Change DefaultHashBuilder to an opaque newtype

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]
@@ -46,7 +47,7 @@ default = ["default-hasher", "inline-more", "allocator-api2", "equivalent", "raw
 
 # Enables use of nightly features. This is only guaranteed to work on the latest
 # version of nightly Rust.
-nightly = ["bumpalo/allocator_api"]
+nightly = ["foldhash?/nightly", "bumpalo/allocator_api"]
 
 # Enables the RustcEntry API used to provide the standard library's Entry API.
 rustc-internal-api = []

src/hasher.rs

@@ -0,0 +1,77 @@
+#[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),
+    }
+
+    // feature(hasher_prefixfree_extras)
+    #[cfg(feature = "nightly")]
+    forward_writes! {
+        write_length_prefix(usize),
+        write_str(&str),
+    }
+
+    #[inline(always)]
+    fn finish(&self) -> u64 {
+        self.inner.finish()
+    }
+}

src/lib.rs

@@ -46,14 +46,10 @@
     all(feature = "nightly", target_arch = "loongarch64"),
     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_attr(
+    all(feature = "nightly", feature = "default-hasher"),
+    feature(hasher_prefixfree_extras)
+)]
 
 #[cfg(test)]
 #[macro_use]
@@ -71,6 +67,7 @@ doc_comment::doctest!("../README.md");
 mod macros;
 
 mod control;
+mod hasher;
 mod raw;
 mod util;
 
@@ -84,6 +81,10 @@ 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::*;