core: Mark DefaultGuard as !Send

[bgw]

Motivation

#2482: Dropping DefaultGuard in another thread doesn't cause it to get cleaned up properly

The Drop impl of DefaultGuard modifies the CURRENT_STATE thread local, assuming it to be the same thread local it was constructed with.

However, because DefaultGuard is Send, that may be a different thread local.

Solution

As discussed in #2482, we should mark DefaultGuard as !Send. This is a breaking API change.

This PR is modeled after #1001, which does the same thing for Entered. One notable change is that I use a different trick for marking the struct as !Send that doesn't involve unsafe.

Entered currently uses:

tracing/tracing/src/span.rs

Lines 1581 to 1591 in a0126b2

	struct PhantomNotSend {
	ghost: PhantomData<*mut ()>,
	}
	#[allow(non_upper_case_globals)]
	const PhantomNotSend: PhantomNotSend = PhantomNotSend { ghost: PhantomData };
	/// # Safety
	///
	/// Trivially safe, as `PhantomNotSend` doesn't have any API.
	unsafe impl Sync for PhantomNotSend {}

My PR proposes the following trick instead:

/// A trait that implements `Sync`, but not `Send`. Used with PhantomData, this
/// can mark a struct as `!Send`.
#[cfg(feature = "std")]
trait NotSend: Sync {}

pub struct DefaultGuard(
    // ...
    PhantomData<dyn NotSend>,
);

If accepted:

• I can follow up with a documentation change for 0.1.x that explains the thread safety issue.
• I can follow up with a change to tracing/src/span.rs that removes the use of unsafe to implement !Send.