Reintroduce GroupingMap[By] with one more generic parameter

This is a breaking change! However, this is mostly inferred.
Note that it breaks (only) laziness tests because the `GroupingMap` object is not used (I could write any type but `u8` is one of the possible returned types). If it was used then it would have been inferred.

I copied/pasted the code of old `into_grouping_map[_by]` methods, added the generic `R` and updated each function body with `_in(.., HashMap::new())`.
The old types are aliases of the new ones.

src/grouping_map.rs

@@ -9,6 +9,35 @@ use std::cmp::Ordering;
 use std::iter::Iterator;
 use std::ops::{Add, Mul};
 
+#[cfg(feature = "use_std")]
+pub use with_hashmap::{GroupingMap, GroupingMapBy};
+
+#[cfg(feature = "use_std")]
+mod with_hashmap {
+    use super::*;
+    use std::collections::HashMap;
+
+    // This is used to infer `K` when `I::Item = (K, V)` since we can't write `I::Item.0`.
+    pub trait KeyValue {
+        type Key;
+    }
+
+    impl<K, V> KeyValue for (K, V) {
+        type Key = K;
+    }
+
+    /// `GroupingMap` is an intermediate struct for efficient group-and-fold operations.
+    ///
+    /// See [`GroupingGenericMap`] for more informations.
+    pub type GroupingMap<I, R> =
+        GroupingGenericMap<I, HashMap<<<I as Iterator>::Item as KeyValue>::Key, R>>;
+
+    /// `GroupingMapBy` is an intermediate struct for efficient group-and-fold operations.
+    ///
+    /// See [`GroupingGenericMap`] for more informations.
+    pub type GroupingMapBy<I, F, R> = GroupingMap<MapForGrouping<I, F>, R>;
+}
+
 /// A wrapper to allow for an easy [`into_grouping_map_by`](crate::Itertools::into_grouping_map_by)
 pub type MapForGrouping<I, F> = MapSpecialCase<I, GroupingMapFn<F>>;
 

src/lib.rs

@@ -113,6 +113,8 @@ pub mod structs {
     pub use crate::groupbylazy::{Chunk, ChunkBy, Chunks, Group, Groups, IntoChunks};
     #[cfg(feature = "use_alloc")]
     pub use crate::grouping_map::{GroupingGenericMap, GroupingGenericMapBy};
+    #[cfg(feature = "use_std")]
+    pub use crate::grouping_map::{GroupingMap, GroupingMapBy};
     pub use crate::intersperse::{Intersperse, IntersperseWith};
     #[cfg(feature = "use_alloc")]
     pub use crate::kmerge_impl::{KMerge, KMergeBy};
@@ -3340,6 +3342,42 @@ pub trait Itertools: Iterator {
         group_map::into_group_map_by(self, f)
     }
 
+    /// Constructs a `GroupingMap` to be used later with one of the efficient
+    /// group-and-fold operations it allows to perform.
+    ///
+    /// The input iterator must yield item in the form of `(K, V)` where the
+    /// value of type `K` will be used as key to identify the groups and the
+    /// value of type `V` as value for the folding operation.
+    ///
+    /// See [`GroupingGenericMap`] for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map<K, V, R>(self) -> GroupingMap<Self, R>
+    where
+        Self: Iterator<Item = (K, V)> + Sized,
+        K: Hash + Eq,
+    {
+        self.into_grouping_map_in(HashMap::new())
+    }
+
+    /// Constructs a `GroupingMap` to be used later with one of the efficient
+    /// group-and-fold operations it allows to perform.
+    ///
+    /// The values from this iterator will be used as values for the folding operation
+    /// while the keys will be obtained from the values by calling `key_mapper`.
+    ///
+    /// See [`GroupingGenericMap`] for more informations
+    /// on what operations are available.
+    #[cfg(feature = "use_std")]
+    fn into_grouping_map_by<K, V, F, R>(self, key_mapper: F) -> GroupingMapBy<Self, F, R>
+    where
+        Self: Iterator<Item = V> + Sized,
+        K: Hash + Eq,
+        F: FnMut(&V) -> K,
+    {
+        self.into_grouping_map_by_in(key_mapper, HashMap::new())
+    }
+
     /// Constructs a `GroupingGenericMap` to be used later with one of the efficient
     /// group-and-fold operations it allows to perform.
     ///

tests/laziness.rs

@@ -229,10 +229,10 @@ must_use_tests! {
     }
     // Not iterator themselves but still lazy.
     into_grouping_map {
-        let _ = Panicking.map(|x| (x, x + 1)).into_grouping_map();
+        let _ = Panicking.map(|x| (x, x + 1)).into_grouping_map::<_, _, u8>();
     }
     into_grouping_map_by {
-        let _ = Panicking.into_grouping_map_by(|x| *x);
+        let _ = Panicking.into_grouping_map_by::<_, _, _, u8>(|x| *x);
     }
     // Macros:
     iproduct {