cmd/cue: mod init should only add @v0 with the experiment

Otherwise, existing users of `cue mod init` who haven't opted into
CUE_EXPERIMENT=modules would get different and likely broken behavior,
made worse by the known breakage in #2945.

We add test cases with and without the experiment for both
a module path argument with and without a major version, for clarity.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I32afc0e669fd5bfd5634ec382111d2e2846dcd9e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1183394
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/mod.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/spf13/cobra"
 
+	"cuelang.org/go/internal/cueexperiment"
 	"cuelang.org/go/mod/modfile"
 	"cuelang.org/go/mod/module"
 	gomodule "golang.org/x/mod/module"
@@ -99,8 +100,10 @@ func runModInit(cmd *Command, args []string) (err error) {
 				}
 				return fmt.Errorf("invalid module name %q: %v", modulePath, err1)
 			}
-			// Default major version to v0.
-			modulePath += "@v0"
+			// Default major version to v0 if the modules experiment is enabled.
+			if cueexperiment.Flags.Modules {
+				modulePath += "@v0"
+			}
 		}
 	}
 

cmd/cue/cmd/testdata/script/modinit_majorversion.txtar

@@ -1,11 +1,28 @@
 env CUE_VERSION_OVERRIDE=v0.1.2
+
+# Without the experiment, the major version is allowed,
+# even though it's not particularly useful.
+# It feels unnecessary to ask the user to re-run with the experiment.
 exec cue mod init foo.com/bar@v1
 cmp cue.mod/module.cue want-module.cue
 exists cue.mod/usr
 exists cue.mod/pkg
 
+# With the experiment, it works as expected.
+env CUE_EXPERIMENT=modules
+rm cue.mod
+exec cue mod init foo.com/bar@v1
+cmp cue.mod/module.cue want-module-experiment.cue
+exists cue.mod/usr
+exists cue.mod/pkg
+
 -- want-module.cue --
 module: "foo.com/bar@v1"
 language: {
 	version: "v0.1.2"
 }
+-- want-module-experiment.cue --
+module: "foo.com/bar@v1"
+language: {
+	version: "v0.1.2"
+}

cmd/cue/cmd/testdata/script/modinit_nomajorversion.txtar

@@ -1,10 +1,25 @@
 env CUE_VERSION_OVERRIDE=v0.1.2
+
+# Without the experiment, we use the module path as-is.
 exec cue mod init foo.com/bar
 cmp cue.mod/module.cue want-module.cue
 exists cue.mod/usr
 exists cue.mod/pkg
 
+# With the experiment, we add the major version v0 by default.
+env CUE_EXPERIMENT=modules
+rm cue.mod
+exec cue mod init foo.com/bar
+cmp cue.mod/module.cue want-module-experiment.cue
+exists cue.mod/usr
+exists cue.mod/pkg
+
 -- want-module.cue --
+module: "foo.com/bar"
+language: {
+	version: "v0.1.2"
+}
+-- want-module-experiment.cue --
 module: "foo.com/bar@v0"
 language: {
 	version: "v0.1.2"