A parte específica do código que você está procurando (e comente) está em memline.c
:
/*
* Change the ".swp" extension to find another file that can be used.
* First decrement the last char: ".swo", ".swn", etc.
* If that still isn't enough decrement the last but one char: ".svz"
* Can happen when editing many "No Name" buffers.
*/
if (fname[n - 1] == 'a') /* ".s?a" */
{
if (fname[n - 2] == 'a') /* ".saa": tried enough, give up */
{
EMSG(_("E326: Too many swap files found"));
vim_free(fname);
fname = NULL;
break;
}
--fname[n - 2]; /* ".svz", ".suz", etc. */
fname[n - 1] = 'z' + 1;
}
--fname[n - 1]; /* ".swo", ".swn", etc. */