vec finer capacity controls

crates/datastructures/src/raw_vec.rs

@@ -18,19 +18,12 @@ impl<T> RawVec<T> {
     }
 
     // See rustonomicon, chapter 9.2
-    pub(crate) fn grow(&mut self) {
-        let (new_cap, new_layout) = if self.capacity == 0 {
-            (1, Layout::array::<T>(1).unwrap())
-        } else {
-            // This can't overflow since self.cap <= isize::MAX.
-            let new_cap = 2 * self.capacity;
-
-            // `Layout::array` checks that the number of bytes is <= usize::MAX,
-            // but this is redundant since old_layout.size() <= isize::MAX,
-            // so the `unwrap` should never fail.
-            let new_layout = Layout::array::<T>(new_cap).unwrap();
-            (new_cap, new_layout)
-        };
+    pub(crate) fn grow_by(&mut self, added_capacity: usize) {
+        let new_cap = self.capacity + added_capacity;
+        // `Layout::array` checks that the number of bytes is <= usize::MAX,
+        // but this is redundant since old_layout.size() <= isize::MAX,
+        // so the `unwrap` should never fail.
+        let new_layout = Layout::array::<T>(new_cap).unwrap();
 
         // Ensure that the new allocation doesn't exceed `isize::MAX` bytes.
         if new_layout.size() > isize::MAX as usize {
@@ -52,6 +45,14 @@ impl<T> RawVec<T> {
         };
         self.capacity = new_cap;
     }
+
+    pub(crate) fn grow(&mut self) {
+        if self.capacity == 0 {
+            self.grow_by(1);
+        } else {
+            self.grow_by(self.capacity);
+        }
+    }
 }
 
 impl<T> Drop for RawVec<T> {

crates/datastructures/src/vec.rs

@@ -37,6 +37,16 @@ impl<T> Vec<T> {
         }
     }
 
+    pub fn with_capacity(capacity: usize) -> Self {
+        if mem::size_of::<T>() == 0 {
+            panic!("We're not ready to handle ZSTs");
+        }
+
+        let mut v = Self::new();
+        v.reserve(capacity);
+        v
+    }
+
     pub fn pop(&mut self) -> Option<T> {
         if self.used == 0 {
             None
@@ -107,6 +117,10 @@ impl<T> Vec<T> {
             result
         }
     }
+
+    pub fn reserve(&mut self, added_capacity: usize) {
+        self.buf.grow_by(added_capacity);
+    }
 }
 
 impl<T> Index<usize> for Vec<T> {