Test and fix Send and Sync traits of BTreeMap artefacts

[ssomers]

Fixes#76686.

I'm not quite sure what all this implies. E.g. comparing with the definitions for NodeRef in node.rs, maybe an extra bound T: 'a is useful for something. The test compiles on stable/beta (apart from drain_filter) so I bet Sync is equally desirable.

r? @Mark-Simulacrum

[jyn514]

Don't you want &mut T to be Send, not T?

Suggested change

	unsafeimpl<'a,T:Sync>SyncforDormantMutRef<'a,T>{}
	unsafeimpl<'a,T:Send>SendforDormantMutRef<'a,T>{}
	unsafeimpl<'a,T>SyncforDormantMutRef<'a,T>where&'amutT:Sync{}
	unsafeimpl<'a,T>SendforDormantMutRef<'a,T>where&'amutT:Send{}

[cuviper]

Is there any tangible difference? That implementation is already:

unsafeimpl<T:Send + ?Sized>Sendfor&mutT{}

And I believe Sync for &mut T is auto-implemented for T: Sync.

[jyn514]

You could have unsafe impl<T> Send for &mut T {}, even if T is not Send.

[jyn514]

And in general this is conceptually a &mut T so it makes sense to use the same rules as &mut T for it.

[Mark-Simulacrum]

Yeah, I think they might match up in practice, but I didn't want to replicate impls when we could just use this.

[cuviper]

I guess core could pull that trick, but nobody else:

structFoo(*consti32);unsafeimplSendfor&mutFoo{}

error[E0119]: conflicting implementations of trait `std::marker::Send` for type `&mut Foo`:
--> src/lib.rs:3:1
|
3 | unsafe impl Send for &mut Foo {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: conflicting implementation in crate `core`:
- impl<T> std::marker::Send for &mut T
where T: std::marker::Send, T: ?Sized;
error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `&mut Foo`
--> src/lib.rs:3:1
|
3 | unsafe impl Send for &mut Foo {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type

[Mark-Simulacrum]

@jyn514 is correct, I think, that we want these in terms of &mut T.

r=me with that change made

[ssomers]

Also got more fine grained testing to compile and added newcomers into_keys and into_values.

r= @Mark-Simulacrum

[jyn514]

@bors r=Mark-Simulacrum

[bors]

📌 Commit 176956c has been approved by Mark-Simulacrum

[RalfJung]

@bors rollup