Add regression test for issue 345

    error[E0599]: `test_no_bound_on_named_fmt::Error<DebugOnly>` doesn't implement `std::fmt::Display`
       --> tests/test_generics.rs:173:22
        |
    168 |     struct Error<T> {
        |     --------------- method `to_string` not found for this struct because it doesn't satisfy `_: Display` or `_: ToString`
    ...
    173 |     assert_eq!(error.to_string(), "...");
        |                      ^^^^^^^^^ `test_no_bound_on_named_fmt::Error<DebugOnly>` cannot be formatted with the default formatter
        |
        = note: the following trait bounds were not satisfied:
                `test_no_bound_on_named_fmt::Error<DebugOnly>: std::fmt::Display`
                which is required by `test_no_bound_on_named_fmt::Error<DebugOnly>: ToString`
        = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
    note: the trait `std::fmt::Display` must be implemented
       --> $RUSTUP_HOME/toolchains/nightly-aarch64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:727:1
        |
    727 | pub trait Display {
        | ^^^^^^^^^^^^^^^^^
        = help: items from traits can only be used if the trait is implemented and in scope
        = note: the following trait defines an item `to_string`, perhaps you need to implement it:
                candidate #1: `ToString`

tests/test_generics.rs

@@ -159,3 +159,16 @@ pub struct StructFromGeneric<E> {
 #[derive(Error, Debug)]
 #[error(transparent)]
 pub struct StructTransparentGeneric<E>(pub E);
+
+// Regression test for https://github.com/dtolnay/thiserror/issues/345
+#[test]
+fn test_no_bound_on_named_fmt() {
+    #[derive(Error, Debug)]
+    #[error("{thing}", thing = "...")]
+    struct Error<T> {
+        thing: T,
+    }
+
+    let error = Error { thing: DebugOnly };
+    assert_eq!(error.to_string(), "...");
+}