Rollup merge of #120936 - ripytide:master, r=Amanieu

improve `btree_cursors` functions documentation

As suggested by ``@Amanieu`` (and others) in #107540 (https://github.com/rust-lang/rust/issues/107540#issuecomment-1937760547)

Improvements:
- Document exact behavior of `{upper/lower}_bound{,_mut}` with each of the three `Bound` types using unambigous words `{greatest,greater,smallest,smaller,before,after}`.
- Added another doc-example for the `Bound::Unbounded` for each of the methods
- Changed doc-example to use From<[T; N]> rather than lots of `insert()`s which requires a mutable map which clutters the example when `mut` may not be required for the method (such as for `{upper,lower}_bound`.
- Removed `# Panics` section from `insert_{before,after}` methods since they were changed to return an error instead a while ago.
- Reworded some phrases to be more consistent with the more regular `BTreeMap` methods such as calling entries "key-value" rather than "element"s.

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

@@ -2522,10 +2522,17 @@ pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
-    /// Returns a [`Cursor`] pointing to the first gap above the given bound.
+    /// Returns a [`Cursor`] pointing at the gap before the smallest key
+    /// greater than the given bound.
     ///
-    /// Passing [`Bound::Unbounded`] will return a cursor pointing to the start
+    /// Passing `Bound::Included(x)` will return a cursor pointing to the
-    /// of the map.
+    /// gap before the smallest key greater than or equal to `x`.
+    ///
+    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
+    /// gap before the smallest key greater than `x`.
+    ///
+    /// Passing `Bound::Unbounded` will return a cursor pointing to the
+    /// gap before the smallest key in the map.
     ///
     /// # Examples
     ///
@@ -2535,17 +2542,24 @@ pub const fn is_empty(&self) -> bool {
     /// use std::collections::BTreeMap;
     /// use std::ops::Bound;
     ///
-    /// let mut a = BTreeMap::new();
+    /// let map = BTreeMap::from([
-    /// a.insert(1, "a");
+    ///     (1, "a"),
-    /// a.insert(2, "b");
+    ///     (2, "b"),
-    /// a.insert(3, "c");
+    ///     (3, "c"),
-    /// a.insert(4, "d");
+    ///     (4, "d"),
-    /// let cursor = a.lower_bound(Bound::Included(&2));
+    /// ]);
+    ///
+    /// let cursor = map.lower_bound(Bound::Included(&2));
     /// assert_eq!(cursor.peek_prev(), Some((&1, &"a")));
     /// assert_eq!(cursor.peek_next(), Some((&2, &"b")));
-    /// let cursor = a.lower_bound(Bound::Excluded(&2));
+    ///
+    /// let cursor = map.lower_bound(Bound::Excluded(&2));
     /// assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
     /// assert_eq!(cursor.peek_next(), Some((&3, &"c")));
+    ///
+    /// let cursor = map.lower_bound(Bound::Unbounded);
+    /// assert_eq!(cursor.peek_prev(), None);
+    /// assert_eq!(cursor.peek_next(), Some((&1, &"a")));
     /// ```
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn lower_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
@@ -2561,11 +2575,17 @@ pub fn lower_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
         Cursor { current: Some(edge), root: self.root.as_ref() }
     }
 
-    /// Returns a [`CursorMut`] pointing to the first gap above the given bound.
+    /// Returns a [`CursorMut`] pointing at the gap before the smallest key
+    /// greater than the given bound.
     ///
+    /// Passing `Bound::Included(x)` will return a cursor pointing to the
+    /// gap before the smallest key greater than or equal to `x`.
     ///
-    /// Passing [`Bound::Unbounded`] will return a cursor pointing to the start
+    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
-    /// of the map.
+    /// gap before the smallest key greater than `x`.
+    ///
+    /// Passing `Bound::Unbounded` will return a cursor pointing to the
+    /// gap before the smallest key in the map.
     ///
     /// # Examples
     ///
@@ -2575,17 +2595,24 @@ pub fn lower_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
     /// use std::collections::BTreeMap;
     /// use std::ops::Bound;
     ///
-    /// let mut a = BTreeMap::new();
+    /// let mut map = BTreeMap::from([
-    /// a.insert(1, "a");
+    ///     (1, "a"),
-    /// a.insert(2, "b");
+    ///     (2, "b"),
-    /// a.insert(3, "c");
+    ///     (3, "c"),
-    /// a.insert(4, "d");
+    ///     (4, "d"),
-    /// let mut cursor = a.lower_bound_mut(Bound::Included(&2));
+    /// ]);
+    ///
+    /// let mut cursor = map.lower_bound_mut(Bound::Included(&2));
     /// assert_eq!(cursor.peek_prev(), Some((&1, &mut "a")));
     /// assert_eq!(cursor.peek_next(), Some((&2, &mut "b")));
-    /// let mut cursor = a.lower_bound_mut(Bound::Excluded(&2));
+    ///
+    /// let mut cursor = map.lower_bound_mut(Bound::Excluded(&2));
     /// assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
     /// assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));
+    ///
+    /// let mut cursor = map.lower_bound_mut(Bound::Unbounded);
+    /// assert_eq!(cursor.peek_prev(), None);
+    /// assert_eq!(cursor.peek_next(), Some((&1, &mut "a")));
     /// ```
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn lower_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
@@ -2618,10 +2645,17 @@ pub fn lower_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_,
         }
     }
 
