cue: reuse adt.OpContext in MarshalJSON recursion

Create the context once in MarshalJSON and reuse it in appendJSON calls.

                            │     old     │                new                 │
                            │   sec/op    │   sec/op     vs base               │
    LargeValueMarshalJSON-8   7.664m ± 1%   6.484m ± 1%  -15.39% (p=0.002 n=6)

                            │     old      │                 new                 │
                            │     B/op     │     B/op      vs base               │
    LargeValueMarshalJSON-8   7.158Mi ± 0%   4.224Mi ± 0%  -40.98% (p=0.002 n=6)

                            │     old     │                new                 │
                            │  allocs/op  │  allocs/op   vs base               │
    LargeValueMarshalJSON-8   70.28k ± 0%   62.27k ± 0%  -11.40% (p=0.002 n=6)

Updates #2470.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Id24eb441c19e5d6028cc7fa8e1eaf0d1551623e9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201824
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mvdan

cue/decode.go

@@ -118,7 +118,7 @@ func (d *decoder) decode(x reflect.Value, v Value, isPtr bool) {
 	ij, it, x := indirect(x, v.IsNull())
 
 	if ij != nil {
-		b, err := v.appendJSON(nil)
+		b, err := v.MarshalJSON()
 		d.addErr(err)
 		d.addErr(ij.UnmarshalJSON(b))
 		return

cue/types.go

@@ -155,7 +155,7 @@ func (o *structValue) appendJSON(b []byte) ([]byte, error) {
 		}
 		b = append(b, s...)
 		b = append(b, ':')
-		b, err = v.appendJSON(b)
+		b, err = v.appendJSON(o.ctx, b)
 		if err != nil {
 			return nil, err
 		}
@@ -299,7 +299,7 @@ func listAppendJSON(b []byte, l *Iterator) ([]byte, error) {
 	if l.Next() {
 		for i := 0; ; i++ {
 			var err error
-			b, err = l.Value().appendJSON(b)
+			b, err = l.Value().appendJSON(l.ctx, b)
 			if err != nil {
 				return nil, err
 			}
@@ -901,19 +901,19 @@ func (v Value) IncompleteKind() Kind {
 
 // MarshalJSON marshalls this value into valid JSON.
 func (v Value) MarshalJSON() (b []byte, err error) {
-	b, err = v.appendJSON(nil)
+	ctx := newContext(v.idx)
+	b, err = v.appendJSON(ctx, nil)
 	if err != nil {
 		return nil, unwrapJSONError(err)
 	}
 	return b, nil
 }
 
-func (v Value) appendJSON(b []byte) ([]byte, error) {
+func (v Value) appendJSON(ctx *adt.OpContext, b []byte) ([]byte, error) {
 	v, _ = v.Default()
 	if v.v == nil {
 		return append(b, "null"...), nil
 	}
-	ctx := newContext(v.idx)
 	x := v.eval(ctx)
 
 	if _, ok := x.(adt.Resolver); ok {