You can use static variables fairly easily as long as they are thread-local.
The downside is that the object will not be visible to other threads your program might spawn. The upside is that unlike truly global state, it is entirely safe and is not a pain to use - true global state is a massive pain in any language. Here's an example:
extern mod sqlite;
use std::cell::RefCell;
thread_local!(static ODB: RefCell<sqlite::database::Database> = RefCell::new(sqlite::open("test.db"));
fn main() {
ODB.with(|odb_cell| {
let odb = odb_cell.borrow_mut();
// code that uses odb goes here
});
}
Here we create a thread-local static variable and then use it in a function. Note that it is static and immutable; this means that the address at which it resides is immutable, but thanks to RefCell the value itself will be mutable.
Unlike regular static, in thread-local!(static ...) you can create pretty much arbitrary objects, including those that require heap allocations for initialization such as Vec, HashMap and others.
If you cannot initialize the value right away, e.g. it depends on user input, you may also have to throw Option in there, in which case accessing it gets a bit unwieldy:
extern mod sqlite;
use std::cell::RefCell;
thread_local!(static ODB: RefCell<Option<sqlite::database::Database>> = RefCell::New(None));
fn main() {
ODB.with(|odb_cell| {
// assumes the value has already been initialized, panics otherwise
let odb = odb_cell.borrow_mut().as_mut().unwrap();
// code that uses odb goes here
});
}