De-stabilize `thread::scoped` and friends

Issue #24292 demonstrates that the `scoped` API as currently offered can
be memory-unsafe: the `JoinGuard` can be moved into a context that will
fail to execute destructors prior to the stack frame being popped (for
example, by creating an `Rc` cycle).

This commit reverts the APIs to `unstable` status while a long-term
solution is worked out.

(There are several possible ways to address this issue; it's not a
fundamental problem with the `scoped` idea, but rather an indication
that Rust doesn't currently provide a good way to ensure that
destructors are run within a particular stack frame.)

[breaking-change]

0 files changed, 0 insertions, 0 deletions