Hoist bsn expressions up to reduce ownership / type constraints

[cart]

Objective

bsn! currently embeds expressions directly into the patch closure, which requires moving all of the "expression items" into the closure. This thrusts constraints like Send + Sync + 'static on anything used in the expression:

/// Workaround 1 (used in Feathers)
fn label(text: impl Into<String>) -> impl Scene {
    let text = Text::new(text.into());
    bsn! {
        template_value(text)
    }
}
/// Workaround 2
fn label(text: impl Into<String> + Send + Sync + 'static) -> impl Scene {
    bsn! {
        Text(text)
    }
}

Solution

Hoist expression above the closures, evaluate them immediately, and move the results into the closure. This significantly reduces the burden on developers defining scenes.

This now works! No workaround necessary!

fn label(text: impl Into<String>) -> impl Scene {
    bsn! {
        Text(text)
    }
}

Borrowing like this is also allowed because of implicit into(), which converts the &str into String, which is then captured in the patch closure:

fn label(text: &str) -> impl Scene {
    bsn! {
        Text(text)
    }
}

Likewise, patterns like this now work:

fn cube(meshes: &mut Assets<Mesh>) -> impl Scene {
    bsn! {
        Mesh3d({meshes.add(Cuboid::default())})
    }
}

[andriyDev]

Nice!