1 files changed, 86 insertions, 0 deletions
diff --git a/src/unexpected.rs b/src/unexpected.rs
new file mode 100644
index 0000000..6e5548b
--- /dev/null
+++ b/src/unexpected.rs
@@ -0,0 +1,86 @@
+use core::fmt::{self, Debug, Display};
+
+#[cfg(not(feature = "std"))]
+use alloc::boxed::Box;
+
+#[cfg(feature = "std")]
+use std::error::Error;
+
+pub trait Errorable: Display + Debug {}
+impl<T: Display + Debug> Errorable for T {}
+
+#[derive(Debug)]
+#[non_exhaustive]
+enum ErrorTy {
+	Message(Box<dyn Errorable + 'static>),
+	#[cfg(feature = "std")]
+	Error(Box<dyn Error + 'static>),
+}
+
+#[derive(Debug)]
+pub struct UnexpectedError {
+	internal: ErrorTy,
+}
+
+impl Display for UnexpectedError {
+	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+		match &self.internal {
+			ErrorTy::Message(m) => Display::fmt(&m, f),
+			#[cfg(feature = "std")]
+			ErrorTy::Error(e) => Display::fmt(&e, f),
+		}
+	}
+}
+
+#[cfg(feature = "std")]
+impl Error for UnexpectedError {
+	fn source(&self) -> Option<&(dyn Error + 'static)> {
+		match &self.internal {
+			ErrorTy::Message(_) => None,
+			#[cfg(feature = "std")]
+			ErrorTy::Error(e) => Some(&**e),
+		}
+	}
+}
+
+impl UnexpectedError {
+	/// Create a new `UnexpectedError` from any [`Error`] type.
+	///
+	/// The error must be `'static` so that the `UnexpectedError` will be too.
+	///
+	/// # Examples
+	///
+	/// Basic usage:
+	///
+	/// ```
+	/// use exun::*;
+	///
+	/// let err = UnexpectedError::new(core::fmt::Error);
+	/// ```
+	#[cfg(feature = "std")]
+	pub fn new<E: Error + 'static>(e: E) -> Self {
+		Self {
+			internal: ErrorTy::Error(Box::new(e)),
+		}
+	}
+
+	/// Create an error message from a printable error message.
+	///
+	/// If the argument implements [`Error`], prefer [`UnexpectedError::new`]
+	/// instead, which preserves the source.
+	///
+	/// # Examples
+	///
+	/// Basic usage:
+	///
+	/// ```
+	/// use exun::*;
+	///
+	/// let err = UnexpectedError::msg("failed");
+	/// ```
+	pub fn msg<E: Errorable + 'static>(e: E) -> Self {
+		Self {
+			internal: ErrorTy::Message(Box::new(e)),
+		}
+	}
+}