cue: reuse adt.OpContext in Value.structValOpts

It called getStruct, which built an entirely new adt.OpContext
rather than reusing the one the parent already had.
The method was used only once too, so inline it to simplify.

                            │     old     │                new                │
                            │   sec/op    │   sec/op     vs base              │
    LargeValueMarshalJSON-8   6.370m ± 1%   6.083m ± 1%  -4.50% (p=0.002 n=6)

                            │     old      │                 new                 │
                            │     B/op     │     B/op      vs base               │
    LargeValueMarshalJSON-8   4.178Mi ± 0%   3.445Mi ± 0%  -17.56% (p=0.002 n=6)

                            │     old     │                new                │
                            │  allocs/op  │  allocs/op   vs base              │
    LargeValueMarshalJSON-8   60.26k ± 0%   58.26k ± 0%  -3.33% (p=0.002 n=6)

Updates #2470.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I28bd8b820bf022e48257b445aa2e6d6e628dfe16
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201826
Reviewed-by: Roger Peppe <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cue/types.go

@@ -1424,8 +1424,7 @@ func (v Value) structValOpts(ctx *adt.OpContext, o options) (s structValue, err
 	// Allow scalar values if hidden or definition fields are requested.
 	case !o.omitHidden, !o.omitDefinitions:
 	default:
-		obj, err = v.getStruct()
-		if err != nil {
+		if err := v.checkKind(ctx, adt.StructKind); err != nil && !err.ChildError {
 			return structValue{}, err
 		}
 	}
@@ -1487,16 +1486,6 @@ func (v hiddenValue) Struct() (*Struct, error) {
 	return &Struct{obj}, nil
 }
 
-func (v Value) getStruct() (*adt.Vertex, *adt.Bottom) {
-	ctx := v.ctx()
-	if err := v.checkKind(ctx, adt.StructKind); err != nil {
-		if !err.ChildError {
-			return nil, err
-		}
-	}
-	return v.v, nil
-}
-
 // Struct represents a CUE struct value.
 type Struct struct {
 	structValue