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;