D13031 changed the criteria for strings which can be evaluated statically. As a result, ArcanistFormattedStringXHPASTLinterRule now fails (throws an exception) on the following text:

printf("  % {$width}d  %s\n", $idx + 1, $line);

A simple solution would be to revert rPHUa9e581f6488fd7db83d4c02edee64db9b1cd3ef9, but maybe we can just fix this somehow.