You can add other types to [user-customizable `filetypes.rust` file](https://github.com/geany/geany/blob/94a944114ba3d9db4e442472c44127fecf642a42...) by editing the [`[keywords]` section](https://www.geany.org/manual/current/index.html#keywords-section). It appears to only have primitive types by default.