Added to_vec_mapped_mut.

Author: LukeMathWalker

src/iterators/mod.rs

@@ -718,7 +718,7 @@ macro_rules! outer_iter_split_at_impl {
             {
                 assert!(index <= self.iter.len);
                 let right_ptr = if index != self.iter.len {
-                    unsafe { self.iter.offset(index) } 
+                    unsafe { self.iter.offset(index) }
                 }
                 else {
                     self.iter.ptr
@@ -1219,3 +1219,27 @@ pub fn to_vec_mapped<I, F, B>(iter: I, mut f: F) -> Vec<B>
     debug_assert_eq!(size, result.len());
     result
 }
+
+/// Like Iterator::collect, but only for trusted length iterators
+pub fn to_vec_mapped_mut<I, F, B>(iter: I, mut f: F) -> Vec<B>
+    where I: TrustedIterator + ExactSizeIterator,
+          F: FnMut(&mut I::Item) -> B,
+{
+    // Use an `unsafe` block to do this efficiently.
+    // We know that iter will produce exactly .size() elements,
+    // and the loop can vectorize if it's clean (without branch to grow the vector).
+    let (size, _) = iter.size_hint();
+    let mut result = Vec::with_capacity(size);
+    let mut out_ptr = result.as_mut_ptr();
+    let mut len = 0;
+    iter.fold((), |(), elt| {
+        unsafe {
+            ptr::write(out_ptr, f(&mut elt));
+            len += 1;
+            result.set_len(len);
+            out_ptr = out_ptr.offset(1);
+        }
+    });
+    debug_assert_eq!(size, result.len());
+    result
+}