Auto merge of #32851 - apasel422:spec-extend, r=alexcrichton

Specialize `Extend` to `append` for `{LinkedList, Vec}`

src/libcollections/lib.rs

@@ -132,3 +132,10 @@ pub enum Bound<T> {
     /// An infinite endpoint. Indicates that there is no bound in this direction.
     Unbounded,
 }
+
+/// An intermediate trait for specialization of `Extend`.
+#[doc(hidden)]
+trait SpecExtend<I: IntoIterator> {
+    /// Extends `self` with the contents of the given iterator.
+    fn spec_extend(&mut self, iter: I);
+}

src/libcollections/linked_list.rs

@@ -30,6 +30,8 @@ use core::mem;
 use core::ops::{BoxPlace, InPlace, Place, Placer};
 use core::ptr::{self, Shared};
 
+use super::SpecExtend;
+
 /// A doubly-linked list.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct LinkedList<T> {
@@ -969,12 +971,24 @@ impl<'a, T> IntoIterator for &'a mut LinkedList<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<A> Extend<A> for LinkedList<A> {
     fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) {
+        <Self as SpecExtend<T>>::spec_extend(self, iter);
+    }
+}
+
+impl<I: IntoIterator> SpecExtend<I> for LinkedList<I::Item> {
+    default fn spec_extend(&mut self, iter: I) {
         for elt in iter {
             self.push_back(elt);
         }
     }
 }
 
+impl<T> SpecExtend<LinkedList<T>> for LinkedList<T> {
+    fn spec_extend(&mut self, ref mut other: LinkedList<T>) {
+        self.append(other);
+    }
+}
+
 #[stable(feature = "extend_ref", since = "1.2.0")]
 impl<'a, T: 'a + Copy> Extend<&'a T> for LinkedList<T> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {

src/libcollections/vec.rs

@@ -75,6 +75,7 @@ use core::ops;
 use core::ptr;
 use core::slice;
 
+use super::SpecExtend;
 use super::range::RangeArgument;
 
 /// A contiguous growable array type, written `Vec<T>` but pronounced 'vector.'
@@ -1390,10 +1391,22 @@ impl<'a, T> IntoIterator for &'a mut Vec<T> {
 impl<T> Extend<T> for Vec<T> {
     #[inline]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
+        <Self as SpecExtend<I>>::spec_extend(self, iter);
+    }
+}
+
+impl<I: IntoIterator> SpecExtend<I> for Vec<I::Item> {
+    default fn spec_extend(&mut self, iter: I) {
         self.extend_desugared(iter.into_iter())
     }
 }
 
+impl<T> SpecExtend<Vec<T>> for Vec<T> {
+    fn spec_extend(&mut self, ref mut other: Vec<T>) {
+        self.append(other);
+    }
+}
+
 impl<T> Vec<T> {
     fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
         // This function should be the moral equivalent of:

src/libcollectionstest/linked_list.rs

@@ -339,6 +339,22 @@ fn test_extend_ref() {
     assert_eq!(a, list_from(&[1, 2, 3, 4, 5, 6]));
 }
 
+#[test]
+fn test_extend() {
+    let mut a = LinkedList::new();
+    a.push_back(1);
+    a.extend(vec![2, 3, 4]); // uses iterator
+
+    assert_eq!(a.len(), 4);
+    assert!(a.iter().eq(&[1, 2, 3, 4]));
+
+    let b: LinkedList<_> = vec![5, 6, 7].into_iter().collect();
+    a.extend(b); // specializes to `append`
+
+    assert_eq!(a.len(), 7);
+    assert!(a.iter().eq(&[1, 2, 3, 4, 5, 6, 7]));
+}
+
 #[bench]
 fn bench_collect_into(b: &mut test::Bencher) {
     let v = &[0; 64];

src/libcollectionstest/vec.rs

@@ -93,6 +93,9 @@ fn test_extend() {
     for i in 3..10 { w.push(i) }
 
     assert_eq!(v, w);
+
+    v.extend(w.clone()); // specializes to `append`
+    assert!(v.iter().eq(w.iter().chain(w.iter())));
 }
 
 #[test]