Change DefaultHashBuilder to an opaque newtype

This hides the underlying hasher as an implementation detail, so it's no
longer a public dependency. A newtype `DefaultHasher` is also added here
to wrap the underlying `<_ as BuildHasher>::Hasher` type.

This is a breaking change now, requiring a semver bump, but it will
allow later changes to the hasher without breaking.

Author: cuviper

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "hashbrown"
-version = "0.15.5"
+version = "0.16.0"
 authors = ["Amanieu d'Antras <[email protected]>"]
 description = "A Rust port of Google's SwissTable hash map"
 license = "MIT OR Apache-2.0"
@@ -30,6 +30,7 @@ allocator-api2 = { version = "0.2.9", optional = true, default-features = false,
 ] }
 
 # Equivalent trait which can be shared with other hash table implementations.
+# NB: this is a public dependency because `Equivalent` is re-exported!
 equivalent = { version = "1.0", optional = true, default-features = false }
 
 [dev-dependencies]

src/hasher.rs

@@ -0,0 +1,70 @@
+#[cfg(feature = "default-hasher")]
+use {
+    core::hash::{BuildHasher, Hasher},
+    foldhash::fast::RandomState,
+};
+
+/// Default hash builder for the `S` type parameter of
+/// [`HashMap`](crate::HashMap) and [`HashSet`](crate::HashSet).
+///
+/// This only implements `BuildHasher` when the "default-hasher" crate feature
+/// is enabled; otherwise it just serves as a placeholder, and a custom `S` type
+/// must be used to have a fully functional `HashMap` or `HashSet`.
+#[derive(Clone, Debug, Default)]
+pub struct DefaultHashBuilder {
+    #[cfg(feature = "default-hasher")]
+    inner: RandomState,
+}
+
+#[cfg(feature = "default-hasher")]
+impl BuildHasher for DefaultHashBuilder {
+    type Hasher = DefaultHasher;
+
+    #[inline(always)]
+    fn build_hasher(&self) -> Self::Hasher {
+        DefaultHasher {
+            inner: self.inner.build_hasher(),
+        }
+    }
+}
+
+/// Default hasher for [`HashMap`](crate::HashMap) and [`HashSet`](crate::HashSet).
+#[cfg(feature = "default-hasher")]
+#[derive(Clone)]
+pub struct DefaultHasher {
+    inner: <RandomState as BuildHasher>::Hasher,
+}
+
+#[cfg(feature = "default-hasher")]
+macro_rules! forward_writes {
+    ($( $write:ident ( $ty:ty ) , )*) => {$(
+        #[inline(always)]
+        fn $write(&mut self, arg: $ty) {
+            self.inner.$write(arg);
+        }
+    )*}
+}
+
+#[cfg(feature = "default-hasher")]
+impl Hasher for DefaultHasher {
+    forward_writes! {
+        write(&[u8]),
+        write_u8(u8),
+        write_u16(u16),
+        write_u32(u32),
+        write_u64(u64),
+        write_u128(u128),
+        write_usize(usize),
+        write_i8(i8),
+        write_i16(i16),
+        write_i32(i32),
+        write_i64(i64),
+        write_i128(i128),
+        write_isize(isize),
+    }
+
+    #[inline(always)]
+    fn finish(&self) -> u64 {
+        self.inner.finish()
+    }
+}

src/lib.rs

@@ -47,14 +47,6 @@
     feature(stdarch_loongarch)
 )]
 
-/// Default hasher for [`HashMap`] and [`HashSet`].
-#[cfg(feature = "default-hasher")]
-pub type DefaultHashBuilder = foldhash::fast::RandomState;
-
-/// Dummy default hasher for [`HashMap`] and [`HashSet`].
-#[cfg(not(feature = "default-hasher"))]
-pub enum DefaultHashBuilder {}
-
 #[cfg(test)]
 #[macro_use]
 extern crate std;
@@ -71,6 +63,7 @@ doc_comment::doctest!("../README.md");
 mod macros;
 
 mod control;
+mod hasher;
 mod raw;
 mod util;
 
@@ -84,6 +77,10 @@ mod scopeguard;
 mod set;
 mod table;
 
+pub use crate::hasher::DefaultHashBuilder;
+#[cfg(feature = "default-hasher")]
+pub use crate::hasher::DefaultHasher;
+
 pub mod hash_map {
     //! A hash map implemented with quadratic probing and SIMD lookup.
     pub use crate::map::*;