chore: Move part of error.rs to winit-core

Signed-off-by: John Nunley <dev@notgull.net>

winit-core/src/error.rs

@@ -0,0 +1,54 @@
+//! Common error types.
+
+use std::{error, fmt};
+
+/// The error type for when the requested operation is not supported by the backend.
+#[derive(Clone)]
+pub struct NotSupportedError {
+    _marker: (),
+}
+
+impl Default for NotSupportedError {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl NotSupportedError {
+    /// Create a new [`NotSupportedError`].
+    #[inline]
+    pub fn new() -> NotSupportedError {
+        NotSupportedError { _marker: () }
+    }
+}
+
+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 error::Error for NotSupportedError {}
+
+#[cfg(test)]
+mod tests {
+    #![allow(clippy::redundant_clone)]
+
+    use super::*;
+
+    // Eat attributes for testing
+    #[test]
+    fn ensure_fmt_does_not_panic() {
+        let _ = format!(
+            "{:?}, {}",
+            NotSupportedError::new(),
+            NotSupportedError::new().clone()
+        );
+    }
+}

winit-core/src/lib.rs

@@ -1,11 +1,13 @@
 //! Base types for a windowing library.
-//! 
+//!
 //! This crate contains types, traits and basic functions from [`winit`] that are platform
 //! independent. It is intended to allow for other crates to build abstractions around [`winit`]
 //! without needing to pull in all of [`winit`]'s dependencies, as well as to provide an
 //! interface for alternative backends for [`winit`] to be constructed.
-//! 
+//!
 //! [`winit`]: https://docs.rs/winit
 
 #[cfg(any(not(feature = "std"), not(feature = "alloc")))]
 compile_error! { "no-std and no-alloc usage are not yet supported" }
+
+pub mod error;