Cache error offset in ParserError for safer position tracking

Instead of requiring downstream consumers to re-derive the byte offset
from pointer arithmetic on the Highlight slice, compute and cache the
offset inside the parser at error-capture time via setErrOffset().

This is safer because:
- The parser is the one place where the backing-array guarantee is known
  to hold (Highlight is always a subslice of the parse buffer)
- Downstream consumers (wrapDecodeError) can use the cached offset
  directly, avoiding the need for pointer comparison
- Errors created outside the parser (strict.go) set the offset from
  existing Raw ranges, which are already correct by construction

Add ParserError.SetOffset/Offset methods for setting and retrieving the
cached offset. Update wrapDecodeError to prefer the cached offset when
available, falling back to subsliceOffset for backward compatibility.

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

unstable/parser.go

@@ -2,6 +2,7 @@ package unstable
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"reflect"
 	"unicode"
@@ -17,6 +18,9 @@ type ParserError struct {
 	Highlight []byte
 	Message   string
 	Key       []string // optional
+
+	offset      int
+	offsetValid bool
 }
 
 // Error is the implementation of the error interface.
@@ -24,6 +28,21 @@ func (e *ParserError) Error() string {
 	return e.Message
 }
 
+// SetOffset records the byte offset of the error highlight within the
+// document. Used by the parser to cache position information so
+// downstream consumers don't need to re-derive it from pointers.
+func (e *ParserError) SetOffset(offset int) {
+	e.offset = offset
+	e.offsetValid = true
+}
+
+// Offset returns the byte offset of the error highlight within the
+// document, if it was previously set by the parser. The boolean
+// indicates whether the offset is valid.
+func (e *ParserError) Offset() (int, bool) {
+	return e.offset, e.offsetValid
+}
+
 // NewParserError is a convenience function to create a ParserError
 //
 // Warning: Highlight needs to be a subslice of Parser.data, so only slices
@@ -137,12 +156,14 @@ func (p *Parser) NextExpression() bool {
 		}
 
 		if len(p.left) == 0 || p.err != nil {
+			p.setErrOffset()
 			return false
 		}
 
 		p.ref, p.left, p.err = p.parseExpression(p.left)
 
 		if p.err != nil {
+			p.setErrOffset()
 			return false
 		}
 
@@ -165,6 +186,23 @@ func (p *Parser) Error() error {
 	return p.err
 }
 
+// setErrOffset computes and caches the byte offset of the error's
+// highlight within p.data, so downstream consumers can use it
+// without pointer arithmetic.
+func (p *Parser) setErrOffset() {
+	if p.err == nil {
+		return
+	}
+	var perr *ParserError
+	if !errors.As(p.err, &perr) {
+		return
+	}
+	if perr.offsetValid || len(perr.Highlight) == 0 {
+		return
+	}
+	perr.SetOffset(p.subsliceOffset(perr.Highlight))
+}
+
 // Position describes a position in the input.
 type Position struct {
 	// Number of bytes from the beginning of the input.