From 582fbb1d62420d8d85f364d06669038f12b7e423 Mon Sep 17 00:00:00 2001
From: The8472 <git@infinite-source.de>
Date: Thu, 21 Nov 2019 13:24:48 +0100
Subject: [PATCH] use From specializations on extend if extended Vec is empty

this enables in-place iteration and allocation reuse in additional cases
---
 library/alloc/src/vec.rs | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/library/alloc/src/vec.rs b/library/alloc/src/vec.rs
index ce7ea2058b5..ffd6f5b31d4 100644
--- a/library/alloc/src/vec.rs
+++ b/library/alloc/src/vec.rs
@@ -2080,7 +2080,16 @@ fn into_iter(self) -> slice::IterMut<'a, T> {
 impl<T> Extend<T> for Vec<T> {
     #[inline]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
+        if self.capacity() > 0 {
+            <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
+        } else {
+            // if self has no allocation then use the more powerful from_iter specializations
+            let other = SpecFrom::from_iter(iter.into_iter());
+            // replace self, don't run drop since self was empty
+            unsafe {
+                ptr::write(self, other);
+            }
+        }
     }
 
     #[inline]
@@ -2121,6 +2130,8 @@ impl<T, I> SpecFrom<T, I> for Vec<T>
                 vector
             }
         };
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
         <Vec<T> as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);
         vector
     }
@@ -2230,7 +2241,9 @@ fn from_iter(iterator: IntoIter<T>) -> Self {
         }
 
         let mut vec = Vec::new();
-        vec.extend(iterator);
+        // must delegate to spec_extend() since extend() itself delegates
+        // to spec_from for empty Vecs
+        vec.spec_extend(iterator);
         vec
     }
 }
@@ -2475,7 +2488,16 @@ pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<'_, T, F>
 #[stable(feature = "extend_ref", since = "1.2.0")]
 impl<'a, T: 'a + Copy> Extend<&'a T> for Vec<T> {
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
-        self.spec_extend(iter.into_iter())
+        if self.capacity() > 0 {
+            self.spec_extend(iter.into_iter())
+        } else {
+            // if self has no allocation then use the more powerful from_iter specializations
+            let other = SpecFrom::from_iter(iter.into_iter());
+            // replace self, don't run drop since self was empty
+            unsafe {
+                ptr::write(self, other);
+            }
+        }
     }
 
     #[inline]