weak: clarify Pointer equality semantics

The docs currently are imprecise about comparisons. This could lead
users to believe that objects of the same type, allocated at the same
address, could produce weak pointers that are equal to
previously-created weak pointers. This is not the case. Weak pointers
map to objects, not addresses.

Update the documentation to state precisely that if two pointers do not
compare equal, then two weak pointers created from those two pointers
are guaranteed not to compare equal. Since a future pointer pointing to
the same address is not comparable with a pointer produced *before* an
object at that address has been reclaimed, this is sufficient to explain
that weak pointers map 1:1 with object offsets, not addresses.

(An object slot cannot be reused unless that slot is unreachable, so
by construction, there's never an opportunity to compare an "old" and
"new" pointer unless one uses unsafe tricks that violate the
unsafe.Pointer rules.)

Fixes #71381.

Change-Id: I5509fd433cde013926d725694d480c697a8bc911
Reviewed-on: https://go-review.googlesource.com/c/go/+/643935
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

0 files changed, 0 insertions, 0 deletions