The constant name `MAX_REFCOUNT` suggests that the value is a
_hard_ limit on the amount of references to an `Arc`. This is
a more soft limit however. This commit adds a comment to the
constant to annotate this.

See also: PR #37605

Servo and Kuchiki have had helper functions doing this for some time.

This can be confusing for newcomers, especially due to the argument
name "this".

Long ago we discovered that threads which outlive main and then exit while the
rest of the program is exiting causes Windows to hang (#20704). That's what was
happening in this test so let's just not run this test any more.

This commit applies the FCP decisions made by the libs team for the 1.10 cycle,
including both new stabilizations and deprecations. Specifically, the list of
APIs is:

Stabilized:

* `os::windows::fs::OpenOptionsExt::access_mode`
* `os::windows::fs::OpenOptionsExt::share_mode`
* `os::windows::fs::OpenOptionsExt::custom_flags`
* `os::windows::fs::OpenOptionsExt::attributes`
* `os::windows::fs::OpenOptionsExt::security_qos_flags`
* `os::unix::fs::OpenOptionsExt::custom_flags`
* `sync::Weak::new`
* `Default for sync::Weak`
* `panic::set_hook`
* `panic::take_hook`
* `panic::PanicInfo`
* `panic::PanicInfo::payload`
* `panic::PanicInfo::location`
* `panic::Location`
* `panic::Location::file`
* `panic::Location::line`
* `ffi::CStr::from_bytes_with_nul`
* `ffi::CStr::from_bytes_with_nul_unchecked`
* `ffi::FromBytesWithNulError`
* `fs::Metadata::modified`
* `fs::Metadata::accessed`
* `fs::Metadata::created`
* `sync::atomic::Atomic{Usize,Isize,Bool,Ptr}::compare_exchange`
* `sync::atomic::Atomic{Usize,Isize,Bool,Ptr}::compare_exchange_weak`
* `collections::{btree,hash}_map::{Occupied,Vacant,}Entry::key`
* `os::unix::net::{UnixStream, UnixListener, UnixDatagram, SocketAddr}`
* `SocketAddr::is_unnamed`
* `SocketAddr::as_pathname`
* `UnixStream::connect`
* `UnixStream::pair`
* `UnixStream::try_clone`
* `UnixStream::local_addr`
* `UnixStream::peer_addr`
* `UnixStream::set_read_timeout`
* `UnixStream::set_write_timeout`
* `UnixStream::read_timeout`
* `UnixStream::write_Timeout`
* `UnixStream::set_nonblocking`
* `UnixStream::take_error`
* `UnixStream::shutdown`
* Read/Write/RawFd impls for `UnixStream`
* `UnixListener::bind`
* `UnixListener::accept`
* `UnixListener::try_clone`
* `UnixListener::local_addr`
* `UnixListener::set_nonblocking`
* `UnixListener::take_error`
* `UnixListener::incoming`
* RawFd impls for `UnixListener`
* `UnixDatagram::bind`
* `UnixDatagram::unbound`
* `UnixDatagram::pair`
* `UnixDatagram::connect`
* `UnixDatagram::try_clone`
* `UnixDatagram::local_addr`
* `UnixDatagram::peer_addr`
* `UnixDatagram::recv_from`
* `UnixDatagram::recv`
* `UnixDatagram::send_to`
* `UnixDatagram::send`
* `UnixDatagram::set_read_timeout`
* `UnixDatagram::set_write_timeout`
* `UnixDatagram::read_timeout`
* `UnixDatagram::write_timeout`
* `UnixDatagram::set_nonblocking`
* `UnixDatagram::take_error`
* `UnixDatagram::shutdown`
* RawFd impls for `UnixDatagram`
* `{BTree,Hash}Map::values_mut`
* `<[_]>::binary_search_by_key`

Deprecated:

* `StaticCondvar` - this, and all other static synchronization primitives
                    below, are usable today through the lazy-static crate on
                    stable Rust today. Additionally, we'd like the non-static
                    versions to be directly usable in a static context one day,
                    so they're unlikely to be the final forms of the APIs in any
                    case.
* `CONDVAR_INIT`
* `StaticMutex`
* `MUTEX_INIT`
* `StaticRwLock`
* `RWLOCK_INIT`
* `iter::Peekable::is_empty`

Closes #27717
Closes #27720
cc #27784 (but encode methods still exist)
Closes #30014
Closes #30425
Closes #30449
Closes #31190
Closes #31399
Closes #31767
Closes #32111
Closes #32281
Closes #32312
Closes #32551
Closes #33018

The fixed issue that allowed this was #12808.

Fixes #31950

This allows printing pointers to unsized types with the {:p} formatting
directive. The following impls are extended to unsized types:
 - impl<'a, T: ?Sized> Pointer for &'a T
 - impl<'a, T: ?Sized> Pointer for &'a mut T
 - impl<T: ?Sized> Pointer for *const T
 - impl<T: ?Sized> Pointer for *mut T
 - impl<T: ?Sized> fmt::Pointer for Box<T>
 - impl<T: ?Sized> fmt::Pointer for Rc<T>
 - impl<T: ?Sized> fmt::Pointer for Arc<T>

Changed the description of the `make_mut` copy-on-write behaviour in arc.rs

The sentence "doesn't have one strong reference and no weak references." is a
hard to understand double negative, which can be much more easily explained.

This was accidentally introduced in
7e2ffc70,
b44ee371 and
36ba96ea.

This hairy conditional doesn't need to be so. It _does_ need to be a
thin pointer, otherwise, it will fail to compile, so let's pull that out
into a temporary for future readers of the source.

Also, after a discussion with @pnkfelix and @gankro, we don't need these
null checks anymore, as zero-on-drop has been gone for a while now.

Lots of cruft to remove!

This is a standard "clean out libstd" commit which removes all 1.5-and-before
deprecated functionality as it's now all been deprecated for at least one entire
cycle.

srong -> strong in liballoc/arc.rs

Closes #30031.

Remove `stable` stability annotations from inherent impls

Sometimes when writing generic code you want to abstract over
owning/pointer type so that calling code isn't restricted by one
concrete owning/pointer type. This commit makes possible such code:
```
fn i_will_work_with_arc<T: Into<Arc<MyTy>>>(t: T) {
    let the_arc = t.into();
    // Do something
}

i_will_work_with_arc(MyTy::new());

i_will_work_with_arc(Box::new(MyTy::new()));

let arc_that_i_already_have = Arc::new(MyTy::new());
i_will_work_with_arc(arc_that_i_already_have);
```

Please note that this patch doesn't work with DSTs.

This change has two consequences:

1. It makes `Arc<T>` and `Rc<T>` covariant in `T`.

2. It causes the compiler to reject code that was unsound with respect
to dropck. See compile-fail/issue-29106.rs for an example of code that
no longer compiles. Because of this, this is a [breaking-change].

Fixes #29037.
Fixes #29106.