cue/cuecontext: fix EvalDefault to follow the default, add EvalStable

When we flipped evalv3 on by default in https://cuelang.org/cl/1211974
we flipped the default for the experiment flag,
but we forgot about cuecontext.EvalDefault, which remained at evalv2.

We could tweak Evaldefault to instead be fixed at evalv3, but that isn't
quite right either; if a user creates an evaluator context via

    cuecontext.New()

the selected version follows the CUE_EXPERIMENT=evalv3 flag,
which is the default behavior implemented in cmd/cue as well.

It would then follow that using

    cuecontext.New(cuecontext.EvaluatorVersion(cuecontext.EvalDefault))

would result in the same behavior, but it does not if EvalDefault
is fixed at either EvalV2 or EvalV3 as a constant.

Make EvalDefault select a default version as described above,
and add EvalStable as an alias for the latest stable version of
the evaluator, which is what EvalDefault used to implement.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I3420fbbadde4e45e3e2c5b27a710a85fda66a12b
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1212505
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mvdan

cue/cuecontext/cuecontext.go

@@ -61,11 +61,17 @@ func Interpreter(i ExternInterpreter) Option {
 type EvalVersion = internal.EvaluatorVersion
 
 const (
-	// EvalDefault is the latest stable version of the evaluator.
+	// EvalDefault is the default version of the evaluator, which is selected based on
+	// the CUE_EXPERIMENT environment variable described in [cue help environment].
+	//
+	// [cue help environment]: https://cuelang.org/docs/reference/command/cue-help-environment/
 	EvalDefault EvalVersion = internal.DefaultVersion
 
-	// EvalExperiment refers to the latest unstable version of the evaluator.
-	// Note that this version may change without notice.
+	// EvalDefault is the latest stable version of the evaluator, currently [EvalV3].
+	EvalStable EvalVersion = internal.StableVersion
+
+	// EvalExperiment refers to the latest in-development version of the evaluator,
+	// currently [EvalV3]. Note that this version may change without notice.
 	EvalExperiment EvalVersion = internal.DevVersion
 
 	// EvalV2 is the previous version of the evaluator. It was introduced in CUE
@@ -74,8 +80,7 @@ const (
 
 	// EvalV3 is the current version of the evaluator. It was introduced in 2024
 	// and brought a new disjunction algorithm, a new closedness algorithm, a
-	// new core scheduler, and adds performance enhancements like structure
-	// sharing.
+	// new core scheduler, and adds performance enhancements like structure sharing.
 	EvalV3 EvalVersion = internal.EvalV3
 )
 

cue/cuecontext/cuecontext_test.go

@@ -93,6 +93,7 @@ func TestEvalVersion(t *testing.T) {
 	// The experiment evaluator version setting does not affect the specific
 	// versions like Stable or V3, as they are fixed.
 	testFixedVersions := func() {
+		test(New(EvaluatorVersion(EvalStable)), internal.EvalV3)
 		// We currently don't have an experimental version, so it's the current version.
 		test(New(EvaluatorVersion(EvalExperiment)), internal.EvalV3)
 		test(New(EvaluatorVersion(EvalV2)), internal.EvalV2)
@@ -102,15 +103,7 @@ func TestEvalVersion(t *testing.T) {
 	// The current and default evaluator version is EvalV3.
 	qt.Assert(t, qt.Equals(cueexperiment.Flags.EvalV3, true))
 	test(New(), internal.EvalV3)
-	// TODO(mvdan): explicitly selecting the default should result in evalv3 here,
-	// just like implicitly selecting the default by not using the EvaluatorVersion flag.
-	// It currently does not, because internally, we treat "unset" vs "default"
-	// as different version selection scenarios.
-	//
-	// Or, if we want an evaluator version to describe "latest stable", opposing
-	// EvalExperiment to describe "latest experimental", we should rename it to EvalStable
-	// and keep its current behavior.
-	test(New(EvaluatorVersion(EvalDefault)), internal.EvalV2)
+	test(New(EvaluatorVersion(EvalDefault)), internal.EvalV3)
 
