Work around trivial bounds being unstable

impl/src/expand.rs

@@ -38,7 +38,9 @@ fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
         #[allow(unused_qualifications)]
         impl #impl_generics std::error::Error for #ty #ty_generics #where_clause
         where
-            #ty #ty_generics: ::core::fmt::Debug,
+            // Work around trivial bounds being unstable.
+            // https://github.com/rust-lang/rust/issues/48214
+            for<'workaround> #ty #ty_generics: ::core::fmt::Debug,
         {}
 
         #[allow(unused_qualifications)]

tests/ui/union.stderr

@@ -6,20 +6,3 @@ error: union as errors are not supported
 6 | |     num: usize,
 7 | | }
   | |_^
-
-error[E0277]: `U` doesn't implement `Debug`
- --> tests/ui/union.rs:3:10
-  |
-3 | #[derive(Error)]
-  |          ^^^^^ `U` cannot be formatted using `{:?}`
-  |
-  = help: the trait `Debug` is not implemented for `U`
-  = note: add `#[derive(Debug)]` to `U` or manually `impl Debug for U`
-  = help: see issue #48214
-  = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
-  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider annotating `U` with `#[derive(Debug)]`
-  |
-4 + #[derive(Debug)]
-5 | pub union U {
-  |