I am obviously not able to understand what, specific, problem this is solving based on the title of "parsing all of C" when the preprocessor is apparently left intact by design

    static int mousedev_open(struct inode *inode, struct file *file)
    {
    int i;

    #ifdef CONFIG_INPUT_MOUSEDEV_PSAUX
    if (imajor(inode) == 10)
    i = 31;
    else
    #endif
    i = iminor(inode) - 32;

    return 0;
    }
    (b) The preprocessed source preserving all configurations

and my experience with C is that there are untold number of "unbound" tokens that are designed to be injected in by -D or auto-generated config.h files, so presumably this works closer to the "ready for compilation" phase versus something one could use to make tree-sitter better (as an example)