Rollup merge of rust-lang#98125 - KarlWithK:entry_add_modify_doc, r=Dylan-DPC

Entry and_modify doc

This PR modifies the documentation for [HashMap](https://doc.rust-lang.org/std/collections/struct.HashMap.html#) and [BTreeMap](https://doc.rust-lang.org/std/collections/struct.BTreeMap.html#) by introducing examples for `and_modify`. `and_modify` is a function that tends to give more idiomatic rust code when dealing with these data structures -- yet it lacked examples and was hidden away. This PR adds that and addresses rust-lang#98122.

I've made some choices which I tried to explain in my commits. This is my first time contributing to rust, so hopefully, I made the right choices.

Author: matthiaskrgr

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

@@ -161,6 +161,9 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// // update a key, guarding against the key possibly not being set
 /// let stat = player_stats.entry("attack").or_insert(100);
 /// *stat += random_stat_buff();
+///
+/// // modify an entry before an insert with in-place mutation
+/// player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "BTreeMap")]
@@ -1211,10 +1214,12 @@ impl<K, V, A: Allocator> BTreeMap<K, V, A> {
     ///
     /// // count the number of occurrences of letters in the vec
     /// for x in ["a", "b", "a", "c", "a", "b"] {
-    ///     *count.entry(x).or_insert(0) += 1;
+    ///     count.entry(x).and_modify(|curr| *curr += 1).or_insert(1);
     /// }
     ///
     /// assert_eq!(count["a"], 3);
+    /// assert_eq!(count["b"], 2);
+    /// assert_eq!(count["c"], 1);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn entry(&mut self, key: K) -> Entry<'_, K, V, A>

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

@@ -164,6 +164,9 @@ use crate::sys;
 /// // update a key, guarding against the key possibly not being set
 /// let stat = player_stats.entry("attack").or_insert(100);
 /// *stat += random_stat_buff();
+///
+/// // modify an entry before an insert with in-place mutation
+/// player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);
 /// ```
 ///
 /// The easiest way to use `HashMap` with a custom key type is to derive [`Eq`] and [`Hash`].
@@ -829,8 +832,7 @@ where
     /// let mut letters = HashMap::new();
     ///
     /// for ch in "a short treatise on fungi".chars() {
-    ///     let counter = letters.entry(ch).or_insert(0);
-    ///     *counter += 1;
+    ///     letters.entry(ch).and_modify(|counter| *counter += 1).or_insert(1);
     /// }
     ///
     /// assert_eq!(letters[&'s'], 2);