Auto merge of #1253 - RalfJung:vec-extend, r=RalfJung

test Vec::extend

Currently fails, until https://github.com/rust-lang/rust/issues/70301 gets fixed.

rust-version

@@ -1 +1 @@
-6050e523bae6de61de4e060facc43dc512adaccd
+e6cef0445779724b469ab7b9a8d3c05d9e848ca8

tests/run-pass/vec.rs

@@ -71,6 +71,39 @@ fn vec_reallocate() -> Vec<u8> {
     v
 }
 
+fn vec_push_ptr_stable() {
+    let mut v = Vec::with_capacity(10);
+    v.push(0);
+    let v0 = unsafe { &mut *(&mut v[0] as *mut _) }; // laundering the lifetime -- we take care that `v` does not reallocate, so that's okay.
+    v.push(1);
+    let _val = *v0;
+}
+
+fn vec_extend_ptr_stable() {
+    let mut v = Vec::with_capacity(10);
+    v.push(0);
+    let v0 = unsafe { &mut *(&mut v[0] as *mut _) }; // laundering the lifetime -- we take care that `v` does not reallocate, so that's okay.
+    // `slice::Iter` (with `T: Copy`) specialization
+    v.extend(&[1]);
+    let _val = *v0;
+    // `vec::IntoIter` specialization
+    v.extend(vec![2]);
+    let _val = *v0;
+    // `TrustedLen` specialization
+    v.extend(std::iter::once(3));
+    let _val = *v0;
+    // base case
+    v.extend(std::iter::once(3).filter(|_| true));
+    let _val = *v0;
+}
+
+fn vec_truncate_ptr_stable() {
+    let mut v = vec![0; 10];
+    let v0 = unsafe { &mut *(&mut v[0] as *mut _) }; // laundering the lifetime -- we take care that `v` does not reallocate, so that's okay.
+    v.truncate(5);
+    let _val = *v0;
+}
+
 fn main() {
     assert_eq!(vec_reallocate().len(), 5);
 
@@ -89,4 +122,9 @@ fn main() {
     // Test interesting empty slice comparison
     // (one is a real pointer, one an integer pointer).
     assert_eq!((200..-5).step_by(1).collect::<Vec<isize>>(), []);
+
+    // liballoc has a more extensive test of this, but let's at least do a smoke test here.
+    vec_push_ptr_stable();
+    vec_extend_ptr_stable();
+    vec_truncate_ptr_stable();
 }