cmd/compile, cmd/link: produce unlinkable object when compile without -p

CL 391014 requires the compiler to be invoked with the -p flag, to
specify the package path. People are used to run "go tool compile"
from the command line with the -p flag. This is mostly for simple
testing, or debugging the compiler. The produced object file is
almost never intended to be linked.

This CL makes the compiler allow "go tool compile" without the -p
flag again. It will produce an unlinkable object. If the linker
sees such an object it will error out.

Change-Id: I7bdb162c3cad61dadd5c456d903b92493a3df20f
Reviewed-on: https://go-review.googlesource.com/c/go/+/394217
Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 560e8e24c4..e7bc45ccdf 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -23,6 +23,8 @@ import (
 	"strings"
 )
 
+const UnlinkablePkg = "<unlinkable>" // invalid package path, used when compiled without -p flag
+
 // Entry point of writing new object file.
 func WriteObjFile(ctxt *Link, b *bio.Writer) {
 
@@ -45,6 +47,9 @@ func WriteObjFile(ctxt *Link, b *bio.Writer) {
 	if ctxt.Flag_shared {
 		flags |= goobj.ObjFlagShared
 	}
+	if w.pkgpath == UnlinkablePkg {
+		flags |= goobj.ObjFlagUnlinkable
+	}
 	if w.pkgpath == "" {
 		flags |= goobj.ObjFlagNeedNameExpansion
 	}