Ensure std::mem::Discriminant is Send + Sync

[bluss]

PhantomData<*const T> has the implication of Send / Syncness following
the *const T type, but the discriminant should always be Send and Sync.

Use PhantomData<fn() -> T> which has the same variance in T, but is Send + Sync

[rust-highfive]

r? @BurntSushi

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[bluss]

Tracking issue #24263

[bluss]

As discussed before on irc, a bit unclear what the correct phantom data really is for this thing.

[BurntSushi]

I am throwing this to @aturon. I don't immediately see anything wrong with this, but I'd like someone else to see it! cc @rust-lang/libs

[dtolnay]

What are the tradeoffs of using a screwy PhantomData marker vs unsafe impl Send?

[ghost]

Sometimes I wish we had a few additional marker structs so we could do:

_marker: PhantomData<Variant<T>>
_marker: PhantomData<Invariant<T>>
_marker: PhantomData<(T, NotSendSync)>

[alexcrichton]

Looks good to me!

@dtolnay I think the usage of fn() -> T helps avoid the usage of unsafe impl, which seems like a bonus to me!

[carols10cents]

Just a note that @aturon is on PTO this week so isn't likely to get to it, but it looks like this one is worth waiting for him to take a look.

[alexcrichton]

I'm gonna go ahead and approve this as I think there won't be too much opposition, and we'll have a long time on nightly before this reaches stable as well!

[alexcrichton]

@bors: r+

[bors]

📌 Commit 3fff2d9 has been approved by alexcrichton