cmd/go: remove the -mod flag from 'go get'

'GOFLAGS=-mod=vendor' currently causes 'go get' to always fail unless
the '-mod' flag is explicitly overwritten. Moreover, as of CL 198319
we plan to set -mod=vendor by default if a vendor directory is
present, so all users with vendor directories will be affected — not
just those who set 'GOFLAGS' explicitly.

Similarly, an explicit '-mod=readonly' argument to 'go get' is
currently ignored as a special case, but the fact that it is ignored
(rather than rejected) can be very surprising.

Rather than adding more special cases, we should remove the '-mod'
flag from 'go get' entirely.

Fixes #30345
Fixes #32502
Updates #33848

Change-Id: Iecd3233ca3ef580ca3a66bd5e6ee8d86d4cbd8a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/198438
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 13 insertions, 4 deletions

diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index 9d6fa0c25b..1fc47a36c7 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -167,8 +167,8 @@ func init() {
 
 	CmdInstall.Flag.BoolVar(&cfg.BuildI, "i", false, "")
 
-	AddBuildFlags(CmdBuild)
-	AddBuildFlags(CmdInstall)
+	AddBuildFlags(CmdBuild, DefaultBuildFlags)
+	AddBuildFlags(CmdInstall, DefaultBuildFlags)
 }
 
 // Note that flags consulted by other parts of the code
@@ -216,9 +216,16 @@ func init() {
 	}
 }
 
+type BuildFlagMask int
+
+const (
+	DefaultBuildFlags BuildFlagMask = 0
+	OmitModFlag       BuildFlagMask = 1 << iota
+)
+
 // addBuildFlags adds the flags common to the build, clean, get,
 // install, list, run, and test commands.
-func AddBuildFlags(cmd *base.Command) {
+func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
 	cmd.Flag.BoolVar(&cfg.BuildN, "n", false, "")
 	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
@@ -230,7 +237,9 @@ func AddBuildFlags(cmd *base.Command) {
 	cmd.Flag.StringVar(&cfg.BuildBuildmode, "buildmode", "default", "")
 	cmd.Flag.Var(&load.BuildGcflags, "gcflags", "")
 	cmd.Flag.Var(&load.BuildGccgoflags, "gccgoflags", "")
-	cmd.Flag.StringVar(&cfg.BuildMod, "mod", "", "")
+	if mask&OmitModFlag == 0 {
+		cmd.Flag.StringVar(&cfg.BuildMod, "mod", "", "")
+	}
 	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
 	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
 	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")