Auto merge of rust-lang#84111 - bstrie:hashfrom, r=joshtriplett

Stabilize `impl From<[(K, V); N]> for HashMap` (and friends)

In addition to allowing HashMap to participate in Into/From conversion, this adds the long-requested ability to use constructor-like syntax for initializing a HashMap:
```rust
let map = HashMap::from([
    (1, 2),
    (3, 4),
    (5, 6)
]);
```
This addition is highly motivated by existing precedence, e.g. it is already possible to similarly construct a Vec from a fixed-size array:
```rust
let vec = Vec::from([1, 2, 3]);
```
...and it is already possible to collect a Vec of tuples into a HashMap (and vice-versa):
```rust
let vec = Vec::from([(1, 2)]);
let map: HashMap<_, _> = vec.into_iter().collect();
let vec: Vec<(_, _)> = map.into_iter().collect();
```
...and of course it is likewise possible to collect a fixed-size array of tuples into a HashMap ([but not vice-versa just yet](rust-lang#81615)):
```rust
let arr = [(1, 2)];
let map: HashMap<_, _> = std::array::IntoIter::new(arr).collect();
```
Therefore this addition seems like a no-brainer.

As for any impl, this would be insta-stable.

library/alloc/src/collections/binary_heap.rs

@@ -209,6 +209,14 @@ use super::SpecExtend;
 /// assert!(heap.is_empty())
 /// ```
 ///
+/// A `BinaryHeap` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::BinaryHeap;
+///
+/// let heap = BinaryHeap::from([1, 5, 2]);
+/// ```
+///
 /// ## Min-heap
 ///
 /// Either `std::cmp::Reverse` or a custom `Ord` implementation can be used to
@@ -1465,6 +1473,22 @@ impl<T: Ord> From<Vec<T>> for BinaryHeap<T> {
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+impl<T: Ord, const N: usize> From<[T; N]> for BinaryHeap<T> {
+    /// ```
+    /// use std::collections::BinaryHeap;
+    ///
+    /// let mut h1 = BinaryHeap::from([1, 4, 2, 3]);
+    /// let mut h2: BinaryHeap<_> = [1, 4, 2, 3].into();
+    /// while let Some((a, b)) = h1.pop().zip(h2.pop()) {
+    ///     assert_eq!(a, b);
+    /// }
+    /// ```
+    fn from(arr: [T; N]) -> Self {
+        core::array::IntoIter::new(arr).collect()
+    }
+}
+
 #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
 impl<T> From<BinaryHeap<T>> for Vec<T> {
     /// Converts a `BinaryHeap<T>` into a `Vec<T>`.

library/alloc/src/collections/btree/map.rs

@@ -109,7 +109,20 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// }
 /// ```
 ///
-/// `BTreeMap` also implements an [`Entry API`], which allows for more complex
+/// A `BTreeMap` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::BTreeMap;
+///
+/// let solar_distance = BTreeMap::from([
+///     ("Mercury", 0.4),
+///     ("Venus", 0.7),
+///     ("Earth", 1.0),
+///     ("Mars", 1.5),
+/// ]);
+/// ```
+///
+/// `BTreeMap` implements an [`Entry API`], which allows for complex
 /// methods of getting, setting, updating and removing keys and their values:
 ///
 /// [`Entry API`]: BTreeMap::entry
@@ -2012,6 +2025,20 @@ where
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+impl<K: Ord, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V> {
+    /// ```
+    /// use std::collections::BTreeMap;
+    ///
+    /// let map1 = BTreeMap::from([(1, 2), (3, 4)]);
+    /// let map2: BTreeMap<_, _> = [(1, 2), (3, 4)].into();
+    /// assert_eq!(map1, map2);
+    /// ```
+    fn from(arr: [(K, V); N]) -> Self {
+        core::array::IntoIter::new(arr).collect()
+    }
+}
+
 impl<K, V> BTreeMap<K, V> {
     /// Gets an iterator over the entries of the map, sorted by key.
     ///