-    /// Returns a [`Cursor`] pointing at the last gap below the given bound.
+    /// Returns a [`Cursor`] pointing at the gap after the greatest key
+    /// smaller than the given bound.
     ///
-    /// Passing [`Bound::Unbounded`] will return a cursor pointing to the end
+    /// Passing `Bound::Included(x)` will return a cursor pointing to the
-    /// of the map.
+    /// gap after the greatest key smaller than or equal to `x`.
+    ///
+    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
+    /// gap after the greatest key smaller than `x`.
+    ///
+    /// Passing `Bound::Unbounded` will return a cursor pointing to the
+    /// gap after the greatest key in the map.
     ///
     /// # Examples
     ///
@@ -2631,17 +2665,24 @@ pub fn lower_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_,
     /// use std::collections::BTreeMap;
     /// use std::ops::Bound;
     ///
-    /// let mut a = BTreeMap::new();
+    /// let map = BTreeMap::from([
-    /// a.insert(1, "a");
+    ///     (1, "a"),
-    /// a.insert(2, "b");
+    ///     (2, "b"),
-    /// a.insert(3, "c");
+    ///     (3, "c"),
-    /// a.insert(4, "d");
+    ///     (4, "d"),
-    /// let cursor = a.upper_bound(Bound::Included(&3));
+    /// ]);
+    ///
+    /// let cursor = map.upper_bound(Bound::Included(&3));
     /// assert_eq!(cursor.peek_prev(), Some((&3, &"c")));
     /// assert_eq!(cursor.peek_next(), Some((&4, &"d")));
-    /// let cursor = a.upper_bound(Bound::Excluded(&3));
+    ///
+    /// let cursor = map.upper_bound(Bound::Excluded(&3));
     /// assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
     /// assert_eq!(cursor.peek_next(), Some((&3, &"c")));
+    ///
+    /// let cursor = map.upper_bound(Bound::Unbounded);
+    /// assert_eq!(cursor.peek_prev(), Some((&4, &"d")));
+    /// assert_eq!(cursor.peek_next(), None);
     /// ```
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn upper_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
@@ -2657,10 +2698,17 @@ pub fn upper_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
         Cursor { current: Some(edge), root: self.root.as_ref() }
     }
 
-    /// Returns a [`CursorMut`] pointing at the last gap below the given bound.
+    /// Returns a [`CursorMut`] pointing at the gap after the greatest key
+    /// smaller than the given bound.
     ///
-    /// Passing [`Bound::Unbounded`] will return a cursor pointing to the end
+    /// Passing `Bound::Included(x)` will return a cursor pointing to the
-    /// of the map.
+    /// gap after the greatest key smaller than or equal to `x`.
+    ///
+    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
+    /// gap after the greatest key smaller than `x`.
+    ///
+    /// Passing `Bound::Unbounded` will return a cursor pointing to the
+    /// gap after the greatest key in the map.
     ///
     /// # Examples
     ///
@@ -2670,17 +2718,24 @@ pub fn upper_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
     /// use std::collections::BTreeMap;
     /// use std::ops::Bound;
     ///
-    /// let mut a = BTreeMap::new();
+    /// let mut map = BTreeMap::from([
-    /// a.insert(1, "a");
+    ///     (1, "a"),
-    /// a.insert(2, "b");
+    ///     (2, "b"),
-    /// a.insert(3, "c");
+    ///     (3, "c"),
-    /// a.insert(4, "d");
+    ///     (4, "d"),
-    /// let mut cursor = a.upper_bound_mut(Bound::Included(&3));
+    /// ]);
+    ///
+    /// let mut cursor = map.upper_bound_mut(Bound::Included(&3));
     /// assert_eq!(cursor.peek_prev(), Some((&3, &mut "c")));
     /// assert_eq!(cursor.peek_next(), Some((&4, &mut "d")));
-    /// let mut cursor = a.upper_bound_mut(Bound::Excluded(&3));
+    ///
+    /// let mut cursor = map.upper_bound_mut(Bound::Excluded(&3));
     /// assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
     /// assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));
+    ///
+    /// let mut cursor = map.upper_bound_mut(Bound::Unbounded);
+    /// assert_eq!(cursor.peek_prev(), Some((&4, &mut "d")));
+    /// assert_eq!(cursor.peek_next(), None);
     /// ```
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn upper_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
@@ -3040,8 +3095,8 @@ pub fn as_cursor(&self) -> Cursor<'_, K, V> {
 
