(fix) SSRPlugin: Don't reference default deferred lighting pass if it doesn't exist

[CrushedPixel]

Fixes a crash when using deferred rendering but disabling the default deferred lighting plugin.

The Issue

The ScreenSpaceReflectionsPlugin references NodePbr::DeferredLightingPass, which hasn't been added when PbrPlugin::add_default_deferred_lighting_plugin is false.

This yields the following crash:

thread 'main' panicked at /Users/marius/Documents/dev/bevy/crates/bevy_render/src/render_graph/graph.rs:155:26:
InvalidNode(DeferredLightingPass)
stack backtrace:
   0: rust_begin_unwind
             at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf/library/core/src/panicking.rs:74:14
   2: bevy_render::render_graph::graph::RenderGraph::add_node_edges
             at /Users/marius/Documents/dev/bevy/crates/bevy_render/src/render_graph/graph.rs:155:26
   3: <bevy_app::sub_app::SubApp as bevy_render::render_graph::app::RenderGraphApp>::add_render_graph_edges
             at /Users/marius/Documents/dev/bevy/crates/bevy_render/src/render_graph/app.rs:66:13
   4: <bevy_pbr::ssr::ScreenSpaceReflectionsPlugin as bevy_app::plugin::Plugin>::finish
             at /Users/marius/Documents/dev/bevy/crates/bevy_pbr/src/ssr/mod.rs:234:9
   5: bevy_app::app::App::finish
             at /Users/marius/Documents/dev/bevy/crates/bevy_app/src/app.rs:255:13
   6: bevy_winit::state::winit_runner
             at /Users/marius/Documents/dev/bevy/crates/bevy_winit/src/state.rs:859:9
   7: core::ops::function::FnOnce::call_once
             at /Users/marius/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   8: core::ops::function::FnOnce::call_once{{vtable.shim}}
             at /Users/marius/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   9: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
             at /Users/marius/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:2454:9
  10: bevy_app::app::App::run
             at /Users/marius/Documents/dev/bevy/crates/bevy_app/src/app.rs:184:9
  11: bevy_deferred_test::main
             at ./src/main.rs:9:5
  12: core::ops::function::FnOnce::call_once
             at /Users/marius/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5

Minimal reproduction example:

use bevy::core_pipeline::prepass::{DeferredPrepass, DepthPrepass};
use bevy::pbr::{DefaultOpaqueRendererMethod, PbrPlugin, ScreenSpaceReflections};
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(PbrPlugin {
            add_default_deferred_lighting_plugin: false,
            ..default()
        }))
        .add_systems(Startup, setup)
        .insert_resource(DefaultOpaqueRendererMethod::deferred())
        .run();
}

/// set up a camera
fn setup(
    mut commands: Commands
) {
    // camera
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
        DepthPrepass,
        DeferredPrepass,
        ScreenSpaceReflections::default(),
    ));
}

The Fix

When no node under the default lighting node's label exists, this label isn't added to the SSR's graph node edges. It's good to keep the SSRPlugin enabled, this way, users can plug in their own lighting system, which I have successfully done on top of this PR.

Workarounds

A current workaround for this issue is to re-use Bevy's NodePbr::DeferredLightingPass as the label for your own custom lighting pass node.

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[CrushedPixel]

Did a force push to run rustfmt, had the IDE setup a bit wrong

[IceSentry]

This should essentially never be reached, but I'd prefer if this used expect with an error message. Alternatively, just use .resource_mut which should already panic with a decent error message

[CrushedPixel]

Used resource_mut, I agree that that is the more idiomatic approach

[CrushedPixel]

A current workaround for this issue is to simply re-use Bevy's NodePbr::DeferredLightingPass as the label for your own custom lighting pass node.