internal/cueexperiment: parse CUE_EXPERIMENT

This allows code the cue command to enable experimentation with new
features, and modules in particular to start with.

In the future, this API might be adapted to allow enabling
experimental features in the Go API too, but for now it's
firmly oriented towards the top level cue command.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I76d5c9343bd791ae0ff9ef39fdf7af9dec5c2d8f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1168930
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

cmd/cue/cmd/root.go

@@ -29,6 +29,7 @@ import (
 	"cuelang.org/go/cue/interpreter/wasm"
 	"cuelang.org/go/cue/stats"
 	"cuelang.org/go/internal/core/adt"
+	"cuelang.org/go/internal/cueexperiment"
 	"cuelang.org/go/internal/encoding"
 	"cuelang.org/go/internal/filetypes"
 )
@@ -84,6 +85,10 @@ func mkRunE(c *Command, f runFunction) func(*cobra.Command, []string) error {
 
 		statsEnc := statsEncoder(c)
 
+		if err := cueexperiment.Init(); err != nil {
+			return err
+		}
+
 		err := f(c, args)
 
 		if statsEnc != nil {

cmd/cue/cmd/testdata/script/experiment_unknown.txtar

@@ -0,0 +1,3 @@
+env CUE_EXPERIMENT=xxx
+! exec cue eval something
+stderr 'unknown CUE_EXPERIMENT xxx'

internal/cueexperiment/exp.go

@@ -0,0 +1,40 @@
+package cueexperiment
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+	"strings"
+)
+
+// Flags holds the set of CUE_EXPERIMENT flags. It is initialized
+// by Init.
+var Flags struct {
+	Modules bool
+}
+
+// Init initializes Flags. Note: this isn't named "init" because we
+// don't always want it to be called (for example we don't want it to be
+// called when running "cue help"), and also because we want the failure
+// mode to be one of error not panic, which would be the only option if
+// it was a top level init function
+func Init() error {
+	exp := os.Getenv("CUE_EXPERIMENT")
+	if exp == "" {
+		return nil
+	}
+	names := make(map[string]int)
+	fv := reflect.ValueOf(&Flags).Elem()
+	ft := fv.Type()
+	for i := 0; i < ft.NumField(); i++ {
+		names[strings.ToLower(ft.Field(i).Name)] = i
+	}
+	for _, uexp := range strings.Split(exp, ",") {
+		index, ok := names[uexp]
+		if !ok {
+			return fmt.Errorf("unknown CUE_EXPERIMENT %s", uexp)
+		}
+		fv.Field(index).SetBool(true)
+	}
+	return nil
+}

internal/cueexperiment/exp_test.go

@@ -0,0 +1,60 @@
+package cueexperiment
+
+import (
+	"testing"
+
+	"github.com/go-quicktest/qt"
+)
+
+var tests = []struct {
+	testName      string
+	cueExperiment string
+	flagVal       *bool
+	want          bool
+	wantError     string
+}{{
+	testName:      "Empty",
+	cueExperiment: "",
+	flagVal:       &Flags.Modules,
+	want:          false,
+}, {
+	testName:      "Unknown",
+	cueExperiment: "foo",
+	flagVal:       &Flags.Modules,
+	wantError:     "unknown CUE_EXPERIMENT foo",
+}, {
+	testName:      "Set",
+	cueExperiment: "modules",
+	flagVal:       &Flags.Modules,
+	want:          true,
+}, {
+	testName:      "SetTwice",
+	cueExperiment: "modules,modules",
+	flagVal:       &Flags.Modules,
+	want:          true,
+}, {
+	testName:      "SetWithUnknown",
+	cueExperiment: "modules,other",
+	flagVal:       &Flags.Modules,
+	wantError:     "unknown CUE_EXPERIMENT other",
+}}
+
+func TestInit(t *testing.T) {
+	for _, test := range tests {
+		t.Run(test.testName, func(t *testing.T) {
+			setZero(&Flags)
+			t.Setenv("CUE_EXPERIMENT", test.cueExperiment)
+			err := Init()
+			if test.wantError != "" {
+				qt.Assert(t, qt.ErrorMatches(err, test.wantError))
+				return
+			}
+			qt.Assert(t, qt.IsNil(err))
+			qt.Assert(t, qt.Equals(*test.flagVal, test.want))
+		})
+	}
+}
+
+func setZero[T any](x *T) {
+	*x = *new(T)
+}