diff --git a/impl/src/expand.rs b/impl/src/expand.rs
index 52782e3..d0b302b 100644
--- a/impl/src/expand.rs
+++ b/impl/src/expand.rs
@@ -1,5 +1,6 @@
 use crate::ast::{Enum, Field, Input, Struct};
 use crate::attr::Trait;
+use crate::fallback;
 use crate::generics::InferredBounds;
 use crate::unraw::MemberUnraw;
 use proc_macro2::{Ident, Span, TokenStream};
@@ -13,7 +14,7 @@ pub fn derive(input: &DeriveInput) -> TokenStream {
         // 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),
+        Err(error) => fallback::expand(input, error),
     }
 }
 
@@ -26,34 +27,6 @@ fn try_expand(input: &DeriveInput) -> Result<TokenStream> {
     })
 }
 
-fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
-    let ty = call_site_ident(&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)]
-        #[automatically_derived]
-        impl #impl_generics ::thiserror::__private::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)]
-        #[automatically_derived]
-        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 = call_site_ident(&input.ident);
     let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
@@ -494,7 +467,7 @@ fn impl_enum(input: Enum) -> TokenStream {
 
 // Create an ident with which we can expand `impl Trait for #ident {}` on a
 // deprecated type without triggering deprecation warning on the generated impl.
-fn call_site_ident(ident: &Ident) -> Ident {
+pub(crate) fn call_site_ident(ident: &Ident) -> Ident {
     let mut ident = ident.clone();
     ident.set_span(ident.span().resolved_at(Span::call_site()));
     ident
diff --git a/impl/src/fallback.rs b/impl/src/fallback.rs
new file mode 100644
index 0000000..e9c429b
--- /dev/null
+++ b/impl/src/fallback.rs
@@ -0,0 +1,32 @@
+use crate::expand::call_site_ident;
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::DeriveInput;
+
+pub(crate) fn expand(input: &DeriveInput, error: syn::Error) -> TokenStream {
+    let ty = call_site_ident(&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)]
+        #[automatically_derived]
+        impl #impl_generics ::thiserror::__private::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)]
+        #[automatically_derived]
+        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!()
+            }
+        }
+    }
+}
diff --git a/impl/src/lib.rs b/impl/src/lib.rs
index 5148323..7f3767e 100644
--- a/impl/src/lib.rs
+++ b/impl/src/lib.rs
@@ -21,6 +21,7 @@ extern crate proc_macro;
 mod ast;
 mod attr;
 mod expand;
+mod fallback;
 mod fmt;
 mod generics;
 mod prop;