cue: support decoding into cue.Value fields

Fixes #2029.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ibe178f5428b2e97b04a1af9d9d9b631fea0a604c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/547369
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/decode.go

@@ -34,6 +34,10 @@ import (
 // An error is returned if x is nil or not a pointer.
 //
 // If x is a struct, Decode will validate the constraints specified in the field tags.
+//
+// If x contains a [Value], that part of x will be set to the value
+// at the corresponding part of v. This allows decoding values
+// that aren't entirely concrete into a Go type.
 func (v Value) Decode(x interface{}) error {
 	var d decoder
 	w := reflect.ValueOf(x)
@@ -71,6 +75,8 @@ func (d *decoder) clear(x reflect.Value) {
 	}
 }
 
+var valueType = reflect.TypeOf(Value{})
+
 func (d *decoder) decode(x reflect.Value, v Value, isPtr bool) {
 	if !x.IsValid() {
 		d.addErr(errors.Newf(v.Pos(), "cannot decode into invalid value"))
@@ -87,6 +93,10 @@ func (d *decoder) decode(x reflect.Value, v Value, isPtr bool) {
 		d.addErr(err)
 		return
 	}
+	if x.Type() == valueType {
+		x.Set(reflect.ValueOf(v))
+		return
+	}
 
 	switch x.Kind() {
 	case reflect.Ptr, reflect.Map, reflect.Slice, reflect.Interface:

cue/decode_test.go

@@ -15,10 +15,12 @@
 package cue
 
 import (
+	"fmt"
 	"reflect"
 	"testing"
 	"time"
 
+	"github.com/go-quicktest/qt"
 	"github.com/google/go-cmp/cmp"
 )
 
@@ -250,6 +252,25 @@ func TestDecode(t *testing.T) {
 	}
 }
 
+func TestDecodeIntoCUEValue(t *testing.T) {
+	// We should be able to decode into a CUE value so we can
+	// decode partially incomplete values into Go.
+	// This test doesn't fit within the table used by TestDecode
+	// because cue values aren't easily comparable with cmp.Diff.
+	var st struct {
+		X Value `json:"x"`
+	}
+	err := getInstance(t, `x: string`).Value().Decode(&st)
+	qt.Assert(t, qt.IsNil(err))
+	qt.Assert(t, qt.Equals(fmt.Sprint(st.X), "string"))
+
+	// Check we can decode into a top level value.
+	var v Value
+	err = getInstance(t, `int`).Value().Decode(&v)
+	qt.Assert(t, qt.IsNil(err))
+	qt.Assert(t, qt.Equals(fmt.Sprint(v), "int"))
+}
+
 type Duration struct {
 	D time.Duration
 }