From a2bea9990ed3a316c89d2826ca271121415124ae Mon Sep 17 00:00:00 2001
From: Cass Fridkin <cass@swag.lgbt>
Date: Sat, 17 Aug 2024 11:55:35 -0600
Subject: [PATCH 1/2] feat(Diagnostic): implement `Diagnostic` for `Infallible`

---
 src/diagnostic_impls.rs | 59 +++++++++++++++++++++++++++++++++++++++++
 src/lib.rs              |  1 +
 2 files changed, 60 insertions(+)
 create mode 100644 src/diagnostic_impls.rs

diff --git a/src/diagnostic_impls.rs b/src/diagnostic_impls.rs
new file mode 100644
index 0000000..c13dc93
--- /dev/null
+++ b/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>;
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index a42f4fa..cbb28bf 100644
--- a/src/lib.rs
+++ b/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")]

From 35d0f077c1afca8eaea792e19e1268945f71fb89 Mon Sep 17 00:00:00 2001
From: Cass Fridkin <cass@swag.lgbt>
Date: Mon, 19 Aug 2024 21:47:30 -0600
Subject: [PATCH 2/2] fix(Diagnostic): remove usage of unsafe code from
 `Infallible` impl

---
 src/diagnostic_impls.rs | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/src/diagnostic_impls.rs b/src/diagnostic_impls.rs
index c13dc93..aebe56b 100644
--- a/src/diagnostic_impls.rs
+++ b/src/diagnostic_impls.rs
@@ -2,45 +2,41 @@
 Default trait implementations for [`Diagnostic`].
 */
 
-use std::{convert::Infallible, fmt::Display, hint::unreachable_unchecked};
+use std::{convert::Infallible, fmt::Display};
 
 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() }
+        match *self {}
     }
 
     fn severity(&self) -> Option<Severity> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn help<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn url<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn source_code(&self) -> Option<&dyn SourceCode> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn related<'a>(&'a self) -> Option<Box<dyn Iterator<Item = &'a dyn Diagnostic> + 'a>> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 
     fn diagnostic_source(&self) -> Option<&dyn Diagnostic> {
-        unsafe { unreachable_unchecked() }
+        match *self {}
     }
 }