Fix documentation for with_capacity and reserve families of methods

Documentation for the following methods

    with_capacity
    with_capacity_in
    with_capacity_and_hasher
    reserve
    reserve_exact
    try_reserve
    try_reserve_exact

was inconsistent and often not entirely correct where they existed on the following types

    Vec
    VecDeque
    String
    OsString
    PathBuf
    BinaryHeap
    HashSet
    HashMap
    BufWriter
    LineWriter

since the allocator is allowed to allocate more than the requested capacity in all such cases, and will frequently "allocate" much more in the case of zero-sized types (I also checked BufReader, but there the docs appear to be accurate as it appears to actually allocate the exact capacity).

Some effort was made to make the documentation more consistent between types as well.

Fix with_capacity* methods for Vec

Fix *reserve*  methods for Vec

Fix docs for *reserve* methods of VecDeque

Fix docs for String::with_capacity

Fix docs for *reserve* methods of String

Fix docs for OsString::with_capacity

Fix docs for *reserve* methods on OsString

Fix docs for with_capacity* methods on HashSet

Fix docs for *reserve methods of HashSet

Fix docs for with_capacity* methods of HashMap

Fix docs for *reserve methods on HashMap

Fix expect messages about OOM in doctests

Fix docs for BinaryHeap::with_capacity

Fix docs for *reserve* methods of BinaryHeap

Fix typos

Fix docs for with_capacity on BufWriter and LineWriter

Fix consistent use of `hasher` between `HashMap` and `HashSet`

Fix warning in doc test

Add test for capacity of vec with ZST

Fix doc test error

2 files changed, 37 insertions, 30 deletions

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 3706300dcfe..197e7aaaccf 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/library/alloc/src/collections/binary_heap.rs
@@ -374,10 +374,11 @@ impl<T: Ord> BinaryHeap<T> {
         BinaryHeap { data: vec![] }
     }
 
-    /// Creates an empty `BinaryHeap` with a specific capacity.
-    /// This preallocates enough memory for `capacity` elements,
-    /// so that the `BinaryHeap` does not have to be reallocated
-    /// until it contains at least that many values.
+    /// Creates an empty `BinaryHeap` with at least the specified capacity.
+    ///
+    /// The binary heap will be able to hold at least `capacity` elements without
+    /// reallocating. This method is allowed to allocate for more elements than
+    /// `capacity`. If `capacity` is 0, the binary heap will not allocate.
     ///
     /// # Examples
     ///
@@ -906,16 +907,18 @@ impl<T> BinaryHeap<T> {
         self.data.capacity()
     }
 
-    /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
-    /// given `BinaryHeap`. Does nothing if the capacity is already sufficient.
+    /// Reserves the minimum capacity for at least `additional` elements more than
+    /// the current length. Unlike [`reserve`], this will not
+    /// deliberately over-allocate to speculatively avoid frequent allocations.
+    /// After calling `reserve_exact`, capacity will be greater than or equal to
+    /// `self.len() + additional`. Does nothing if the capacity is already
+    /// sufficient.
     ///
-    /// Note that the allocator may give the collection more space than it requests. Therefore
-    /// capacity can not be relied upon to be precisely minimal. Prefer [`reserve`] if future
-    /// insertions are expected.
+    /// [`reserve`]: BinaryHeap::reserve
     ///
     /// # Panics
     ///
-    /// Panics if the new capacity overflows `usize`.
+    /// Panics if the new capacity overflows [`usize`].
     ///
     /// # Examples
     ///
@@ -935,12 +938,15 @@ impl<T> BinaryHeap<T> {
         self.data.reserve_exact(additional);
     }
 
