cmd/compile: enable -d=zerocopy by default

Fixes #2205.

Change-Id: Ib0802fee2b274798b35f0ebbd0b736b1be5ae00a
Reviewed-on: https://go-review.googlesource.com/c/go/+/520600
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

Author: mdempsky

src/cmd/compile/internal/base/flag.go

@@ -181,6 +181,7 @@ func ParseFlags() {
 	Debug.PGOInline = 1
 	Debug.PGODevirtualize = 1
 	Debug.SyncFrames = -1 // disable sync markers by default
+	Debug.ZeroCopy = 1
 
 	Debug.Checkptr = -1 // so we can tell whether it is set explicitly
 

test/escape2.go

@@ -1729,7 +1729,7 @@ func intstring2() {
 
 func stringtoslicebyte0() {
 	s := "foo"
-	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$" "zero-copy string->\[\]byte conversion"
 	_ = x
 }
 

test/escape2n.go

@@ -1729,7 +1729,7 @@ func intstring2() {
 
 func stringtoslicebyte0() {
 	s := "foo"
-	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape$" "zero-copy string->\[\]byte conversion"
 	_ = x
 }