cmd/go/internal/modcmd: allow mod download without go.mod

Fixes #29522

Change-Id: I48f3a945d24c23c7c7ef5c7f1fe5046b6b2898e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/157937
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/download.go

@@ -5,13 +5,14 @@
 package modcmd
 
 import (
+	"encoding/json"
+	"os"
+
 	"cmd/go/internal/base"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
-	"encoding/json"
-	"os"
 )
 
 var cmdDownload = &base.Command{

src/cmd/go/internal/modload/init.go

@@ -253,9 +253,11 @@ func Init() {
 func init() {
 	load.ModInit = Init
 
-	// Set modfetch.PkgMod unconditionally, so that go clean -modcache can run even without modules enabled.
+	// Set modfetch.PkgMod and codehost.WorkRoot unconditionally,
+	// so that go clean -modcache and go mod download can run even without modules enabled.
 	if list := filepath.SplitList(cfg.BuildContext.GOPATH); len(list) > 0 && list[0] != "" {
 		modfetch.PkgMod = filepath.Join(list[0], "pkg/mod")
+		codehost.WorkRoot = filepath.Join(list[0], "pkg/mod/cache/vcs")
 	}
 }
 

src/cmd/go/testdata/script/mod_download.txt

@@ -82,5 +82,12 @@ exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
 go mod download -json rsc.io/quote@v1.5.1
 exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
 
+# allow go mod download without go.mod
+env GO111MODULE=auto
+rm go.mod
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
+go mod download rsc.io/quote@v1.2.1
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
+
 -- go.mod --
 module m