Would you be open to changing the SharedState API not require use of Default for setting the initial value? (This would also make it more like the RwLock API.) In my case, I am using SharedState to wrap an object which I instantiate with a constructor called load because it needs to do filesystem IO. I could make it implement Default, but I'd prefer not to have a Default implementation that does IO because I want it to be clear that creating new instances of this object should be done sparingly. So I'd like to have some way to create a SharedState by passing in an initial value that I've already created.

I'd propose an API that better mirror's RwLock's API:

impl<Data> SharedState<Data> {
    pub const fn new(t: Data) -> SharedState<Data>;
}

impl<Data> Default for SharedState<Data> {
    fn default() -> SharedState<Data>;
}

Maybe a problem with that proposal that SharedState::default() is not const which I'm gathering is maybe a use case you want to make easy for Relm users? If that's the case, maybe also adding pub const fn from_default() -> SharedState<Data> would solve that? (I'm pretty new to Rust, and not very familiar with const and with when lazy initialized statics are needed.)

An alternative (or additional) proposal would be to leave the existing SharedState::new, and add impl<Data> From<Data> for SharedState<Data>. Imo, this still leaves SharedState::new as being a bit weird for someone familiar with RwLock, but it would solve my need, and would avoid a breaking change to SharedState::new.

If there's agreement on an API change, I expect I'd be able to make a PR to implement it if that's helpful.