Zero vector contents after popping

Since Alloy is a conservative GC, this prevents memory leaks kept alive
by stale vector elements.

library/alloc/src/vec/mod.rs

@@ -1955,7 +1955,9 @@ impl<T, A: Allocator> Vec<T, A> {
         } else {
             unsafe {
                 self.len -= 1;
-                Some(ptr::read(self.as_ptr().add(self.len())))
+                let value = Some(ptr::read(self.as_ptr().add(self.len())));
+                ptr::write_bytes(self.as_mut_ptr().add(self.len()), 0, 1);
+                value
             }
         }
     }

tests/ui/runtime/gc/zero_vecs.rs

@@ -0,0 +1,45 @@
+// run-pass
+// ignore-tidy-linelength
+#![feature(gc)]
+#![feature(rustc_private)]
+#![feature(negative_impls)]
+#![feature(allocator_api)]
+#![allow(unused_assignments)]
+#![allow(unused_variables)]
+
+use std::gc::{Gc, GcAllocator};
+use std::sync::atomic::{self, AtomicUsize};
+
+struct Finalizable(usize);
+
+impl Drop for Finalizable {
+    fn drop(&mut self) {
+        FINALIZER_COUNT.fetch_add(1, atomic::Ordering::Relaxed);
+    }
+}
+
+static FINALIZER_COUNT: AtomicUsize = AtomicUsize::new(0);
+
+fn test_pop(v: &mut Vec<Gc<Finalizable>, GcAllocator>) {
+    for i in 0..10 {
+        let mut gc = Some(Gc::new(Finalizable(i)));
+        v.push(gc.unwrap());
+        gc = None;
+    }
+
+    for _ in 0..10 {
+        let mut _gc = Some(v.pop());
+        _gc = None;
+    }
+}
+
+fn main() {
+    let mut v1 = Vec::with_capacity_in(10, GcAllocator);
+    test_pop(&mut v1);
+    test_pop(&mut v1);
+
+    GcAllocator::force_gc();
+
+    assert_eq!(FINALIZER_COUNT.load(atomic::Ordering::Relaxed), 20);
+
+}