They're called "modes" and were a way of controlling whether parameters were copied or passed by reference etc (I think). It's been deprecated for a while and there's an (ongoing) effort to remove them from the code base, e.g. the example you give has been removed already: https://github.com/mozilla/rust/blob/incoming/src/librustc/m... (along with most of the modes from the whole source tree, a big patch landed just a few days ago).
(Also, the master branch is not the one to be looking at for the latest changes at the moment: patches are merged into 'incoming', and master hasn't been updated for more than a month.)
(Also, the master branch is not the one to be looking at for the latest changes at the moment: patches are merged into 'incoming', and master hasn't been updated for more than a month.)