Function setloopvar 

pub fn setloopvar(name: &str, value: &str)

Param pm = (Param) gethashnode2(realparamtab, name);
if (pm && (pm->node.flags & PM_NAMEREF)) {
    if (pm->node.flags & PM_READONLY) {
        zerr("read-only reference: %s", pm->node.nam); return;
    }
    pm->base = pm->width = 0;
    SETREFNAME(pm, ztrdup(value));
    pm->node.flags &= ~PM_UNSET;
    setscope(pm);
} else
    setsparam(name, ztrdup(value));

gethashnode2 is the no-autoload paramtab lookup. The nameref branch updates the alias target in-place; the normal branch falls through to setsparam. Port of setloopvar(char *name, char *value) from Src/params.c:6362.