 // Now the tree editing operations
 impl<'a, K: Ord, V, A: Allocator + Clone> CursorMutKey<'a, K, V, A> {
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMutKey` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap before the
     /// newly inserted element.
@@ -3083,8 +3138,8 @@ pub unsafe fn insert_after_unchecked(&mut self, key: K, value: V) {
         *self.length += 1;
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMutKey` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap after the
     /// newly inserted element.
@@ -3129,19 +3184,16 @@ pub unsafe fn insert_before_unchecked(&mut self, key: K, value: V) {
         *self.length += 1;
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMutKey` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap before the
     /// newly inserted element.
     ///
-    /// # Panics
+    /// If the inserted key is not greater than the key before the cursor
-    ///
+    /// (if any), or if it not less than the key after the cursor (if any),
-    /// This function panics if:
+    /// then an [`UnorderedKeyError`] is returned since this would
-    /// - the given key compares less than or equal to the current element (if
+    /// invalidate the [`Ord`] invariant between the keys of the map.
-    ///   any).
-    /// - the given key compares greater than or equal to the next element (if
-    ///   any).
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn insert_after(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
         if let Some((prev, _)) = self.peek_prev() {
@@ -3160,19 +3212,16 @@ pub fn insert_after(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError
         Ok(())
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMutKey` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap after the
     /// newly inserted element.
     ///
-    /// # Panics
+    /// If the inserted key is not greater than the key before the cursor
-    ///
+    /// (if any), or if it not less than the key after the cursor (if any),
-    /// This function panics if:
+    /// then an [`UnorderedKeyError`] is returned since this would
-    /// - the given key compares greater than or equal to the current element
+    /// invalidate the [`Ord`] invariant between the keys of the map.
-    ///   (if any).
-    /// - the given key compares less than or equal to the previous element (if
-    ///   any).
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn insert_before(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
         if let Some((prev, _)) = self.peek_prev() {
@@ -3239,10 +3288,10 @@ pub fn remove_prev(&mut self) -> Option<(K, V)> {
 }
 
 impl<'a, K: Ord, V, A: Allocator + Clone> CursorMut<'a, K, V, A> {
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMut` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
-    /// After the insertion the cursor will be pointing at the gap before the
+    /// After the insertion the cursor will be pointing at the gap after the
     /// newly inserted element.
     ///
     /// # Safety
@@ -3257,8 +3306,8 @@ pub unsafe fn insert_after_unchecked(&mut self, key: K, value: V) {
         unsafe { self.inner.insert_after_unchecked(key, value) }
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMut` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap after the
     /// newly inserted element.
@@ -3275,37 +3324,31 @@ pub unsafe fn insert_before_unchecked(&mut self, key: K, value: V) {
         unsafe { self.inner.insert_before_unchecked(key, value) }
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMut` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap before the
     /// newly inserted element.
     ///
-    /// # Panics
+    /// If the inserted key is not greater than the key before the cursor
-    ///
+    /// (if any), or if it not less than the key after the cursor (if any),
-    /// This function panics if:
+    /// then an [`UnorderedKeyError`] is returned since this would
-    /// - the given key compares less than or equal to the current element (if
+    /// invalidate the [`Ord`] invariant between the keys of the map.
-    ///   any).
-    /// - the given key compares greater than or equal to the next element (if
-    ///   any).
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn insert_after(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
         self.inner.insert_after(key, value)
     }
 
-    /// Inserts a new element into the `BTreeMap` in the gap that the
+    /// Inserts a new key-value pair into the map in the gap that the
-    /// `CursorMut` is currently pointing to.
+    /// cursor is currently pointing to.
     ///
     /// After the insertion the cursor will be pointing at the gap after the
     /// newly inserted element.
     ///
-    /// # Panics
+    /// If the inserted key is not greater than the key before the cursor
-    ///
+    /// (if any), or if it not less than the key after the cursor (if any),
-    /// This function panics if:
+    /// then an [`UnorderedKeyError`] is returned since this would
-    /// - the given key compares greater than or equal to the current element
+    /// invalidate the [`Ord`] invariant between the keys of the map.
-    ///   (if any).
-    /// - the given key compares less than or equal to the previous element (if
-    ///   any).
     #[unstable(feature = "btree_cursors", issue = "107540")]
     pub fn insert_before(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
         self.inner.insert_before(key, value)