clean: clean mod cache should respect "-n" option.

Clean mod cache should print remove commands and not run them when with set "-n" option.
Fixes #27458.

Change-Id: I97242cb40c062b347784cdb61653c84a3a7eab44
GitHub-Last-Rev: 5a6f10cad8c5f2c3916a74ca5eea27b1fdd1dc38
GitHub-Pull-Request: golang/go#27710
Reviewed-on: https://go-review.googlesource.com/135695
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/clean/clean.go

@@ -112,9 +112,10 @@ func runClean(cmd *base.Command, args []string) {
 		}
 	}
 
+	var b work.Builder
+	b.Print = fmt.Print
+
 	if cleanCache {
-		var b work.Builder
-		b.Print = fmt.Print
 		dir := cache.DefaultDir()
 		if dir != "off" {
 			// Remove the cache subdirectories but not the top cache directory.
@@ -156,8 +157,13 @@ func runClean(cmd *base.Command, args []string) {
 		if modfetch.PkgMod == "" {
 			base.Fatalf("go clean -modcache: no module cache")
 		}
-		if err := removeAll(modfetch.PkgMod); err != nil {
-			base.Errorf("go clean -modcache: %v", err)
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "rm -rf %s", modfetch.PkgMod)
+		}
+		if !cfg.BuildN {
+			if err := removeAll(modfetch.PkgMod); err != nil {
+				base.Errorf("go clean -modcache: %v", err)
+			}
 		}
 	}
 }

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

+env GO111MODULE=on
+
+go mod download rsc.io/quote@v1.5.0
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+go clean -modcache -n
+stdout '^rm -rf .*pkg.mod$'
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+go clean -modcache
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
\ No newline at end of file