How to detect at compile time what backend im using

[UnknownSuperficialNight]

I'm writing a little screen to show information on what backend the program is compiled for, but I want to be able to move between glow or wgpu and detect what backend I'm using so I can enable and disable code based on that

example for wgpu I want this code to be disabled because if I'm using glow as those methods do not exist in glow

if let Some(render_state) = frame.wgpu_render_state() {
  let wgpu_adapter_details_ui = |ui: &mut egui::Ui, adapter: &eframe::wgpu::Adapter| {
      let info = &adapter.get_info();
  
      let wgpu::AdapterInfo {
          name,
          vendor,
          device,
          device_type,
          driver,
          driver_info,
          backend,
      } = &info;

      ui.label("Backend:");
      ui.label(format!("{backend:?}"));
      ui.end_row();

[YgorSouza]

It seems like you want to do conditional compilation based on another crate's features. This is not possible in Rust. The best you can do is create the features in your crate and forward them to eframe, then you'll know for sure that the feature you are using is enabled, because you enabled it. This is what the demo app does.

egui/crates/egui_demo_app/Cargo.toml

Lines 39 to 40 in a9e9252

	glow = ["eframe/glow"]
	wgpu = ["eframe/wgpu", "bytemuck", "dep:wgpu"]

egui/crates/egui_demo_app/src/backend_panel.rs

Lines 201 to 265 in a9e9252

	#[cfg(feature = "glow")]
	if _frame.gl().is_some() {
	ui.horizontal(|ui| {
	ui.label("Renderer:");
	ui.hyperlink_to("glow", "https://github.com/grovesNL/glow");
	});
	}
	#[cfg(feature = "wgpu")]
	if let Some(render_state) = _frame.wgpu_render_state() {
	let wgpu_adapter_details_ui = |ui: &mut egui::Ui, adapter: &eframe::wgpu::Adapter| {
	let info = &adapter.get_info();
	let wgpu::AdapterInfo {
	name,
	vendor,
	device,
	device_type,
	driver,
	driver_info,
	backend,
	} = &info;
	// Example values:
	// > name: "llvmpipe (LLVM 16.0.6, 256 bits)", device_type: Cpu, backend: Vulkan, driver: "llvmpipe", driver_info: "Mesa 23.1.6-arch1.4 (LLVM 16.0.6)"
	// > name: "Apple M1 Pro", device_type: IntegratedGpu, backend: Metal, driver: "", driver_info: ""
	// > name: "ANGLE (Apple, Apple M1 Pro, OpenGL 4.1)", device_type: IntegratedGpu, backend: Gl, driver: "", driver_info: ""
	egui::Grid::new("adapter_info").show(ui, |ui| {
	ui.label("Backend:");
	ui.label(format!("{backend:?}"));
	ui.end_row();
	ui.label("Device Type:");
	ui.label(format!("{device_type:?}"));
	ui.end_row();
	if !name.is_empty() {
	ui.label("Name:");
	ui.label(format!("{name:?}"));
	ui.end_row();
	}
	if !driver.is_empty() {
	ui.label("Driver:");
	ui.label(format!("{driver:?}"));
	ui.end_row();
	}
	if !driver_info.is_empty() {
	ui.label("Driver info:");
	ui.label(format!("{driver_info:?}"));
	ui.end_row();
	}
	if *vendor != 0 {
	// TODO(emilk): decode using https://github.com/gfx-rs/wgpu/blob/767ac03245ee937d3dc552edc13fe7ab0a860eec/wgpu-hal/src/auxil/mod.rs#L7
	ui.label("Vendor:");
	ui.label(format!("0x{vendor:04X}"));
	ui.end_row();
	}
	if *device != 0 {
	ui.label("Device:");
	ui.label(format!("0x{device:02X}"));
	ui.end_row();
	}
	});
	};

The drawback is that if something else in the project depends on eframe and enables some backends without enabling them in your crate, your crate won't be able to know that they are enabled and won't use the code. I don't think there is a way around it.