Support hashbrown hash maps and sets with or without use_std

Add an optional `hashbrown` feature that switches the underlying hash
map implementation from `std::collections` to `hashbrown` and makes
items that depend on hash maps available without `use_std` (although it
still requires and implicitly enables `use_alloc`).

This addresses the motivating use case of [rust-itertools#605][] (which is the same
use case that I am interested in) but does not implement a generic
approach as the issue suggests. [rust-itertools#901][] would be a better solution long
term and would make these changes obsolete, but the PR appears to be
stalled and I wanted a simple solution that allows using APIs like
`unique`/`unique_by` with `no_std` now.

It also partially addresses [rust-itertools#322][] because enabling this feature does
switch to *a* faster hash algorithm, it just doesn't allow to specify
which one exactly (currently the default hasher in `hashbrown` is
`foldhash`, so this is what's going to be used until/unless changed
upstream in the future).

[rust-itertools#605]: rust-itertools#605
[rust-itertools#901]: rust-itertools#901
[rust-itertools#322]: rust-itertools#322

Author: aqrln

Cargo.toml

@@ -24,6 +24,10 @@ test = false
 
 [dependencies]
 either = { version = "1.0", default-features = false }
+hashbrown = { version = "0.15", optional = true, default-features = false, features = [
+    "alloc",
+    "default-hasher",
+] }
 
 [dev-dependencies]
 rand = "0.7"
@@ -36,6 +40,7 @@ quickcheck = { version = "0.9", default-features = false }
 default = ["use_std"]
 use_std = ["use_alloc", "either/use_std"]
 use_alloc = []
+hashbrown = ["dep:hashbrown", "use_alloc"]
 
 [profile]
 bench = { debug = true }

src/duplicates_impl.rs

@@ -1,6 +1,9 @@
 use std::hash::Hash;
 
 mod private {
+    #[cfg(feature = "hashbrown")]
+    use hashbrown::HashMap;
+    #[cfg(not(feature = "hashbrown"))]
     use std::collections::HashMap;
     use std::fmt;
     use std::hash::Hash;

src/group_map.rs

@@ -1,5 +1,8 @@
-#![cfg(feature = "use_std")]
+#![cfg(any(feature = "use_std", feature = "hashbrown"))]
 
+#[cfg(feature = "hashbrown")]
+use hashbrown::HashMap;
+#[cfg(not(feature = "hashbrown"))]
 use std::collections::HashMap;
 use std::hash::Hash;
 use std::iter::Iterator;

src/grouping_map.rs

@@ -2,7 +2,10 @@ use crate::{
     adaptors::map::{MapSpecialCase, MapSpecialCaseFn},
     MinMaxResult,
 };
+#[cfg(feature = "hashbrown")]
+use hashbrown::HashMap;
 use std::cmp::Ordering;
+#[cfg(not(feature = "hashbrown"))]
 use std::collections::HashMap;
 use std::hash::Hash;
 use std::iter::Iterator;

src/lib.rs

@@ -47,6 +47,13 @@
 //!   - Enabled by default.
 //!   - Enables any item that depend on allocations (like `chunk_by`,
 //!     `kmerge`, `join` and many more).
+//! - `hashbrown`
+//!   - Disabled by default.
+//!   - Enables `use_alloc` and uses `hashbrown::HashMap` instead of
+//!     `std::collections::HashMap` to enable some items that depend on hash maps
+//!     without `use_std`. It is also possible to use this feature together with
+//!     `use_std` if you prefer `hashbrown::HashMap` (for example, because of the
+//!     different default hasher).
 //!
 //! ## Rust Version
 //!
@@ -64,11 +71,11 @@ use alloc::{collections::VecDeque, string::String, vec::Vec};
 pub use either::Either;
 
