api: unify error handling

Make error infrastructure more backend agnostic and let backends
just forward the os errors opaquely.

src/error.rs

@@ -1,45 +1,40 @@
-use std::{error, fmt};
+use std::error::Error;
+use std::fmt::{self, Display};
 
-use crate::platform_impl;
-
-// TODO: Rename
-/// An error that may be generated when requesting Winit state
-#[derive(Debug)]
-pub enum ExternalError {
-    /// The operation is not supported by the backend.
-    NotSupported(NotSupportedError),
-    /// The operation was ignored.
-    Ignored,
-    /// The OS cannot perform the operation.
-    Os(OsError),
-}
-
-/// The error type for when the requested operation is not supported by the backend.
-#[derive(Clone, Copy, Default, Eq, Hash, PartialEq)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
-pub struct NotSupportedError {
-    _marker: (),
-}
-
-/// The error type for when the OS cannot perform the requested operation.
-#[derive(Debug)]
-pub struct OsError {
-    line: u32,
-    file: &'static str,
-    error: platform_impl::OsError,
-}
-
-/// A general error that may occur while running the Winit event loop
+/// A general error that may occur while running or creating
+/// the event loop.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum EventLoopError {
-    /// The operation is not supported by the backend.
-    NotSupported(NotSupportedError),
-    /// The OS cannot perform the operation.
-    Os(OsError),
     /// The event loop can't be re-created.
     RecreationAttempt,
     /// Application has exit with an error status.
     ExitFailure(i32),
+    /// Got unspecified OS-specific error during the request.
+    Os(OsError),
+    /// Creating the event loop with the requested configuration is not supported.
+    NotSupported(NotSupportedError),
+}
+
+impl fmt::Display for EventLoopError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::RecreationAttempt => write!(f, "EventLoop can't be recreated"),
+            Self::Os(err) => err.fmt(f),
+            Self::ExitFailure(status) => write!(f, "Exit Failure: {status}"),
+            Self::NotSupported(err) => err.fmt(f),
+        }
+    }
+}
+
+impl Error for EventLoopError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        if let Self::Os(err) = self {
+            err.source()
+        } else {
+            None
+        }
+    }
 }
 
 impl From<OsError> for EventLoopError {
@@ -48,18 +43,102 @@ impl From<OsError> for EventLoopError {
     }
 }
 
-impl NotSupportedError {
-    #[inline]
-    #[allow(dead_code)]
-    pub(crate) fn new() -> NotSupportedError {
-        NotSupportedError { _marker: () }
+impl From<NotSupportedError> for EventLoopError {
+    fn from(value: NotSupportedError) -> Self {
+        Self::NotSupported(value)
     }
 }
 
+/// A general error that may occur during a request to the windowing system.
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum RequestError {
+    /// The request is not supported.
+    NotSupported(NotSupportedError),
+    /// The request was ignored by the operating system.
+    Ignored,
+    /// Got unspecified OS specific error during the request.
+    Os(OsError),
+}
+
+impl Display for RequestError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NotSupported(err) => err.fmt(f),
+            Self::Ignored => write!(f, "The request was ignored"),
+            Self::Os(err) => err.fmt(f),
+        }
+    }
+}
+impl Error for RequestError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        if let Self::Os(err) = self {
+            err.source()
+        } else {
+            None
+        }
+    }
+}
+
+impl From<NotSupportedError> for RequestError {
+    fn from(value: NotSupportedError) -> Self {
+        Self::NotSupported(value)
+    }
+}
+
+impl From<OsError> for RequestError {
+    fn from(value: OsError) -> Self {
+        Self::Os(value)
+    }
+}
+
+/// The requested operation is not supported.
+#[derive(Debug)]
+pub struct NotSupportedError {
+    /// The reason why a certain operation is not supported.
+    reason: &'static str,
+}
+
+impl NotSupportedError {
+    pub(crate) fn new(reason: &'static str) -> Self {
+        Self { reason }
+    }
+}
+
+impl fmt::Display for NotSupportedError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Operation is not supported: {}", self.reason)
+    }
+}
+impl Error for NotSupportedError {}
+
+/// Unclassified error from the OS.
+#[derive(Debug)]
+pub struct OsError {
+    line: u32,
+    file: &'static str,
+    error: Box<dyn Error + Send + Sync + 'static>,
+}
+
 impl OsError {
     #[allow(dead_code)]
-    pub(crate) fn new(line: u32, file: &'static str, error: platform_impl::OsError) -> OsError {
-        OsError { line, file, error }
+    pub(crate) fn new(
+        line: u32,
+        file: &'static str,
+        error: impl Into<Box<dyn Error + Send + Sync + 'static>>,
+    ) -> Self {
+        Self { line, file, error: error.into() }
+    }
+}
+
+impl Display for OsError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad(&format!("os error at {}:{}: {}", self.file, self.line, self.error))
+    }
+}
+impl Error for OsError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        Some(self.error.as_ref())
     }
 }
 
@@ -69,64 +148,3 @@ macro_rules! os_error {
         crate::error::OsError::new(line!(), file!(), $error)
     }};
 }
-
-impl fmt::Display for OsError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        f.pad(&format!("os error at {}:{}: {}", self.file, self.line, self.error))
-    }
-}
-
-impl fmt::Display for ExternalError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        match self {
-            ExternalError::NotSupported(e) => e.fmt(f),
-            ExternalError::Ignored => write!(f, "Operation was ignored"),
-            ExternalError::Os(e) => e.fmt(f),
-        }
-    }
-}
-
-impl fmt::Debug for NotSupportedError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        f.debug_struct("NotSupportedError").finish()
-    }
-}
-
-impl fmt::Display for NotSupportedError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        f.pad("the requested operation is not supported by Winit")
-    }
-}
-
-impl fmt::Display for EventLoopError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        match self {
-            EventLoopError::RecreationAttempt => write!(f, "EventLoop can't be recreated"),
-            EventLoopError::NotSupported(e) => e.fmt(f),
-            EventLoopError::Os(e) => e.fmt(f),
-            EventLoopError::ExitFailure(status) => write!(f, "Exit Failure: {status}"),
-        }
-    }
-}
-
-impl error::Error for OsError {}
-impl error::Error for ExternalError {}
-impl error::Error for NotSupportedError {}
-impl error::Error for EventLoopError {}
-
-#[cfg(test)]
-#[allow(clippy::redundant_clone)]
-mod tests {
-    use super::*;
-
-    // Eat attributes for testing
-    #[test]
-    fn ensure_fmt_does_not_panic() {
-        let _ = format!("{:?}, {}", NotSupportedError::new(), NotSupportedError::new().clone());
-        let _ = format!(
-            "{:?}, {}",
-            ExternalError::NotSupported(NotSupportedError::new()),
-            ExternalError::NotSupported(NotSupportedError::new())
-        );
-    }
-}