Add ui test where fallback impl conflicts with handwritten Display

tests/ui/fallback-impl-with-display.rs

@@ -0,0 +1,14 @@
+use std::fmt::{self, Display};
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error]
+pub struct MyError;
+
+impl Display for MyError {
+    fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+        unimplemented!()
+    }
+}
+
+fn main() {}

tests/ui/fallback-impl-with-display.stderr

@@ -0,0 +1,16 @@
+error: expected attribute arguments in parentheses: #[error(...)]
+ --> tests/ui/fallback-impl-with-display.rs:5:3
+  |
+5 | #[error]
+  |   ^^^^^
+
+error[E0119]: conflicting implementations of trait `std::fmt::Display` for type `MyError`
+ --> tests/ui/fallback-impl-with-display.rs:4:10
+  |
+4 | #[derive(Error, Debug)]
+  |          ^^^^^ conflicting implementation for `MyError`
+...
+8 | impl Display for MyError {
+  | ------------------------ first implementation here
+  |
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)