feat(Diagnostic): implement `Diagnostic` for `Infallible`

src/diagnostic_impls.rs

@@ -0,0 +1,59 @@
+/*!
+Default trait implementations for [`Diagnostic`].
+*/
+
+use std::{convert::Infallible, fmt::Display, hint::unreachable_unchecked};
+
+use crate::{Diagnostic, LabeledSpan, Severity, SourceCode};
+
+// Since all trait methods for [`Diagnostic`] take a reference to `self`, there must be an instance of `Self`.
+// However, since an instance of [`Infallible`] can never be constructed, these methods can never be called.
+// Therefore, these methods are unreachable, and can be safely optimized away by the compiler.
+#[allow(clippy::undocumented_unsafe_blocks)]
+impl Diagnostic for Infallible {
+    fn code<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn severity(&self) -> Option<Severity> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn help<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn url<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn source_code(&self) -> Option<&dyn SourceCode> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn related<'a>(&'a self) -> Option<Box<dyn Iterator<Item = &'a dyn Diagnostic> + 'a>> {
+        unsafe { unreachable_unchecked() }
+    }
+
+    fn diagnostic_source(&self) -> Option<&dyn Diagnostic> {
+        unsafe { unreachable_unchecked() }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::Report;
+
+    /// Test that [`Infallible`] implements [`Diagnostic`] by seeing if a function that's generic over `Diagnostic`
+    /// will accept `Infallible` as a type parameter.
+    #[test]
+    fn infallible() {
+        let _ = Report::new::<Infallible>;
+    }
+}

src/lib.rs

@@ -788,6 +788,7 @@ pub use protocol::*;
 
 mod chain;
 mod diagnostic_chain;
+mod diagnostic_impls;
 mod error;
 mod eyreish;
 #[cfg(feature = "fancy-base")]