-    /// Reserves capacity for at least `additional` more elements to be inserted in the
-    /// `BinaryHeap`. The collection may reserve more space to avoid frequent reallocations.
+    /// Reserves capacity for at least `additional` elements more than the
+    /// current length. The allocator may reserve more space to speculatively
+    /// avoid frequent allocations. After calling `reserve`,
+    /// capacity will be greater than or equal to `self.len() + additional`.
+    /// Does nothing if capacity is already sufficient.
     ///
     /// # Panics
     ///
-    /// Panics if the new capacity overflows `usize`.
+    /// Panics if the new capacity overflows [`usize`].
     ///
     /// # Examples
     ///
@@ -958,10 +964,11 @@ impl<T> BinaryHeap<T> {
         self.data.reserve(additional);
     }
 
-    /// Tries to reserve the minimum capacity for exactly `additional`
-    /// elements to be inserted in the given `BinaryHeap<T>`. After calling
-    /// `try_reserve_exact`, capacity will be greater than or equal to
-    /// `self.len() + additional` if it returns `Ok(())`.
+    /// Tries to reserve the minimum capacity for at least `additional` elements
+    /// more than the current length. Unlike [`try_reserve`], this will not
+    /// deliberately over-allocate to speculatively avoid frequent allocations.
+    /// After calling `try_reserve_exact`, capacity will be greater than or
+    /// equal to `self.len() + additional` if it returns `Ok(())`.
     /// Does nothing if the capacity is already sufficient.
     ///
     /// Note that the allocator may give the collection more space than it
@@ -999,11 +1006,11 @@ impl<T> BinaryHeap<T> {
         self.data.try_reserve_exact(additional)
     }
 
-    /// Tries to reserve capacity for at least `additional` more elements to be inserted
-    /// in the given `BinaryHeap<T>`. The collection may reserve more space to avoid
-    /// frequent reallocations. After calling `try_reserve`, capacity will be
-    /// greater than or equal to `self.len() + additional`. Does nothing if
-    /// capacity is already sufficient.
+    /// Tries to reserve capacity for at least `additional` elements more than the
+    /// current length. The allocator may reserve more space to speculatively
+    /// avoid frequent allocations. After calling `try_reserve`, capacity will be
+    /// greater than or equal to `self.len() + additional` if it returns
+    /// `Ok(())`. Does nothing if capacity is already sufficient.
     ///
     /// # Errors
     ///
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index f92e5759b6f..4d895d83745 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -688,7 +688,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
         self.cap() - 1
     }
 
-    /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
+    /// Reserves the minimum capacity for at least `additional` more elements to be inserted in the
     /// given deque. Does nothing if the capacity is already sufficient.
     ///
     /// Note that the allocator may give the collection more space than it requests. Therefore
@@ -716,7 +716,7 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Reserves capacity for at least `additional` more elements to be inserted in the given
-    /// deque. The collection may reserve more space to avoid frequent reallocations.
+    /// deque. The collection may reserve more space to speculatively avoid frequent reallocations.
     ///
     /// # Panics
     ///
@@ -748,10 +748,10 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
     }
 
-    /// Tries to reserve the minimum capacity for exactly `additional` more elements to
+    /// Tries to reserve the minimum capacity for at least `additional` more elements to
     /// be inserted in the given deque. After calling `try_reserve_exact`,
-    /// capacity will be greater than or equal to `self.len() + additional`.
-    /// Does nothing if the capacity is already sufficient.
+    /// capacity will be greater than or equal to `self.len() + additional` if
+    /// it returns `Ok(())`. Does nothing if the capacity is already sufficient.
     ///
     /// Note that the allocator may give the collection more space than it
     /// requests. Therefore, capacity can not be relied upon to be precisely
@@ -791,10 +791,10 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Tries to reserve capacity for at least `additional` more elements to be inserted
-    /// in the given deque. The collection may reserve more space to avoid
+    /// in the given deque. The collection may reserve more space to speculatively avoid
     /// frequent reallocations. After calling `try_reserve`, capacity will be
-    /// greater than or equal to `self.len() + additional`. Does nothing if
-    /// capacity is already sufficient.
+    /// greater than or equal to `self.len() + additional` if it returns
+    /// `Ok(())`. Does nothing if capacity is already sufficient.
     ///
     /// # Errors
     ///