feat: carry the byte offset and column in SyntaxError

Assisted-by: GLM 5.3 Flash

interpres.go

@@ -21,24 +21,52 @@
 package interpres
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
 	"os"
 	"slices"
+	"strings"
 )
 
-// A SyntaxError describes a malformed TOML document, including the 1-based
-// line on which the problem was detected.
+// A SyntaxError describes a malformed TOML document. Line is the 1-based line
+// the problem was detected on. Offset is the byte offset in the input the scan
+// stopped at, and Column is the 1-based column on that line; both are new in
+// 2.0 and a struct literal that names Line and Msg alone still builds.
 type SyntaxError struct {
-	Line int
-	Msg  string
+	Line   int
+	Offset int
+	Column int
+	Msg    string
 }
 
 func (e *SyntaxError) Error() string {
 	return fmt.Sprintf("interpres: line %d: %s", e.Line, e.Msg)
 }
 
+// SourceLine returns the source line the error points at, rendered from src,
+// followed by a caret line marking the column. It is meant for a message the
+// reader sees under the input:
+//
+//	port = = 8080
+//	       ^
+//
+// The caret sits at Offset when it falls inside src, and at the start of the
+// line when the error carries no position.
+func (e *SyntaxError) SourceLine(src []byte) string {
+	off := min(e.Offset, len(src))
+	start := 0
+	if i := bytes.LastIndexByte(src[:off], '\n'); i >= 0 {
+		start = i + 1
+	}
+	end := len(src)
+	if i := bytes.IndexByte(src[start:], '\n'); i >= 0 {
+		end = start + i
+	}
+	return string(src[start:end]) + "\n" + strings.Repeat(" ", off-start) + "^"
+}
+
 // A DecodeError wraps a decoding failure with the key path at which it
 // happened. Path lists one segment per level from the document root, the
 // outermost key first: a key contributes its name and an array element its