 	testFixedVersions()
 

internal/core/runtime/imports.go

@@ -22,7 +22,6 @@ import (
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/internal"
 	"cuelang.org/go/internal/core/adt"
-	"cuelang.org/go/internal/cueexperiment"
 )
 
 type PackageFunc func(ctx adt.Runtime) (*adt.Vertex, errors.Error)
@@ -59,12 +58,7 @@ var SharedRuntime = sync.OnceValue(func() *Runtime {
 	// but future evaluator versions may not be compatible at that level.
 	// We should consider using one SharedRuntime per evaluator version,
 	// or getting rid of SharedRuntime altogether.
-	cueexperiment.Init()
-	if cueexperiment.Flags.EvalV3 {
-		r.version = internal.EvalV3
-	} else {
-		r.version = internal.EvalV2
-	}
+	r.SetVersion(internal.DefaultVersion)
 	return r
 })
 

internal/core/runtime/runtime.go

@@ -80,7 +80,17 @@ func NewWithSettings(v internal.EvaluatorVersion, flags cuedebug.Config) *Runtim
 // SetVersion sets the version to use for the Runtime. This should only be set
 // before first use.
 func (r *Runtime) SetVersion(v internal.EvaluatorVersion) {
-	r.version = v
+	switch v {
+	case internal.EvalV2, internal.EvalV3:
+		r.version = v
+	case internal.EvalVersionUnset, internal.DefaultVersion:
+		cueexperiment.Init()
+		if cueexperiment.Flags.EvalV3 {
+			r.version = internal.EvalV3
+		} else {
+			r.version = internal.EvalV2
+		}
+	}
 }
 
 // SetTopologicalSort sets whether or not to use topological sorting
@@ -114,12 +124,7 @@ func (r *Runtime) Init() {
 
 	r.loaded = map[*build.Instance]interface{}{}
 
-	cueexperiment.Init()
-	if cueexperiment.Flags.EvalV3 {
-		r.version = internal.EvalV3
-	} else {
-		r.version = internal.EvalV2
-	}
+	r.SetVersion(internal.DefaultVersion)
 	r.topoSort = cueexperiment.Flags.TopoSort
 
 	// By default we follow the environment's CUE_DEBUG settings,

internal/internal.go

@@ -118,17 +118,23 @@ const (
 	// EvalVersionUnset is the zero value, which signals that no evaluator version is provided.
 	EvalVersionUnset EvaluatorVersion = 0
 
+	// DefaultVersion is a special value as it selects a version depending on the current
+	// value of CUE_EXPERIMENT. It exists separately to [EvalVersionUnset], even though both
+	// implement the same version selection logic, so that we can distinguish between
+	// a user explicitly asking for the default version versus an entirely unset version.
+	DefaultVersion EvaluatorVersion = -1 // TODO(mvdan): rename to EvalDefault for consistency with cuecontext
+
 	// The values below are documented under [cuelang.org/go/cue/cuecontext.EvalVersion].
 	// We should never change or delete the values below, as they describe all known past versions
 	// which is useful for understanding old debug output.
 
 	EvalV2 EvaluatorVersion = 2
 	EvalV3 EvaluatorVersion = 3
 
-	// The current default and experimental versions.
+	// The current default, stable, and experimental versions.
 
-	DefaultVersion = EvalV2 // TODO(mvdan): rename to EvalDefault for consistency with cuecontext
-	DevVersion     = EvalV3 // TODO(mvdan): rename to EvalExperiment for consistency with cuecontext
+	StableVersion = EvalV3 // TODO(mvdan): rename to EvalStable for consistency with cuecontext
+	DevVersion    = EvalV3 // TODO(mvdan): rename to EvalExperiment for consistency with cuecontext
 )
 
 // ListEllipsis reports the list type and remaining elements of a list. If we