library/alloc/src/collections/btree/map/tests.rs

@@ -2173,3 +2173,10 @@ fn test_insert_remove_intertwined_ord_chaos() {
     }
     map.check_invariants();
 }
+
+#[test]
+fn from_array() {
+    let map = BTreeMap::from([(1, 2), (3, 4)]);
+    let unordered_duplicates = BTreeMap::from([(3, 4), (1, 2), (1, 2)]);
+    assert_eq!(map, unordered_duplicates);
+}

library/alloc/src/collections/btree/set.rs

@@ -59,6 +59,14 @@ use super::Recover;
 ///     println!("{}", book);
 /// }
 /// ```
+///
+/// A `BTreeSet` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::BTreeSet;
+///
+/// let set = BTreeSet::from([1, 2, 3]);
+/// ```
 #[derive(Hash, PartialEq, Eq, Ord, PartialOrd)]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "BTreeSet")]
@@ -1057,6 +1065,20 @@ impl<T: Ord> FromIterator<T> for BTreeSet<T> {
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+impl<T: Ord, const N: usize> From<[T; N]> for BTreeSet<T> {
+    /// ```
+    /// use std::collections::BTreeSet;
+    ///
+    /// let set1 = BTreeSet::from([1, 2, 3, 4]);
+    /// let set2: BTreeSet<_> = [1, 2, 3, 4].into();
+    /// assert_eq!(set1, set2);
+    /// ```
+    fn from(arr: [T; N]) -> Self {
+        core::array::IntoIter::new(arr).collect()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> IntoIterator for BTreeSet<T> {
     type Item = T;

library/alloc/src/collections/btree/set/tests.rs

@@ -738,3 +738,10 @@ fn test_split_off_large_random_sorted() {
     assert!(set.into_iter().eq(data.clone().into_iter().filter(|x| *x < key)));
     assert!(right.into_iter().eq(data.into_iter().filter(|x| *x >= key)));
 }
+
+#[test]
+fn from_array() {
+    let set = BTreeSet::from([1, 2, 3, 4]);
+    let unordered_duplicates = BTreeSet::from([4, 1, 4, 3, 2]);
+    assert_eq!(set, unordered_duplicates);
+}

library/alloc/src/collections/linked_list.rs

@@ -31,6 +31,13 @@ mod tests;
 /// The `LinkedList` allows pushing and popping elements at either end
 /// in constant time.
 ///
+/// A `LinkedList` with a known list of items can be initialized from an array:
+/// ```
+/// use std::collections::LinkedList;
+///
+/// let list = LinkedList::from([1, 2, 3]);
+/// ```
+///
 /// NOTE: It is almost always better to use `Vec` or `VecDeque` because
 /// array-based containers are generally faster,
 /// more memory efficient, and make better use of CPU cache.
@@ -1901,6 +1908,20 @@ impl<T: Hash> Hash for LinkedList<T> {
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+impl<T, const N: usize> From<[T; N]> for LinkedList<T> {
+    /// ```
+    /// use std::collections::LinkedList;
+    ///
+    /// let list1 = LinkedList::from([1, 2, 3, 4]);
+    /// let list2: LinkedList<_> = [1, 2, 3, 4].into();
+    /// assert_eq!(list1, list2);
+    /// ```
+    fn from(arr: [T; N]) -> Self {
+        core::array::IntoIter::new(arr).collect()
+    }
+}
+
 // Ensure that `LinkedList` and its read-only iterators are covariant in their type parameters.
 #[allow(dead_code)]
 fn assert_covariance() {

