perf(decode): parse struct destinations without the value tree

Assisted-by: GLM 5.3 Flash

interpres.go

@@ -305,14 +305,22 @@ func NewSchema[T any]() {
 
 // UnmarshalContext is the cancellable variant of Unmarshal.
 func UnmarshalContext(ctx context.Context, data []byte, v any) error {
+	dec := newDecoder()
+	dec.ctx = ctx
+	if canTargetDecode(v) {
+		// The targeted parse fills struct destinations without the
+		// intermediate tree; a document or destination it cannot model falls
+		// back to the tree path, whose contracts it keeps.
+		if err := parseIntoTargeted(ctx, data, dec, false, 0, v); err != errTargetFallback {
+			return err
+		}
+	}
 	// Only a destination that can reach an OrderedMap needs the node tree the
 	// written key order is read from; every other decode skips building it.
 	tree, doc, err := parseWithOptions(ctx, data, parseOptions{}, typeWantsOrder(reflect.TypeOf(v)))
 	if err != nil {
 		return err
 	}
-	dec := newDecoder()
-	dec.ctx = ctx
 	dec.nodes = indexNodes(doc.Root())
 	return dec.decode(tree, v)
 }
@@ -390,6 +398,22 @@ func (d *Decoder) Decode(data []byte, v any) error {
 
 // DecodeContext is the cancellable variant of Decode.
 func (d *Decoder) DecodeContext(ctx context.Context, data []byte, v any) error {
+	dec := newDecoder()
+	dec.disallowUnknown = d.disallowUnknown
+	dec.ctx = ctx
+	dec.loc = d.localLoc
+	if canTargetDecode(v) {
+		// The targeted parse fills struct destinations without the
+		// intermediate tree; a document or destination it cannot model falls
+		// back to the tree path, whose contracts it keeps. The size limit is
+		// checked here, the targeted parse being the parse itself.
+		if d.maxInputSize > 0 && len(data) > d.maxInputSize {
+			return fmt.Errorf("interpres: input is %d bytes, over the limit of %d", len(data), d.maxInputSize)
+		}
+		if err := parseIntoTargeted(ctx, data, dec, d.useNumber, d.maxDepth, v); err != errTargetFallback {
+			return err
+		}
+	}
 	opts := parseOptions{
 		maxDepth:     d.maxDepth,
 		maxInputSize: d.maxInputSize,
@@ -399,11 +423,7 @@ func (d *Decoder) DecodeContext(ctx context.Context, data []byte, v any) error {
 	if err != nil {
 		return err
 	}
-	dec := newDecoder()
-	dec.disallowUnknown = d.disallowUnknown
-	dec.ctx = ctx
 	dec.nodes = indexNodes(doc.Root())
-	dec.loc = d.localLoc
 	return dec.decode(tree, v)
 }