fix(graphical): render cause chains for inner errors

The default `GraphicalReportHandler` disables the printing of cause
chains for any inner errors (errors `related()` to a source diagnostic)
when it disables nested footer printing. This results in lost cause
chain information when printing with the default report handler.

src/handlers/graphical.rs

@@ -317,9 +317,10 @@ impl GraphicalReportHandler {
                     ErrorKind::Diagnostic(diag) => {
                         let mut inner = String::new();
 
-                        // Don't print footer for inner errors
                         let mut inner_renderer = self.clone();
+                        // Don't print footer for inner errors
                         inner_renderer.footer = None;
+                        // Cause chains are already flattened, so don't double-print the nested error
                         inner_renderer.with_cause_chain = false;
                         inner_renderer.render_report(&mut inner, diag)?;
 
@@ -362,6 +363,9 @@ impl GraphicalReportHandler {
         parent_src: Option<&dyn SourceCode>,
     ) -> fmt::Result {
         if let Some(related) = diagnostic.related() {
+            let mut inner_renderer = self.clone();
+            // Re-enable the printing of nested cause chains for related errors
+            inner_renderer.with_cause_chain = true;
             writeln!(f)?;
             for rel in related {
                 match rel.severity() {
@@ -369,12 +373,12 @@ impl GraphicalReportHandler {
                     Some(Severity::Warning) => write!(f, "Warning: ")?,
                     Some(Severity::Advice) => write!(f, "Advice: ")?,
                 };
-                self.render_header(f, rel)?;
-                self.render_causes(f, rel)?;
+                inner_renderer.render_header(f, rel)?;
+                inner_renderer.render_causes(f, rel)?;
                 let src = rel.source_code().or(parent_src);
-                self.render_snippets(f, rel, src)?;
-                self.render_footer(f, rel)?;
-                self.render_related(f, rel, src)?;
+                inner_renderer.render_snippets(f, rel, src)?;
+                inner_renderer.render_footer(f, rel)?;
+                inner_renderer.render_related(f, rel, src)?;
             }
         }
         Ok(())

tests/test_diagnostic_source_macro.rs

@@ -194,3 +194,85 @@ fn test_nested_diagnostic_source_is_output() {
 
     assert_eq!(expected, out);
 }
+
+#[derive(Debug, miette::Diagnostic, thiserror::Error)]
+#[error("A multi-error happened")]
+struct MultiError {
+    #[related]
+    related_errs: Vec<Box<dyn Diagnostic>>,
+}
+
+#[cfg(feature = "fancy-no-backtrace")]
+#[test]
+fn test_nested_cause_chains_for_related_errors_are_output() {
+    let inner_error = TestStructError {
+        asdf_inner_foo: SourceError {
+            code: String::from("This is another error"),
+            help: String::from("You should fix this"),
+            label: (3, 4),
+        },
+    };
+    let first_error = NestedError {
+        code: String::from("right here"),
+        label: (6, 4),
+        the_other_err: Box::new(inner_error),
+    };
+    let second_error = SourceError {
+        code: String::from("You're actually a mess"),
+        help: String::from("Get a grip..."),
+        label: (3, 4),
+    };
+    let multi_error = MultiError {
+        related_errs: vec![Box::new(first_error), Box::new(second_error)],
+    };
+    let diag = NestedError {
+        code: String::from("the outside world"),
+        label: (6, 4),
+        the_other_err: Box::new(multi_error),
+    };
+    let mut out = String::new();
+    miette::GraphicalReportHandler::new_themed(miette::GraphicalTheme::unicode_nocolor())
+        .with_width(80)
+        .with_footer("Yooo, a footer".to_string())
+        .render_report(&mut out, &diag)
+        .unwrap();
+    println!("{}", out);
+
+    let expected = r#"  × A nested error happened
+  ╰─▶   × A multi-error happened
+      
+      Error:   × A nested error happened
+        ├─▶   × TestError
+        │
+        ╰─▶   × A complex error happened
+               ╭────
+             1 │ This is another error
+               ·    ──┬─
+               ·      ╰── here
+               ╰────
+              help: You should fix this
+      
+         ╭────
+       1 │ right here
+         ·       ──┬─
+         ·         ╰── here
+         ╰────
+      Error:   × A complex error happened
+         ╭────
+       1 │ You're actually a mess
+         ·    ──┬─
+         ·      ╰── here
+         ╰────
+        help: Get a grip...
+      
+   ╭────
+ 1 │ the outside world
+   ·       ──┬─
+   ·         ╰── here
+   ╰────
+
+  Yooo, a footer
+"#;
+
+    assert_eq!(expected, out);
+}