library/alloc/src/collections/vec_deque/mod.rs

@@ -67,6 +67,14 @@ const MAXIMUM_ZST_CAPACITY: usize = 1 << (usize::BITS - 1); // Largest possible
 /// push onto the back in this manner, and iterating over `VecDeque` goes front
 /// to back.
 ///
+/// A `VecDeque` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::VecDeque;
+///
+/// let deq = VecDeque::from([-1, 0, 1]);
+/// ```
+///
 /// Since `VecDeque` is a ring buffer, its elements are not necessarily contiguous
 /// in memory. If you want to access the elements as a single slice, such as for
 /// efficient sorting, you can use [`make_contiguous`]. It rotates the `VecDeque`
@@ -2915,3 +2923,17 @@ impl<T> From<VecDeque<T>> for Vec<T> {
         }
     }
 }
+
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+impl<T, const N: usize> From<[T; N]> for VecDeque<T> {
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let deq1 = VecDeque::from([1, 2, 3, 4]);
+    /// let deq2: VecDeque<_> = [1, 2, 3, 4].into();
+    /// assert_eq!(deq1, deq2);
+    /// ```
+    fn from(arr: [T; N]) -> Self {
+        core::array::IntoIter::new(arr).collect()
+    }
+}

library/alloc/src/vec/mod.rs

@@ -174,12 +174,13 @@ mod spec_extend;
 /// assert_eq!(vec, [7, 1, 2, 3]);
 /// ```
 ///
-/// The [`vec!`] macro is provided to make initialization more convenient:
+/// The [`vec!`] macro is provided for convenient initialization:
 ///
 /// ```
-/// let mut vec = vec![1, 2, 3];
-/// vec.push(4);
-/// assert_eq!(vec, [1, 2, 3, 4]);
+/// let mut vec1 = vec![1, 2, 3];
+/// vec1.push(4);
+/// let vec2 = Vec::from([1, 2, 3, 4]);
+/// assert_eq!(vec1, vec2);
 /// ```
 ///
 /// It can also initialize each element of a `Vec<T>` with a given value.

library/std/src/collections/hash/map.rs

@@ -124,8 +124,21 @@ use crate::sys;
 /// }
 /// ```
 ///
-/// `HashMap` also implements an [`Entry API`](#method.entry), which allows
-/// for more complex methods of getting, setting, updating and removing keys and
+/// A `HashMap` with a known list of items can be initialized from an array:
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// let solar_distance = HashMap::from([
+///     ("Mercury", 0.4),
+///     ("Venus", 0.7),
+///     ("Earth", 1.0),
+///     ("Mars", 1.5),
+/// ]);
+/// ```
+///
+/// `HashMap` implements an [`Entry API`](#method.entry), which allows
+/// for complex methods of getting, setting, updating and removing keys and
 /// their values:
 ///
 /// ```
@@ -179,27 +192,17 @@ use crate::sys;
 /// }
 ///
 /// // Use a HashMap to store the vikings' health points.
-/// let mut vikings = HashMap::new();
-///
-/// vikings.insert(Viking::new("Einar", "Norway"), 25);
-/// vikings.insert(Viking::new("Olaf", "Denmark"), 24);
-/// vikings.insert(Viking::new("Harald", "Iceland"), 12);
+/// let vikings = HashMap::from([
+///     (Viking::new("Einar", "Norway"), 25),
+///     (Viking::new("Olaf", "Denmark"), 24),
+///     (Viking::new("Harald", "Iceland"), 12),
+/// ]);
 ///
 /// // Use derived implementation to print the status of the vikings.
 /// for (viking, health) in &vikings {
 ///     println!("{:?} has {} hp", viking, health);
 /// }
 /// ```
-///
-/// A `HashMap` with fixed list of elements can be initialized from an array:
-///
-/// ```
-/// use std::collections::HashMap;
-///
-/// let timber_resources: HashMap<&str, i32> = [("Norway", 100), ("Denmark", 50), ("Iceland", 10)]
-///     .iter().cloned().collect();
-/// // use the values stored in map
-/// ```
 
