cmd/cue: hard-code a fallback CUE semver version

As described in https://cuelang.org/issue/2908, the behavior now is:

1) Users explicity setting a version at build time via
   -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=<version>'
   would use that version as-is, just like today
2) Users building via GOPROXY would get the module version
   reported by the proxy as-is, just like today
3) Other users (i.e. those using `go install ./cmd/cue`) would now get
   the fallback version added by this patch and updated manually,
   even if they are currently a few commits after that last tag,
   since `cue version` will still print the stamped VCS information

That is, up until now and in master, we had

    $ go install ./cmd/cue
    $ cue version
    cue version v0.0.0-20240307083748-cdcb6fa19bf3

    go version devel go1.23-58052fe8e7 2024-03-07 05:27:04 +0000
          -buildmode exe
           -compiler gc
      DefaultGODEBUG httplaxcontentlength=1,httpmuxgo121=1,tls10server=1,tlsrsakex=1,tlsunsafeekm=1,winreadlinkvolume=0,winsymlink=0
         CGO_ENABLED 1
              GOARCH amd64
                GOOS linux
             GOAMD64 v3
                 vcs git
        vcs.revision cdcb6fa
            vcs.time 2024-03-07T08:37:48Z
        vcs.modified false

Whereas now, we have

    $ go install ./cmd/cue
    $ cue version
    cue version v0.8.0-alpha.5

    go version devel go1.23-58052fe8e7 2024-03-07 05:27:04 +0000
          -buildmode exe
           -compiler gc
      DefaultGODEBUG httplaxcontentlength=1,httpmuxgo121=1,tls10server=1,tlsrsakex=1,tlsunsafeekm=1,winreadlinkvolume=0,winsymlink=0
         CGO_ENABLED 1
              GOARCH amd64
                GOOS linux
             GOAMD64 v3
                 vcs git
        vcs.revision 9d9b786f0996468bf527a9412d1389a7a17a0562
            vcs.time 2024-03-07T15:13:21Z
        vcs.modified false

Such a local build will also now be able to add language.version
to cue.mod/module.cue via `cue mod init` and `cue mod tidy`,
as up until now a local cmd/cue build did not know its own
semver version, from which we derive the language version.

Add a TODO to this added fallback version, as once upstream Go
implements the proposal at https://go.dev/issue/50603,
we can remove this manually updated version and keep semver versions
in local builds via the Go toolchain automatically.

Fixes #2908.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I9e6189fcdaff5b493c9065e5bb74f4dd9a023800

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

@@ -5,3 +5,6 @@ exists cue.mod/pkg
 
 -- want-module.cue --
 module: "foo.com/bar@v1"
+language: {
+	version: "v0.8.0-alpha.5"
+}

cmd/cue/cmd/testdata/script/modinit_noargs.txtar

@@ -7,3 +7,6 @@ exists cue.mod/pkg
 
 -- want-module.cue --
 module: ""
+language: {
+	version: "v0.8.0-alpha.5"
+}

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

@@ -5,3 +5,6 @@ exists cue.mod/pkg
 
 -- want-module.cue --
 module: "foo.com/bar@v0"
+language: {
+	version: "v0.8.0-alpha.5"
+}

cmd/cue/cmd/testdata/script/modtidy_initial.txtar

@@ -11,6 +11,9 @@ exec cue export .
 cmp stdout want-stdout
 -- want-module --
 module: "main.org@v0"
+language: {
+	version: "v0.8.0-alpha.5"
+}
 deps: {
 	"bar.com@v0": {
 		v: "v0.5.0"

cmd/cue/cmd/testdata/script/modtidy_non_root.txtar

@@ -8,6 +8,9 @@ cmp cue.mod/module.cue want-module
 
 -- want-module --
 module: "main.org@v0"
+language: {
+	version: "v0.8.0-alpha.5"
+}
 -- cue.mod/module.cue --
 module: "main.org@v0"
 -- x/y/z.cue --

cmd/cue/cmd/testdata/script/modtidy_with_generated_code.txtar

@@ -11,6 +11,9 @@ exec cue export .
 cmp stdout want-stdout
 -- want-module --
 module: "main.org@v0"
+language: {
+	version: "v0.8.0-alpha.5"
+}
 deps: {
 	"example.com@v0": {
 		v: "v0.0.1"

cmd/cue/cmd/testdata/script/modtidy_with_vcsinfo.txtar

@@ -15,6 +15,9 @@ cmp stdout want-stdout
 }
 -- want-module --
 module: "main.org@v0"
+language: {
+	version: "v0.8.0-alpha.5"
+}
 -- cue.mod/module.cue --
 module: "main.org@v0"
 

cmd/cue/cmd/testdata/script/version.txtar

@@ -1,14 +1,12 @@
 # Version output without VCS information.
-#
-# TODO: this test may fail if someone runs the tests via GOPROXY,
-# as then the module version won't be "devel".
 exec cue version
-stdout -count=1 'cue version \(devel\)'
+stdout -count=1 '^cue version v0.[1-9]'
+stdout -count=1 '^go version (devel )?go1.'
 
