perf(parse): cut allocations and validate UTF-8 in the scan

interpres.go

@@ -24,7 +24,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"unicode/utf8"
 )
 
 // A SyntaxError describes a malformed TOML document, including the 1-based
@@ -143,9 +142,9 @@ func parseWithOptions(ctx context.Context, data []byte, opts parseOptions, wantD
 	if opts.maxInputSize > 0 && len(data) > opts.maxInputSize {
 		return nil, nil, fmt.Errorf("interpres: input is %d bytes, over the limit of %d", len(data), opts.maxInputSize)
 	}
-	if !utf8.Valid(data) {
-		return nil, nil, &SyntaxError{Line: 1, Msg: "input is not valid UTF-8"}
-	}
+	// UTF-8 validity is not checked in a pass of its own: the scanner
+	// validates the multi-byte sequences where it meets them, so an invalid
+	// byte is reported on its own line instead of always on line 1.
 	maxDepth := opts.maxDepth
 	if maxDepth <= 0 {
 		maxDepth = maxNestingDepth