Add basic Display test

tests/test_display.rs

@@ -0,0 +1,39 @@
+use std::fmt::Display;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("braced error: {}", msg)]
+struct BracedError {
+    msg: String,
+}
+
+#[derive(Error, Debug)]
+#[error("braced error")]
+struct BracedUnused {
+    extra: usize,
+}
+
+#[derive(Error, Debug)]
+#[error("tuple error: {}", .0)]
+struct TupleError(usize);
+
+#[derive(Error, Debug)]
+#[error("unit error")]
+struct UnitError;
+
+fn assert<T: Display>(expected: &str, value: T) {
+    assert_eq!(expected, value.to_string());
+}
+
+#[test]
+fn test_display() {
+    assert(
+        "braced error: T",
+        BracedError {
+            msg: "T".to_owned(),
+        },
+    );
+    assert("braced error", BracedUnused { extra: 0 });
+    assert("tuple error: 0", TupleError(0));
+    assert("unit error", UnitError);
+}