cmd/go: enable -x in go mod tidy

Updates #35849

Change-Id: I8b40a2de6a05880a9f939349a714b631888f5f94
Reviewed-on: https://go-review.googlesource.com/c/go/+/448915
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -1285,7 +1285,7 @@
 //
 // Usage:
 //
-//	go mod tidy [-e] [-v] [-go=version] [-compat=version]
+//	go mod tidy [-e] [-v] [-x] [-go=version] [-compat=version]
 //
 // Tidy makes sure go.mod matches the source code in the module.
 // It adds any missing modules necessary to build the current module's
@@ -1313,6 +1313,8 @@
 // version prior to the one indicated by the 'go' directive in the go.mod
 // file.
 //
+// The -x flag causes tidy to print the commands download executes.
+//
 // See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 //
 // # Make vendored copy of dependencies

src/cmd/go/internal/modcmd/tidy.go

@@ -19,7 +19,7 @@ import (
 )
 
 var cmdTidy = &base.Command{
-	UsageLine: "go mod tidy [-e] [-v] [-go=version] [-compat=version]",
+	UsageLine: "go mod tidy [-e] [-v] [-x] [-go=version] [-compat=version]",
 	Short:     "add missing and remove unused modules",
 	Long: `
 Tidy makes sure go.mod matches the source code in the module.
@@ -48,6 +48,8 @@ version. By default, tidy acts as if the -compat flag were set to the
 version prior to the one indicated by the 'go' directive in the go.mod
 file.
 
+The -x flag causes tidy to print the commands download executes.
+
 See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
 	`,
 	Run: runTidy,
@@ -61,6 +63,7 @@ var (
 
 func init() {
 	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	cmdTidy.Flag.BoolVar(&cfg.BuildX, "x", false, "")
 	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
 	cmdTidy.Flag.Var(&tidyGo, "go", "")
 	cmdTidy.Flag.Var(&tidyCompat, "compat", "")

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

@@ -0,0 +1,18 @@
+# This test checks that "go mod tidy -x" print
+# commands tidy executes.
+# Verifies golang.org/issue/35849
+
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote
+go mod tidy
+! stderr 'get '$GOPROXY
+
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote
+go mod tidy -x
+stderr 'get '$GOPROXY
+
+-- go.mod --
+module example.com/mod
+
+-- a.go --
+package mod
+import _ "rsc.io/quote"