::ABuildHasher;
+
+/// Dummy default hasher for `HashMap`.
+#[cfg(not(feature = "ahash"))]
+pub enum DefaultHashBuilder {}
 
 /// A hash map implemented with quadratic probing and SIMD lookup.
 ///
@@ -183,7 +189,6 @@ pub use crate::fx::FxHashBuilder as DefaultHashBuilder;
 ///     // use the values stored in map
 /// }
 /// ```
-
 #[derive(Clone)]
 pub struct HashMap<K, V, S = DefaultHashBuilder> {
     pub(crate) hash_builder: S,
@@ -197,6 +202,7 @@ pub(crate) fn make_hash<K: Hash + ?Sized>(hash_builder: &impl BuildHasher, val:
     state.finish()
 }
 
+#[cfg(feature = "ahash")]
 impl<K, V> HashMap<K, V, DefaultHashBuilder> {
     /// Creates an empty `HashMap`.
     ///
@@ -227,7 +233,7 @@ impl<K, V> HashMap<K, V, DefaultHashBuilder> {
     /// ```
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
-        Self::with_capacity_and_hasher(capacity, DefaultHashBuilder::default())
+        Self::with_capacity_and_hasher(capacity, Default::default())
     }
 }
 

src/map.rs

@@ -116,6 +116,7 @@ pub struct HashSet<T, S = DefaultHashBuilder> {
     pub(crate) map: HashMap<T, (), S>,
 }
 
+#[cfg(feature = "ahash")]
 impl<T: Hash + Eq> HashSet<T, DefaultHashBuilder> {
     /// Creates an empty `HashSet`.
     ///