Emit an Error impl even in the presence of bad attributes

impl/src/expand.rs

@@ -7,15 +7,51 @@ use quote::{format_ident, quote, quote_spanned, ToTokens};
 use std::collections::BTreeSet as Set;
 use syn::{DeriveInput, GenericArgument, Member, PathArguments, Result, Token, Type};
 
-pub fn derive(node: &DeriveInput) -> Result<TokenStream> {
-    let input = Input::from_syn(node)?;
+pub fn derive(input: &DeriveInput) -> TokenStream {
+    match try_expand(input) {
+        Ok(expanded) => expanded,
+        // If there are invalid attributes in the input, expand to an Error impl
+        // anyway to minimize spurious knock-on errors in other code that uses
+        // this type as an Error.
+        Err(error) => fallback(input, error),
+    }
+}
+
+fn try_expand(input: &DeriveInput) -> Result<TokenStream> {
+    let input = Input::from_syn(input)?;
     input.validate()?;
     Ok(match input {
         Input::Struct(input) => impl_struct(input),
         Input::Enum(input) => impl_enum(input),
     })
 }
 
+fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
+    let ty = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let error = error.to_compile_error();
+
+    quote! {
+        #error
+
+        #[allow(unused_qualifications)]
+        impl #impl_generics std::error::Error for #ty #ty_generics #where_clause
+        where
+            // 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)]
+        impl #impl_generics ::core::fmt::Display for #ty #ty_generics #where_clause {
+            fn fmt(&self, __formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+                ::core::unreachable!()
+            }
+        }
+    }
+}
+
 fn impl_struct(input: Struct) -> TokenStream {
     let ty = &input.ident;
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

impl/src/lib.rs

@@ -33,7 +33,5 @@ use syn::{parse_macro_input, DeriveInput};
 #[proc_macro_derive(Error, attributes(backtrace, error, from, source))]
 pub fn derive_error(input: TokenStream) -> TokenStream {
     let input = parse_macro_input!(input as DeriveInput);
-    expand::derive(&input)
-        .unwrap_or_else(|err| err.to_compile_error())
-        .into()
+    expand::derive(&input).into()
 }

tests/ui/invalid-input-impl-anyway.rs

@@ -5,7 +5,7 @@ use thiserror::Error;
 pub struct MyError;
 
 fn main() {
-    // FIXME: there should be no error on the following line. Thiserror should
-    // emit an Error impl regardless of the bad attribute.
+    // No error on the following line. Thiserror emits an Error impl despite the
+    // bad attribute.
     _ = &MyError as &dyn std::error::Error;
 }

tests/ui/invalid-input-impl-anyway.stderr

@@ -3,11 +3,3 @@ error: expected attribute arguments in parentheses: #[error(...)]
   |
 4 | #[error]
   |   ^^^^^
-
-error[E0277]: the trait bound `MyError: std::error::Error` is not satisfied
-  --> tests/ui/invalid-input-impl-anyway.rs:10:9
-   |
-10 |     _ = &MyError as &dyn std::error::Error;
-   |         ^^^^^^^^ the trait `std::error::Error` is not implemented for `MyError`
-   |
-   = note: required for the cast from `&MyError` to `&dyn std::error::Error`