 #[cfg_attr(not(test), rustc_diagnostic_item = "hashmap_type")]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1148,6 +1151,37 @@ where
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+// Note: as what is currently the most convenient built-in way to construct
+// a HashMap, a simple usage of this function must not *require* the user
+// to provide a type annotation in order to infer the third type parameter
+// (the hasher parameter, conventionally "S").
+// To that end, this impl is defined using RandomState as the concrete
+// type of S, rather than being generic over `S: BuildHasher + Default`.
+// It is expected that users who want to specify a hasher will manually use
+// `with_capacity_and_hasher`.
+// If type parameter defaults worked on impls, and if type parameter
+// defaults could be mixed with const generics, then perhaps
+// this could be generalized.
+// See also the equivalent impl on HashSet.
+impl<K, V, const N: usize> From<[(K, V); N]> for HashMap<K, V, RandomState>
+where
+    K: Eq + Hash,
+{
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let map1 = HashMap::from([(1, 2), (3, 4)]);
+    /// let map2: HashMap<_, _> = [(1, 2), (3, 4)].into();
+    /// assert_eq!(map1, map2);
+    /// ```
+    fn from(arr: [(K, V); N]) -> Self {
+        crate::array::IntoIter::new(arr).collect()
+    }
+}
+
 /// An iterator over the entries of a `HashMap`.
 ///
 /// This `struct` is created by the [`iter`] method on [`HashMap`]. See its

library/std/src/collections/hash/map/tests.rs

@@ -1085,3 +1085,15 @@ mod test_drain_filter {
         assert_eq!(map.len(), 2);
     }
 }
+
+#[test]
+fn from_array() {
+    let map = HashMap::from([(1, 2), (3, 4)]);
+    let unordered_duplicates = HashMap::from([(3, 4), (1, 2), (1, 2)]);
+    assert_eq!(map, unordered_duplicates);
+
+    // This next line must infer the hasher type parameter.
+    // If you make a change that causes this line to no longer infer,
+    // that's a problem!
+    let _must_not_require_type_annotation = HashMap::from([(1, 2)]);
+}

library/std/src/collections/hash/set.rs

@@ -95,14 +95,12 @@ use super::map::{map_try_reserve_error, RandomState};
 /// }
 /// ```
 ///
-/// A `HashSet` with fixed list of elements can be initialized from an array:
+/// A `HashSet` with a known list of items can be initialized from an array:
 ///
 /// ```
 /// use std::collections::HashSet;
 ///
-/// let viking_names: HashSet<&'static str> =
-///     [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
-/// // use the values stored in the set
+/// let viking_names = HashSet::from(["Einar", "Olaf", "Harald"]);
 /// ```
 ///
 /// [hash set]: crate::collections#use-the-set-variant-of-any-of-these-maps-when
@@ -997,6 +995,37 @@ where
     }
 }
 
+#[stable(feature = "std_collections_from_array", since = "1.56.0")]
+// Note: as what is currently the most convenient built-in way to construct
+// a HashSet, a simple usage of this function must not *require* the user
+// to provide a type annotation in order to infer the third type parameter
+// (the hasher parameter, conventionally "S").
+// To that end, this impl is defined using RandomState as the concrete
+// type of S, rather than being generic over `S: BuildHasher + Default`.
+// It is expected that users who want to specify a hasher will manually use
+// `with_capacity_and_hasher`.
+// If type parameter defaults worked on impls, and if type parameter
+// defaults could be mixed with const generics, then perhaps
+// this could be generalized.
+// See also the equivalent impl on HashMap.
+impl<T, const N: usize> From<[T; N]> for HashSet<T, RandomState>
+where
+    T: Eq + Hash,
+{
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::HashSet;
+    ///
+    /// let set1 = HashSet::from([1, 2, 3, 4]);
+    /// let set2: HashSet<_> = [1, 2, 3, 4].into();
+    /// assert_eq!(set1, set2);
+    /// ```
+    fn from(arr: [T; N]) -> Self {
+        crate::array::IntoIter::new(arr).collect()
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, S> Extend<T> for HashSet<T, S>
 where