Sync/Send mismatch for Iter?

[sparecycles]

There are a few places where an Iter implements Send when the item/key/value traits are Sync. The other related types (IterMut, etc...) implement Sync for Sync items and Send for Send items.

See line 1209 in

rust/src/liballoc/collections/linked_list.rs

Lines 1202 to 1218 in 917945d

	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<T: Send> Send for LinkedList<T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<T: Sync> Sync for LinkedList<T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Sync> Send for Iter<'a, T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Sync> Sync for Iter<'a, T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Send> Send for IterMut<'a, T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Sync> Sync for IterMut<'a, T> {}

and also in

rust/src/libcore/slice/mod.rs

Lines 2650 to 2653 in 917945d

	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Sync> Sync for Iter<'a, T> {}
	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<'a, T: Sync> Send for Iter<'a, T> {}

as well as

rust/src/libstd/collections/hash/table.rs

Lines 916 to 917 in 917945d

	unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
	unsafe impl<'a, K: Sync, V: Sync> Send for Iter<'a, K, V> {}

( Noticed this pattern in hyperium/http#239 )

[qnighy]

They just inherit the item type's sendability/syncability, not the generic parameter's. &mut T: Send if T: Send and &T: Send if T: Sync.

[sparecycles]

Thanks, I see now why Iter<Sync> could be Send, but I'm not sure about related structs like Drain or IntoIters.

It's interesting that Iter<T: Send> is not Send, but IterMut<T: Send> is.

[Dylan-DPC]

I am closing this issue as it is not a bug and working as intended.

[sparecycles]

You really are seeing Sync implying Send on, e.g.,

rust/library/alloc/src/collections/linked_list.rs

Lines 1990 to 1991 in b685242

	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<T: Sync> Send for Iter<'_, T> {}

rust/library/alloc/src/collections/linked_list.rs

Lines 2002 to 2003 in b685242

	#[unstable(feature = "linked_list_cursors", issue = "58533")]
	unsafe impl<T: Sync> Send for Cursor<'_, T> {}

and that's fine?

[sparecycles]

Okay, looks like this is actually intentional, but it's still a bit cryptic.

#27615 (comment)