I want to use the `expand` function in an `autocmd`. In particular, I am adapting a [tip](https://stackoverflow.com/questions/1240037/recommended-build-system-for-latex/1394702#1394702) by Paul Biggar and I wanted to use the following auto command in my `.gvimrc`:

```
autocmd FileType tex set makeprg=rubber-info\ expand("%:t:r").log

```

– Which of course doesn’t work. So what I actually want this to mean is that upon opening a `tex` file, the `makeprg` variable is set to the value of `rubber-info filename.log` where `filename` is the name of the TeX file without its file extension.

How can I accomplish this?