seal the Diagnose trait (#107)

* seal the `Diagnose` trait

CHANGELOG.md

@@ -8,7 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 
 ### Changed
-
+-   Sealed the `Diagnose` trait.
 -   Implementation of the `Default` trait for `Pointer` now doesn't constrain the lifetime.
 
 ## [0.7.1] 2025-02-16

src/diagnostic.rs

@@ -163,7 +163,7 @@ pub(crate) use diagnostic_url;
 /// An extension trait for `Result<_, E>`, where `E` is an implementation of
 /// [`Diagnostic`], that converts `E` into [`Report<E>`](`Report`), yielding
 /// `Result<_, Report<E>>`.
-pub trait Diagnose<'s, T> {
+pub trait Diagnose<'s, T>: private::Sealed {
     /// The error type returned from `diagnose` and `diagnose_with`.
     type Error: Diagnostic;
 
@@ -224,6 +224,11 @@ where
     }
 }
 
+mod private {
+    pub trait Sealed {}
+    impl<T, E> Sealed for Result<T, E> {}
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;