Add regression test for issue 335

    error[E0277]: `PathBuf` doesn't implement `std::fmt::Display`
       --> tests/test_expr.rs:105:14
        |
    104 |     #[derive(Error, Debug)]
        |              ----- in this derive macro expansion
    105 |     #[error("{A} {b}", b = &0 as &dyn Trait<i32, A = i32>)]
        |              ^^^ `PathBuf` cannot be formatted with the default formatter; call `.display()` on it
        |
        = help: the trait `std::fmt::Display` is not implemented for `PathBuf`
        = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
        = note: call `.display()` or `.to_string_lossy()` to safely print paths, as they may contain non-Unicode data
        = note: this error originates in the macro `$crate::format_args` which comes from the expansion of the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/test_expr.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::iter_cloned_collect, clippy::uninlined_format_args)]
 
 use core::fmt::Display;
+use std::path::PathBuf;
 use thiserror::Error;
 
 // Some of the elaborate cases from the rcc codebase, which is a C compiler in
@@ -87,3 +88,29 @@ fn test_rustup() {
         },
     );
 }
+
+// Regression test for https://github.com/dtolnay/thiserror/issues/335
+#[test]
+#[allow(non_snake_case)]
+fn test_assoc_type_equality_constraint() {
+    pub trait Trait<T>: Display {
+        type A;
+    }
+
+    impl<T> Trait<T> for i32 {
+        type A = i32;
+    }
+
+    #[derive(Error, Debug)]
+    #[error("{A} {b}", b = &0 as &dyn Trait<i32, A = i32>)]
+    pub struct Error {
+        pub A: PathBuf,
+    }
+
+    assert(
+        "... 0",
+        Error {
+            A: PathBuf::from("..."),
+        },
+    );
+}