interpreter/embed: do not require CUE_EXPERIMENT=embed

In general, if someone is using `embed.New`, they are explicitly
asking to enable embedding, so there should be no need to additionally
set CUE_EXPERIMENT.

This means that we do need to explicitly check `CUE_EXPERIMENT`
inside `cmd/cue` to decide whether to include the embed interpreter.
Take the opportunity to remove the mutable `rootContextOptions`
variable, avoiding appending to it each time `cmd.New` is called,
and moving the `Command.ctx` initialization to `mkRunE`
where we've already invoked `cueexperiment.Init`.

Also change `internal/core/runtime` to always error if an unknown
interpreter is mentioned in an `@extern` attribute. This code path
was not encountered previously because `cmd/cue` always
added at least one interpreter (embed), but seems to make more
sense because in general `@extern` attributes should mean something
and if there's no interpreter found, it probably signals a real problem.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Id52402cf8e2d7fc0965e38b126c4058338614508
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202903
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: rogpeppe

cmd/cue/cmd/root.go

@@ -55,6 +55,9 @@ import (
 
 type runFunction func(cmd *Command, args []string) error
 
+// wasmInterp is set when the cuewasm build tag is enbabled.
+var wasmInterp cuecontext.ExternInterpreter
+
 func statsEncoder(cmd *Command) (*encoding.Encoder, error) {
 	file := os.Getenv("CUE_STATS_FILE")
 	if file == "" {
@@ -105,6 +108,14 @@ func mkRunE(c *Command, f runFunction) func(*cobra.Command, []string) error {
 		if err := cuedebug.Init(); err != nil {
 			return err
 		}
+		var opts []cuecontext.Option
+		if wasmInterp != nil {
+			opts = append(opts, cuecontext.Interpreter(wasmInterp))
+		}
+		if cueexperiment.Flags.Embed {
+			opts = append(opts, cuecontext.Interpreter(embed.New()))
+		}
+		c.ctx = cuecontext.New(opts...)
 		// Some init work, such as in internal/filetypes, evaluates CUE by design.
 		// We don't want that work to count towards $CUE_STATS.
 		adt.ResetStats()
@@ -198,13 +209,9 @@ func New(args []string) (*Command, error) {
 		DisableSuggestions: true,
 	}
 
-	embedding := cuecontext.Interpreter(embed.New())
-	rootContextOptions = append(rootContextOptions, embedding)
-
 	c := &Command{
 		Command: cmd,
 		root:    cmd,
-		ctx:     cuecontext.New(rootContextOptions...),
 	}
 	c.cmdCmd = newCmdCmd(c)
 
@@ -252,8 +259,6 @@ func New(args []string) (*Command, error) {
 	return c, nil
 }
 
-var rootContextOptions []cuecontext.Option
-
 // rootWorkingDir avoids repeated calls to [os.Getwd] in cmd/cue.
 // If we can't figure out the current directory, something is very wrong,
 // and there's no point in continuing to run a command.

cmd/cue/cmd/root_cuewasm.go

@@ -17,12 +17,11 @@
 package cmd
 
 import (
-	"cuelang.org/go/cue/cuecontext"
 	"cuelang.org/go/cue/interpreter/wasm"
 )
 
-// The wasm interpreter can be enabled by default once we are ready to ship the feature.
-// For now, it's not ready, and makes cue binaries heavier by over 2MiB.
 func init() {
-	rootContextOptions = append(rootContextOptions, cuecontext.Interpreter(wasm.New()))
+	// The wasm interpreter can be enabled by default once we are ready to ship the feature.
+	// For now, it's not ready, and makes cue binaries heavier by over 2MiB.
+	wasmInterp = wasm.New()
 }

cmd/cue/cmd/testdata/script/embed.txtar

@@ -1,5 +1,5 @@
-exec cue eval
-cmp stdout out/noembed
+! exec cue eval
+cmp stderr out/noembed
 
 env CUE_EXPERIMENT=embed
 
@@ -108,19 +108,8 @@ language: version: "v0.9.0"
     }
 }
 -- out/noembed --
-a: _
-b: _
-c: _
-d: _
-f: _
-g: _
-special: {
-    underscoreFile: _
-    dotFile:        _
-    dotdotFile:     _
-    underscoreDir:  _
-    dotDir:         _
-}
+no interpreter defined for "embed":
+    ./test.cue:1:1
 -- out/eval --
 a: {
     x: 34

cue/interpreter/embed/embed.go

@@ -96,7 +96,6 @@ import (
 	"cuelang.org/go/internal"
 	"cuelang.org/go/internal/core/adt"
 	"cuelang.org/go/internal/core/runtime"
-	"cuelang.org/go/internal/cueexperiment"
 	"cuelang.org/go/internal/encoding"
 	"cuelang.org/go/internal/filetypes"
 	"cuelang.org/go/internal/value"
@@ -149,11 +148,6 @@ type compiler struct {
 // (@embed(file=...)) or a glob of files (@embed(glob=...)).
 // and decodes the given files.
 func (c *compiler) Compile(funcName string, scope adt.Value, a *internal.Attr) (adt.Expr, errors.Error) {
-	// This is a really weird spot to disable embedding, but I could not get
-	// the wasm tests to pass without doing it like this.
-	if !cueexperiment.Flags.Embed {
-		return &adt.Top{}, nil
-	}
 
 	file, _, err := a.Lookup(0, "file")
 	if err != nil {

internal/core/runtime/extern.go

@@ -61,9 +61,6 @@ type Compiler interface {
 // injectImplementations modifies v to include implementations of functions
 // for fields associated with the @extern attributes.
 func (r *Runtime) injectImplementations(b *build.Instance, v *adt.Vertex) (errs errors.Error) {
-	if r.interpreters == nil {
-		return nil
-	}
 
 	d := &externDecorator{
 		runtime: r,