internal/cueversion: construct pseudoversion when possible

This change copies some old logic that was previously deleted
(in https://cuelang.org/cl/1176194) to construct a pseudoversion
when one isn't directly present.

It's a little tricky to test but has been manually verified to work.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I508011561c0dd3f3b212c777919f64f67411ad35
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194091
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

internal/cueversion/version.go

@@ -8,6 +8,9 @@ import (
 	"runtime/debug"
 	"strings"
 	"sync"
+	"time"
+
+	"golang.org/x/mod/module"
 )
 
 // LanguageVersion returns the CUE language version.
@@ -40,7 +43,32 @@ var moduleVersionOnce = sync.OnceValue(func() string {
 		// module name; it also happens when running the cue tests.
 		return "(no-cue-module)"
 	}
-	return cueMod.Version
+	version := cueMod.Version
+	if version != "(devel)" {
+		return version
+	}
+	// A specific version was not provided by the buildInfo
+	// so 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 != "" {
+		version = module.PseudoVersion("", "", vcsTime, vcsRevision)
+	}
+	return version
 })
 
 func findCUEModule(bi *debug.BuildInfo) *debug.Module {