…ointers

Interfaces often contain pointers. Implement a fast path for this case.

name                   old time/op  new time/op  delta
MapInterfaceString-16  21.4ns ±19%  20.5ns ±10%     ~     (p=0.361 n=10+10)
MapInterfacePtr-16     25.8ns ± 8%  17.3ns ± 7%  -33.11%  (p=0.000 n=10+9)

We need this CL as well to fix 37721.
Update #37721
Fixes #37613

Change-Id: Ice52820e6259a3edeafcbbbeb25b1e363bef00d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/219338
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit afd691c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/222779
Run-TryBot: Alexander Rakoczy <alex@golang.org>

…ed hashes exactly

If typehash (used by reflect) does not match the built-in map's hash,
then problems occur. If a map is built using reflect, and then
assigned to a variable of map type, the hash function can change. That
causes very bad things.

This issue is rare. MapOf consults a cache of all types that occur in
the binary before making a new one. To make a true new map type (with
a hash function derived from typehash) that map type must not occur in
the binary anywhere. But to cause the bug, we need a variable of that
type in order to assign to it. The only way to make that work is to
use a named map type for the variable, so it is distinct from the
unnamed version that MapOf looks for.

Fixes #37721

Change-Id: I3537bfceca8cbfa1af84202f432f3c06953fe0ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/222357
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 2b8e60d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/222778