feat!(blanket_impls): impl `Diagnostic` for &T and Box<T>

Brings things in line with best practices as described in "Rust for
Rustaceans" when it comes to "Ergonomic Trait Implementations" in Chapter
3. Practically means that people can pass more types to any functions
taking either `dyn Diagnostic` or `impl Diagnostic`.

BREAKING CHANGE: Added blanket impls may overlap with manual user impls.
If these impls are just hacks to achieve the same as the blanket ones do,
then they can simply be removed. If the impls for `T` and `&T` differ
semantically, then the user would need to employ the newtype pattern to
avoid overlap.

src/protocol.rs

@@ -69,6 +69,48 @@ pub trait Diagnostic: std::error::Error {
     }
 }
 
+macro_rules! blanket_ref_impls {
+    ($($ref_type:ty),+ $(,)?) => {
+        $(
+            impl<T: Diagnostic> Diagnostic for $ref_type {
+                fn code<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+                    (**self).code()
+                }
+
+                fn severity(&self) -> Option<Severity> {
+                    (**self).severity()
+                }
+
+                fn help<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+                    (**self).help()
+                }
+
+                fn url<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
+                    (**self).url()
+                }
+
+                fn source_code(&self) -> Option<&dyn SourceCode> {
+                    (**self).source_code()
+                }
+
+                fn labels(&self) -> Option<Box<dyn Iterator<Item = LabeledSpan> + '_>> {
+                    (**self).labels()
+                }
+
+                fn related<'a>(&'a self) -> Option<Box<dyn Iterator<Item = &'a dyn Diagnostic> + 'a>> {
+                    (**self).related()
+                }
+
+                fn diagnostic_source(&self) -> Option<&dyn Diagnostic> {
+                    (**self).diagnostic_source()
+                }
+            }
+        )+
+    };
+}
+
+blanket_ref_impls!(&T, Box<T>);
+
 macro_rules! box_error_impls {
     ($($box_type:ty),*) => {
         $(

tests/test_blanket.rs

@@ -0,0 +1,13 @@
+use miette::{Diagnostic, MietteDiagnostic};
+
+fn assert_diagnostic<T: Diagnostic>() {}
+
+#[test]
+fn test_ref() {
+    assert_diagnostic::<&MietteDiagnostic>()
+}
+
+#[test]
+fn test_box() {
+    assert_diagnostic::<Box<MietteDiagnostic>>()
+}