Reintroduce backend selection through ICED_BACKEND env var

graphics/src/compositor.rs

@@ -20,6 +20,18 @@ pub trait Compositor: Sized {
     fn new<W: Window + Clone>(
         settings: Settings,
         compatible_window: W,
+    ) -> impl Future<Output = Result<Self, Error>> {
+        Self::with_backend(settings, compatible_window, None)
+    }
+
+    /// Creates a new [`Compositor`] with a backend preference.
+    ///
+    /// If the backend does not match the preference, it will return
+    /// [`Error::GraphicsAdapterNotFound`].
+    fn with_backend<W: Window + Clone>(
+        _settings: Settings,
+        _compatible_window: W,
+        _backend: Option<&str>,
     ) -> impl Future<Output = Result<Self, Error>>;
 
     /// Creates a [`Self::Renderer`] for the [`Compositor`].
@@ -130,9 +142,10 @@ impl Compositor for () {
     type Renderer = ();
     type Surface = ();
 
-    async fn new<W: Window + Clone>(
+    async fn with_backend<W: Window + Clone>(
         _settings: Settings,
         _compatible_window: W,
+        _preffered_backend: Option<&str>,
     ) -> Result<Self, Error> {
         Ok(())
     }

graphics/src/error.rs

@@ -1,5 +1,7 @@
+//! See what can go wrong when creating graphical backends.
+
 /// An error that occurred while creating an application's graphical context.
-#[derive(Debug, thiserror::Error)]
+#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
 pub enum Error {
     /// The requested backend version is not supported.
     #[error("the requested backend version is not supported")]
@@ -11,9 +13,30 @@ pub enum Error {
 
     /// A suitable graphics adapter or device could not be found.
     #[error("a suitable graphics adapter or device could not be found")]
-    GraphicsAdapterNotFound,
+    GraphicsAdapterNotFound {
+        /// The name of the backend where the error happened
+        backend: &'static str,
+        /// The reason why this backend could not be used
+        reason: Reason,
+    },
 
     /// An error occurred in the context's internal backend
     #[error("an error occurred in the context's internal backend")]
     BackendError(String),
+
+    /// Multiple errors occurred
+    #[error("multiple errors occurred: {0:?}")]
+    List(Vec<Self>),
+}
+
+/// The reason why a graphics adapter could not be found
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Reason {
+    /// The backend did not match the preference
+    DidNotMatch {
+        /// The preferred backend
+        preferred_backend: String,
+    },
+    /// The request to create the backend failed
+    RequestFailed(String),
 }

graphics/src/lib.rs

@@ -18,7 +18,6 @@
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 mod antialiasing;
 mod cached;
-mod error;
 mod primitive;
 mod settings;
 mod viewport;
@@ -27,6 +26,7 @@ pub mod backend;
 pub mod color;
 pub mod compositor;
 pub mod damage;
+pub mod error;
 pub mod gradient;
 pub mod mesh;
 pub mod renderer;

renderer/src/fallback.rs

@@ -204,18 +204,55 @@ where
     type Renderer = Renderer<L::Renderer, R::Renderer>;
     type Surface = Surface<L::Surface, R::Surface>;
 
-    async fn new<W: compositor::Window + Clone>(
+    async fn with_backend<W: compositor::Window + Clone>(
         settings: graphics::Settings,
         compatible_window: W,
+        backend: Option<&str>,
     ) -> Result<Self, graphics::Error> {
-        if let Ok(left) = L::new(settings, compatible_window.clone())
-            .await
-            .map(Self::Left)
-        {
-            return Ok(left);
+        use std::env;
+
+        let backends = backend
+            .map(str::to_owned)
+            .or_else(|| env::var("ICED_BACKEND").ok());
+
+        let mut candidates: Vec<_> = backends
+            .map(|backends| {
+                backends
+                    .split(',')
+                    .filter(|candidate| !candidate.is_empty())
+                    .map(str::to_owned)
+                    .map(Some)
+                    .collect()
+            })
+            .unwrap_or_default();
+
+        if candidates.is_empty() {
+            candidates.push(None);
         }
 
-        R::new(settings, compatible_window).await.map(Self::Right)
+        let mut errors = vec![];
+
+        for backend in candidates.iter().map(Option::as_deref) {
+            match L::with_backend(settings, compatible_window.clone(), backend)
+                .await
+            {
+                Ok(compositor) => return Ok(Self::Left(compositor)),
+                Err(error) => {
+                    errors.push(error);
+                }
+            }
+
+            match R::with_backend(settings, compatible_window.clone(), backend)
+                .await
+            {
+                Ok(compositor) => return Ok(Self::Right(compositor)),
+                Err(error) => {
+                    errors.push(error);
+                }
+            }
+        }
+
+        Err(graphics::Error::List(errors))
     }
 
     fn create_renderer(&self) -> Self::Renderer {

tiny_skia/src/window/compositor.rs

@@ -1,11 +1,11 @@
 use crate::core::{Color, Rectangle, Size};
 use crate::graphics::compositor::{self, Information};
 use crate::graphics::damage;
-use crate::graphics::{self, Error, Viewport};
+use crate::graphics::error::{self, Error};
+use crate::graphics::{self, Viewport};
 use crate::{Backend, Primitive, Renderer, Settings};
 
 use std::collections::VecDeque;
-use std::future::{self, Future};
 use std::num::NonZeroU32;
 
 pub struct Compositor {
@@ -28,11 +28,22 @@ impl crate::graphics::Compositor for Compositor {
     type Renderer = Renderer;
     type Surface = Surface;
 
-    fn new<W: compositor::Window>(
+    async fn with_backend<W: compositor::Window>(
         settings: graphics::Settings,
         compatible_window: W,
-    ) -> impl Future<Output = Result<Self, Error>> {
-        future::ready(Ok(new(settings.into(), compatible_window)))
+        backend: Option<&str>,
+    ) -> Result<Self, Error> {
+        match backend {
+            None | Some("tiny-skia") | Some("tiny_skia") => {
+                Ok(new(settings.into(), compatible_window))
+            }
+            Some(backend) => Err(Error::GraphicsAdapterNotFound {
+                backend: "tiny-skia",
+                reason: error::Reason::DidNotMatch {
+                    preferred_backend: backend.to_owned(),
+                },
+            }),
+        }
     }
 
     fn create_renderer(&self) -> Self::Renderer {

wgpu/Cargo.toml

@@ -32,6 +32,7 @@ glyphon.workspace = true
 guillotiere.workspace = true
 log.workspace = true
 once_cell.workspace = true
+thiserror.workspace = true
 wgpu.workspace = true
 
 lyon.workspace = true

wgpu/src/settings.rs

@@ -29,30 +29,6 @@ pub struct Settings {
     pub antialiasing: Option<Antialiasing>,
 }
 
-impl Settings {
-    /// Creates new [`Settings`] using environment configuration.
-    ///
-    /// Specifically:
-    ///
-    /// - The `internal_backend` can be configured using the `WGPU_BACKEND`
-    /// environment variable. If the variable is not set, the primary backend
-    /// will be used. The following values are allowed:
-    ///     - `vulkan`
-    ///     - `metal`
-    ///     - `dx12`
-    ///     - `dx11`
-    ///     - `gl`
-    ///     - `webgpu`
-    ///     - `primary`
-    pub fn from_env() -> Self {
-        Settings {
-            internal_backend: wgpu::util::backend_bits_from_env()
-                .unwrap_or(wgpu::Backends::all()),
-            ..Self::default()
-        }
-    }
-}
-
 impl Default for Settings {
     fn default() -> Settings {
         Settings {