 use core::borrow::Borrow;
+#[cfg(feature = "hashbrown")]
+use hashbrown::{HashMap, HashSet};
 use std::cmp::Ordering;
-#[cfg(feature = "use_std")]
-use std::collections::HashMap;
-#[cfg(feature = "use_std")]
-use std::collections::HashSet;
+#[cfg(all(feature = "use_std", not(feature = "hashbrown")))]
+use std::collections::{HashMap, HashSet};
 use std::fmt;
 #[cfg(feature = "use_alloc")]
 use std::fmt::Write;
@@ -102,7 +109,7 @@ pub mod structs {
     #[cfg(feature = "use_alloc")]
     pub use crate::combinations_with_replacement::CombinationsWithReplacement;
     pub use crate::cons_tuples_impl::ConsTuples;
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     pub use crate::duplicates_impl::{Duplicates, DuplicatesBy};
     pub use crate::exactly_one_err::ExactlyOneError;
     pub use crate::flatten_ok::FlattenOk;
@@ -140,7 +147,7 @@ pub mod structs {
     #[cfg(feature = "use_alloc")]
     pub use crate::tee::Tee;
     pub use crate::tuple_impl::{CircularTupleWindows, TupleBuffer, TupleWindows, Tuples};
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     pub use crate::unique_impl::{Unique, UniqueBy};
     pub use crate::with_position::WithPosition;
     pub use crate::zip_eq_impl::ZipEq;
@@ -187,18 +194,18 @@ mod combinations_with_replacement;
 mod concat_impl;
 mod cons_tuples_impl;
 mod diff;
-#[cfg(feature = "use_std")]
+#[cfg(any(feature = "use_std", feature = "hashbrown"))]
 mod duplicates_impl;
 mod exactly_one_err;
 #[cfg(feature = "use_alloc")]
 mod extrema_set;
 mod flatten_ok;
 mod format;
-#[cfg(feature = "use_alloc")]
+#[cfg(any(feature = "use_std", feature = "hashbrown"))]
 mod group_map;
 #[cfg(feature = "use_alloc")]
 mod groupbylazy;
-#[cfg(feature = "use_std")]
+#[cfg(any(feature = "use_std", feature = "hashbrown"))]
 mod grouping_map;
 mod intersperse;
 mod iter_index;
@@ -1415,7 +1422,7 @@ pub trait Itertools: Iterator {
     /// itertools::assert_equal(data.into_iter().duplicates(),
     ///                         vec![20, 10]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn duplicates(self) -> Duplicates<Self>
     where
         Self: Sized,
@@ -1441,7 +1448,7 @@ pub trait Itertools: Iterator {
     /// itertools::assert_equal(data.into_iter().duplicates_by(|s| s.len()),
     ///                         vec!["aa", "c"]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn duplicates_by<V, F>(self, f: F) -> DuplicatesBy<Self, V, F>
     where
         Self: Sized,
@@ -1469,7 +1476,7 @@ pub trait Itertools: Iterator {
     /// itertools::assert_equal(data.into_iter().unique(),
     ///                         vec![10, 20, 30, 40, 50]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn unique(self) -> Unique<Self>
     where
         Self: Sized,
@@ -1496,7 +1503,7 @@ pub trait Itertools: Iterator {
     /// itertools::assert_equal(data.into_iter().unique_by(|s| s.len()),
     ///                         vec!["a", "bb", "ccc"]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn unique_by<V, F>(self, f: F) -> UniqueBy<Self, V, F>
     where
         Self: Sized,
@@ -3744,7 +3751,7 @@ pub trait Itertools: Iterator {
     /// assert_eq!(lookup[&2], vec![12, 42]);
     /// assert_eq!(lookup[&3], vec![13, 33]);
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn into_group_map<K, V>(self) -> HashMap<K, Vec<V>>
     where
         Self: Iterator<Item = (K, V)> + Sized,
@@ -3780,7 +3787,7 @@ pub trait Itertools: Iterator {
     ///     30,
     /// );
     /// ```
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn into_group_map_by<K, V, F>(self, f: F) -> HashMap<K, Vec<V>>
     where
         Self: Iterator<Item = V> + Sized,
@@ -3799,7 +3806,7 @@ pub trait Itertools: Iterator {
     ///
     /// See [`GroupingMap`] for more informations
     /// on what operations are available.
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn into_grouping_map<K, V>(self) -> GroupingMap<Self>
     where
         Self: Iterator<Item = (K, V)> + Sized,
@@ -3816,7 +3823,7 @@ pub trait Itertools: Iterator {
     ///
     /// See [`GroupingMap`] for more informations
     /// on what operations are available.
-    #[cfg(feature = "use_std")]
+    #[cfg(any(feature = "use_std", feature = "hashbrown"))]
     fn into_grouping_map_by<K, V, F>(self, key_mapper: F) -> GroupingMapBy<Self, F>
     where
         Self: Iterator<Item = V> + Sized,

src/unique_impl.rs

@@ -1,5 +1,7 @@
-use std::collections::hash_map::Entry;
-use std::collections::HashMap;
+#[cfg(feature = "hashbrown")]
+use hashbrown::{hash_map::Entry, HashMap};
+#[cfg(not(feature = "hashbrown"))]
+use std::collections::{hash_map::Entry, HashMap};
 use std::fmt;
 use std::hash::Hash;
 use std::iter::FusedIterator;