Add test of Display impl nested inside display attribute

tests/test_display.rs

@@ -1,4 +1,4 @@
-use std::fmt::Display;
+use std::fmt::{self, Display};
 use thiserror::Error;
 
 fn assert<T: Display>(expected: &str, value: T) {
@@ -141,6 +141,35 @@ fn test_match() {
     );
 }
 
+#[test]
+fn test_nested_display() {
+    // Same behavior as the one in `test_match`, but without String allocations.
+    #[derive(Error, Debug)]
+    #[error("{}", {
+        struct Msg<'a>(&'a String, &'a Option<usize>);
+        impl<'a> Display for Msg<'a> {
+            fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                match self.1 {
+                    Some(n) => write!(formatter, "error occurred with {}", n),
+                    None => write!(formatter, "there was an empty error"),
+                }?;
+                write!(formatter, ": {}", self.0)
+            }
+        }
+        Msg(.0, .1)
+    })]
+    struct Error(String, Option<usize>);
+
+    assert(
+        "error occurred with 1: ...",
+        Error("...".to_owned(), Some(1)),
+    );
+    assert(
+        "there was an empty error: ...",
+        Error("...".to_owned(), None),
+    );
+}
+
 #[test]
 fn test_void() {
     #[allow(clippy::empty_enum)]