From 0d5eac6dcaffb12098ffd314ba929c01e88c61e8 Mon Sep 17 00:00:00 2001
From: Mads Marquart <mads@marquart.dk>
Date: Tue, 15 Jun 2021 12:34:40 +0200
Subject: [PATCH 1/2] Correct Send and Sync implementations on Id

See comments in the code for reasoning.

The impl for `Id<T, Shared>` is the same as `Arc<T>` and the impl for `Id<T, Owned>` is the same as `Box<T>`.
---
 src/id.rs | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/id.rs b/src/id.rs
index b679f3b..203cd45 100644
--- a/src/id.rs
+++ b/src/id.rs
@@ -80,11 +80,30 @@ impl<T> Clone for Id<T, Shared> where T: Message {
     }
 }
 
-unsafe impl<T, O> Sync for Id<T, O> where T: Sync { }
-
-unsafe impl<T> Send for Id<T, Owned> where T: Send { }
-
-unsafe impl<T> Send for Id<T, Shared> where T: Sync { }
+/// The `Send` implementation requires `T: Sync` because `Id<T, Shared>` give
+/// access to `&T`.
+///
+/// Additiontally, it requires `T: Send` because if `T: !Send`, you could
+/// clone a `Id<T, Shared>`, send it to another thread, and drop the clone
+/// last, making `dealloc` get called on the other thread, and violate
+/// `T: !Send`.
+unsafe impl<T: Sync + Send> Send for Id<T, Shared> {}
+
+/// The `Sync` implementation requires `T: Sync` because `&Id<T, Shared>` give
+/// access to `&T`.
+///
+/// Additiontally, it requires `T: Send`, because if `T: !Send`, you could
+/// clone a `&Id<T, Shared>` from another thread, and drop the clone last,
+/// making `dealloc` get called on the other thread, and violate `T: !Send`.
+unsafe impl<T: Sync + Send> Sync for Id<T, Shared> {}
+
+/// `Id<T, Owned>` are `Send` if `T` is `Send` because they give the same
+/// access as having a T directly.
+unsafe impl<T: Send> Send for Id<T, Owned> {}
+
+/// `Id<T, Owned>` are `Sync` if `T` is `Sync` because they give the same
+/// access as having a `T` directly.
+unsafe impl<T: Sync> Sync for Id<T, Owned> {}
 
 impl<T, O> Deref for Id<T, O> {
     type Target = T;

From 8833c22ea3f7e199756721f11cceb090aaa992f7 Mon Sep 17 00:00:00 2001
From: Mads Marquart <mads@marquart.dk>
Date: Tue, 15 Jun 2021 12:42:55 +0200
Subject: [PATCH 2/2] Correct Send and Sync implementations on WeakId

The implementations follow the same reasoning as `Id<T, Shared>`, since you can upgrade `WeakId` to `Id<T, Shared>`.
---
 src/id.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/id.rs b/src/id.rs
index 203cd45..f6eee1c 100644
--- a/src/id.rs
+++ b/src/id.rs
@@ -180,9 +180,11 @@ impl<T> WeakId<T> where T: Message {
     }
 }
 
-unsafe impl<T> Sync for WeakId<T> where T: Sync { }
+/// This implementation follows the same reasoning as `Id<T, Shared>`.
+unsafe impl<T: Sync + Send> Sync for WeakId<T> {}
 
-unsafe impl<T> Send for WeakId<T> where T: Sync { }
+/// This implementation follows the same reasoning as `Id<T, Shared>`.
+unsafe impl<T: Sync + Send> Send for WeakId<T> {}
 
 #[cfg(test)]
 mod tests {