remove empty Vec extend optimization

The optimization meant that every extend code path had to emit llvm
IR for from_iter and extend spec_extend, which likely impacts
compile times while only improving a few edge-cases

Author: the8472

library/alloc/src/vec.rs

@@ -2082,13 +2082,7 @@ 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) {
-        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
-            // and overwrite self
-            *self = SpecFrom::from_iter(iter.into_iter());
-        }
+        <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
     }
 
     #[inline]
@@ -2544,13 +2538,7 @@ impl<T> Vec<T> {
 #[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) {
-        if self.capacity() > 0 {
-            self.spec_extend(iter.into_iter())
-        } else {
-            // if self has no allocation then use the more powerful from_iter specializations
-            // and overwrite self
-            *self = SpecFrom::from_iter(iter.into_iter());
-        }
+        self.spec_extend(iter.into_iter())
     }
 
     #[inline]

library/alloc/tests/vec.rs

@@ -798,16 +798,6 @@ fn test_from_iter_partially_drained_in_place_specialization() {
     assert_eq!(srcptr, sinkptr);
 }
 
-#[test]
-fn test_extend_in_place_specialization() {
-    let src: Vec<usize> = vec![0usize; 1];
-    let srcptr = src.as_ptr();
-    let mut dst = Vec::new();
-    dst.extend(src.into_iter());
-    let dstptr = dst.as_ptr();
-    assert_eq!(srcptr, dstptr);
-}
-
 #[test]
 fn test_from_iter_specialization_with_iterator_adapters() {
     fn assert_in_place_trait<T: InPlaceIterable>(_: &T) {};