 # Version output with VCS information.
 env CUE_VERSION_TEST_CFG='[{"Key":"vcs","Value":"git"},{"Key":"vcs.revision","Value":"47b7032385cb490fab7d47b89fca36835cf13d39"},{"Key":"vcs.time","Value":"2022-05-10T04:58:46Z"},{"Key":"vcs.modified","Value":"true"}]'
 exec cue version
-stdout -count=1 '^cue version v0.0.0-20220510045846-47b7032385cb$'
+stdout -count=1 '^cue version v0.[1-9]'
 stdout -count=1 '^go version (devel )?go1.'
 stdout -count=1 'vcs git'
 stdout -count=1 'vcs\.revision 47b7032385cb490fab7d47b89fca36835cf13d39'

cmd/cue/cmd/version.go

@@ -21,10 +21,8 @@ import (
 	"os"
 	"runtime"
 	"runtime/debug"
-	"time"
 
 	"github.com/spf13/cobra"
-	"golang.org/x/mod/module"
 )
 
 func newVersionCmd(c *Command) *cobra.Command {
@@ -37,8 +35,6 @@ func newVersionCmd(c *Command) *cobra.Command {
 	return cmd
 }
 
-const defaultVersion = "(devel)"
-
 // version can be set by a builder using
 // -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=<version>'.
 // However, people should prefer building via a mechanism which
@@ -48,6 +44,15 @@ const defaultVersion = "(devel)"
 // considered legacy.
 var version string
 
+// fallbackVersion is used as-is when the -ldflags above isn't used
+// and when there isn't a recorded main module version,
+// for example when building via `go install ./cmd/cue`.
+// It should reflect the last release in the current branch.
+//
+// TODO: remove once Go stamps local builds with a main module version
+// derived from the local VCS information per https://go.dev/issue/50603.
+const fallbackVersion = "v0.8.0-alpha.5"
+
 func runVersion(cmd *Command, args []string) error {
 	w := cmd.OutOrStdout()
 
@@ -57,11 +62,7 @@ func runVersion(cmd *Command, args []string) error {
 		// shouldn't happen
 		return errors.New("unknown error reading build-info")
 	}
-	version := cueVersion(bi)
-	if version == "" {
-		version = defaultVersion
-	}
-	fmt.Fprintf(w, "cue version %s\n\n", version)
+	fmt.Fprintf(w, "cue version %s\n\n", cueVersion(bi))
 	fmt.Fprintf(w, "go version %s\n", runtime.Version())
 	for _, s := range bi.Settings {
 		if s.Value == "" {
@@ -88,41 +89,19 @@ func cueVersion(bi *debug.BuildInfo) string {
 	}
 	v := version
 	if v != "" {
-		// The global version variable has been
-		// configured via ldflags.
+		// The global version variable has been configured via ldflags.
 		return v
 	}
 	if bi == nil {
-		return ""
+		return fallbackVersion
 	}
-	if bi.Main.Version != "" && bi.Main.Version != defaultVersion {
+	if bi.Main.Version != "" && bi.Main.Version != "(devel)" {
 		v = bi.Main.Version
 	}
 	if v != "" {
 		return v
 	}
-	// a specific version was not provided by ldflags or buildInfo
-	// attempt to make our own
-	var vcsTime time.Time
-	var vcsRevision string
-	for _, s := range bi.Settings {
-		switch s.Key {
-		case "vcs.time":
-			// If the format is invalid, we'll print a zero timestamp.
-			vcsTime, _ = time.Parse(time.RFC3339Nano, s.Value)
-		case "vcs.revision":
-			vcsRevision = s.Value
-			// module.PseudoVersion recommends the revision to be a 12-byte
-			// commit hash prefix, which is what cmd/go uses as well.
-			if len(vcsRevision) > 12 {
-				vcsRevision = vcsRevision[:12]
-			}
-		}
-	}
-	if vcsRevision != "" {
-		return module.PseudoVersion("", "", vcsTime, vcsRevision)
-	}
-	return ""
+	return fallbackVersion
 }
 
 func readBuildInfo() (*debug.BuildInfo, bool) {