Allow non-static eframe::App lifetime

[timstr]

I love egui! Thank you Emil <3

This request specifically enables an eframe::App which stores a lifetime.

In general, I believe this is necessary because eframe::App currently does not seem to provide a good place to allocate and then borrow from long-lived data between update() calls. To attempt to borrow such long-lived data from a field of the App itself would be to create a self-referential struct. A hacky alternative is to allocate long-lived data with Box::leak, but that's a code smell and would cause problems if a program ever creates multiple Apps.

As a more specific motivating example, I am developing with the inkwell crate which requires creating a inkwell::context::Context instance which is then borrowed from by a bazillion things with a dedicated 'ctx lifetime. I need such a inkwell::context::Context for the duration of my eframe::App but I can't store it as a field of the app. The most natural solution to me is to simply to lift the inkwell context outside of the App and borrow from it, but that currently fails because the AppCreator implicitly has a 'static lifetime requirement due to the use of dyn trait objects.

Here is a simpler, self-contained motivating example adapted from the current hello world example:

use eframe::egui;

struct LongLivedThing {
    message: String,
}

fn main() {
    let long_lived_thing = LongLivedThing {
        message: "Hello World!".to_string(),
    };

    let native_options = eframe::NativeOptions::default();
    eframe::run_native(
        "My egui App",
        native_options,
        Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc, &long_lived_thing)))),
        //                                           ^^^^^^^^^^^^^^^^^
        //                                           BORROWING from long_lived_thing in App
    );
}

struct MyEguiApp<'a> {
    long_lived_thing: &'a LongLivedThing,
}

impl<'a> MyEguiApp<'a> {
    fn new(cc: &eframe::CreationContext<'_>, long_lived_thing: &'a LongLivedThing) -> Self {
        // Customize egui here with cc.egui_ctx.set_fonts and cc.egui_ctx.set_visuals.
        // Restore app state using cc.storage (requires the "persistence" feature).
        // Use the cc.gl (a glow::Context) to create graphics shaders and buffers that you can use
        // for e.g. egui::PaintCallback.
        MyEguiApp { long_lived_thing }
    }
}

impl<'a> eframe::App for MyEguiApp<'a> {
    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading(&self.long_lived_thing.message);
        });
    }
}

This currently fails to compile with:

error[E0597]: `long_lived_thing` does not live long enough
  --> src/main.rs:16:55
   |
16 |         Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc, &long_lived_thing)))),
   |         ----------------------------------------------^^^^^^^^^^^^^^^^----
   |         |        |                                    |
   |         |        |                                    borrowed value does not live long enough
   |         |        value captured here
   |         cast requires that `long_lived_thing` is borrowed for `'static`
17 |     );
18 | }
   | - `long_lived_thing` dropped here while still borrowed
   |
   = note: due to object lifetime defaults, `Box<dyn for<'a, 'b> FnOnce(&'a CreationContext<'b>) -> Result<Box<dyn App>, Box<dyn std::error::Error + Send + Sync>>>` actually means `Box<(dyn for<'a, 'b> FnOnce(&'a CreationContext<'b>) -> Result<Box<dyn App>, Box<dyn std::error::Error + Send + Sync>> + 'static)>`

With the added lifetimes in this request, I'm able to compile and run this as expected on Ubuntu + Wayland. I see the CI has been emailing me about some build failures and I'll do what I can to address those. Currently running the check.sh script as well.

This is intended to resolve #2152

• Closes Allow the use of non-static App #2152
• I have followed the instructions in the PR template

[timstr]

I've added lifetime parameters to the AppCreator as well as the native backends. I'm not familiar with the web backend and whether it makes sense for them to store lifetimes, so I've required AppCreator<'static> for the web backend without loss of existing functionality.

[emilk]

Thanks - this is great to have!