Freetype get list of all valid chars/glyphs

I'm using https://docs.rs/freetype-rs/0.21.0/freetype/ and I've already looked at https://github.com/PistonDevelopers/freetype-rs/tree/master/examples

Given a ttf, I can load the ttf and get a freetype::Face object.

Now, I want a way to iterate over all glyphs/chars of the Face -- is there a way to do this?

Thanks!