There are two places where the compiler generates memclrHasPointers
calls. These are effectively write barriers, but the compiler doesn't
currently record them as such in the function. As a result code like

  for i := range a {
    a[i] = nil
  }

inserts a write barrier for the assignment to a[i], but the compiler
doesn't report this. Hence, it's not reported in the -d=wb output, and
it's not checked against //go:nowritebarrier annotations.

Change-Id: I40299ebc9824f05cf516cba494d4c086b80ffb53
Reviewed-on: https://go-review.googlesource.com/c/152722
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>