Auto merge of #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/rust#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.

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.
     ///

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);
+}

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;

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);
+}