Implement `shrink_to` method on collections

4 files changed, 113 insertions, 2 deletions

diff --git a/src/liballoc/binary_heap.rs b/src/liballoc/binary_heap.rs
index 8aaac5d6e08..f6a666b599b 100644
--- a/src/liballoc/binary_heap.rs
+++ b/src/liballoc/binary_heap.rs
@@ -509,6 +509,31 @@ impl<T: Ord> BinaryHeap<T> {
         self.data.shrink_to_fit();
     }
 
+    /// Discards capacity with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// Panics if the current capacity is smaller than the supplied
+    /// minimum capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(shrink_to)]
+    /// use std::collections::BinaryHeap;
+    /// let mut heap: BinaryHeap<i32> = BinaryHeap::with_capacity(100);
+    ///
+    /// assert!(heap.capacity() >= 100);
+    /// heap.shrink_to(10);
+    /// assert!(heap.capacity() >= 10);
+    /// ```
+    #[inline]
+    #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.data.shrink_to(min_capacity)
+    }
+
     /// Removes the greatest item from the binary heap and returns it, or `None` if it
     /// is empty.
     ///
diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index e253122ffd6..2bb60a50679 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1015,6 +1015,34 @@ impl String {
         self.vec.shrink_to_fit()
     }
 
+    /// Shrinks the capacity of this `String` with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// Panics if the current capacity is smaller than the supplied
+    /// minimum capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(shrink_to)]
+    /// let mut s = String::from("foo");
+    ///
+    /// s.reserve(100);
+    /// assert!(s.capacity() >= 100);
+    ///
+    /// s.shrink_to(10);
+    /// assert!(s.capacity() >= 10);
+    /// s.shrink_to(0);
+    /// assert!(s.capacity() >= 3);
+    /// ```
+    #[inline]
+    #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.vec.shrink_to(min_capacity)
+    }
+
     /// Appends the given [`char`] to the end of this `String`.
     ///
     /// [`char`]: ../../std/primitive.char.html
diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 953f95876be..c9c6cf1cb66 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -66,7 +66,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use core::cmp::Ordering;
+use core::cmp::{self, Ordering};
 use core::fmt;
 use core::hash::{self, Hash};
 use core::intrinsics::{arith_offset, assume};
@@ -586,6 +586,31 @@ impl<T> Vec<T> {
         self.buf.shrink_to_fit(self.len);
     }
 
+    /// Shrinks the capacity of the vector with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// Panics if the current capacity is smaller than the supplied
+    /// minimum capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(shrink_to)]
+    /// let mut vec = Vec::with_capacity(10);
+    /// vec.extend([1, 2, 3].iter().cloned());
+    /// assert_eq!(vec.capacity(), 10);
+    /// vec.shrink_to(4);
+    /// assert!(vec.capacity() >= 4);
+    /// vec.shrink_to(0);
+    /// assert!(vec.capacity() >= 3);
+    /// ```
+    #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
+    }
+
     /// Converts the vector into [`Box<[T]>`][owned slice].
     ///
     /// Note that this will drop any excess capacity.
diff --git a/src/liballoc/vec_deque.rs b/src/liballoc/vec_deque.rs
index 0658777f0a0..be6e8d0f22f 100644
--- a/src/liballoc/vec_deque.rs
+++ b/src/liballoc/vec_deque.rs
@@ -676,9 +676,42 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn shrink_to_fit(&mut self) {
+        self.shrink_to(0);
+    }
+
+    /// Shrinks the capacity of the `VecDeque` with a lower bound.
+    ///
+    /// The capacity will remain at least as large as both the length
+    /// and the supplied value.
+    ///
+    /// Panics if the current capacity is smaller than the supplied
+    /// minimum capacity.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(shrink_to)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::with_capacity(15);
+    /// buf.extend(0..4);
+    /// assert_eq!(buf.capacity(), 15);
+    /// buf.shrink_to(6);
+    /// assert!(buf.capacity() >= 6);
+    /// buf.shrink_to(0);
+    /// assert!(buf.capacity() >= 4);
+    /// ```
+    #[unstable(feature = "shrink_to", reason = "new API", issue="0")]
+    pub fn shrink_to(&mut self, min_capacity: usize) {
+        assert!(self.capacity() >= min_capacity, "Tried to shrink to a larger capacity");
+
         // +1 since the ringbuffer always leaves one space empty
         // len + 1 can't overflow for an existing, well-formed ringbuffer.
-        let target_cap = cmp::max(self.len() + 1, MINIMUM_CAPACITY + 1).next_power_of_two();
+        let target_cap = cmp::max(
+            cmp::max(min_capacity, self.len()) + 1,
+            MINIMUM_CAPACITY + 1
+        ).next_power_of_two();
+
         if target_cap < self.cap() {
             // There are three cases of interest:
             //   All elements are out of desired bounds