From 578e7143936158a0130f17bedcc946cae62583f3 Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Sat, 5 Sep 2020 14:09:33 +0200
Subject: [PATCH 1/2] Fix dropck issue of SyncOnceCell.

Fixes #76367.
---
 library/std/src/lazy.rs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/library/std/src/lazy.rs b/library/std/src/lazy.rs
index 845e9d76a772b..fc485f0cd47ba 100644
--- a/library/std/src/lazy.rs
+++ b/library/std/src/lazy.rs
@@ -6,6 +6,7 @@ mod tests;
 use crate::{
     cell::{Cell, UnsafeCell},
     fmt,
+    marker::PhantomData,
     mem::{self, MaybeUninit},
     ops::{Deref, Drop},
     panic::{RefUnwindSafe, UnwindSafe},
@@ -46,6 +47,8 @@ pub struct SyncOnceCell<T> {
     once: Once,
     // Whether or not the value is initialized is tracked by `state_and_queue`.
     value: UnsafeCell<MaybeUninit<T>>,
+    // Make sure dropck understands we're dropping T in our Drop impl.
+    _marker: PhantomData<T>,
 }
 
 // Why do we need `T: Send`?
@@ -119,7 +122,11 @@ impl<T> SyncOnceCell<T> {
     /// Creates a new empty cell.
     #[unstable(feature = "once_cell", issue = "74465")]
     pub const fn new() -> SyncOnceCell<T> {
-        SyncOnceCell { once: Once::new(), value: UnsafeCell::new(MaybeUninit::uninit()) }
+        SyncOnceCell {
+            once: Once::new(),
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+            _marker: PhantomData,
+        }
     }
 
     /// Gets the reference to the underlying value.

From e56ea68db50d23f4a7efa712c53ba02e506fd61a Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Sat, 5 Sep 2020 15:55:20 +0200
Subject: [PATCH 2/2] Add compile_fail test for SyncOnceCell's dropck issue.

---
 library/std/src/lazy.rs | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/library/std/src/lazy.rs b/library/std/src/lazy.rs
index fc485f0cd47ba..d171231b0f1e6 100644
--- a/library/std/src/lazy.rs
+++ b/library/std/src/lazy.rs
@@ -47,7 +47,25 @@ pub struct SyncOnceCell<T> {
     once: Once,
     // Whether or not the value is initialized is tracked by `state_and_queue`.
     value: UnsafeCell<MaybeUninit<T>>,
-    // Make sure dropck understands we're dropping T in our Drop impl.
+    /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
+    ///
+    /// ```compile_fail,E0597
+    /// #![feature(once_cell)]
+    ///
+    /// use std::lazy::SyncOnceCell;
+    ///
+    /// struct A<'a>(&'a str);
+    ///
+    /// impl<'a> Drop for A<'a> {
+    ///     fn drop(&mut self) {}
+    /// }
+    ///
+    /// let cell = SyncOnceCell::new();
+    /// {
+    ///     let s = String::new();
+    ///     let _ = cell.set(A(&s));
+    /// }
+    /// ```
     _marker: PhantomData<T>,
 }