Add errors on invalid forward usage and compile_fail tests for them

impl/src/error.rs

@@ -1,4 +1,4 @@
-use proc_macro2::TokenStream;
+use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::{spanned::Spanned as _, Error, Result};
 
@@ -390,7 +390,12 @@ fn parse_fields_impl<'input, 'state, P>(
 where
     P: Fn(&str, &syn::Field, usize) -> bool,
 {
-    let MultiFieldData { fields, infos, .. } = state.enabled_fields_data();
+    let MultiFieldData {
+        fields,
+        infos,
+        variant_info,
+        ..
+    } = state.enabled_fields_data();
 
     let iter = fields
         .iter()
@@ -418,6 +423,11 @@ where
 
     if let Some((index, _, _)) = source {
         parsed_fields.source = Some(index);
+    } else if variant_info.forward {
+        return Err(syn::Error::new(
+            Span::call_site(),
+            "`#[error(forward)]` cannot be used when an error has no source",
+        ));
     }
 
     if let Some((index, _, _)) = backtrace {

tests/compile_fail/error/forward_no_source_enum.rs

@@ -0,0 +1,18 @@
+use derive_more::Error;
+
+#[derive(Debug, Error)]
+#[error(forward)]
+enum Foo {
+    Bar,
+    Baz {
+        source: Box<dyn Error + Send + 'static>,
+    },
+}
+
+impl ::core::fmt::Display for Foo {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+        write!(f, "")
+    }
+}
+
+fn main() {}

tests/compile_fail/error/forward_no_source_enum.stderr

@@ -0,0 +1,7 @@
+error: `#[error(forward)]` cannot be used when an error has no source
+ --> tests/compile_fail/error/forward_no_source_enum.rs:3:17
+  |
+3 | #[derive(Debug, Error)]
+  |                 ^^^^^
+  |
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/compile_fail/error/forward_no_source_struct.rs

@@ -0,0 +1,13 @@
+use derive_more::Error;
+
+#[derive(Debug, Error)]
+#[error(forward)]
+struct Foo;
+
+impl ::core::fmt::Display for Foo {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+        write!(f, "")
+    }
+}
+
+fn main() {}

tests/compile_fail/error/forward_no_source_struct.stderr

@@ -0,0 +1,7 @@
+error: `#[error(forward)]` cannot be used when an error has no source
+ --> tests/compile_fail/error/forward_no_source_struct.rs:3:17
+  |
+3 | #[derive(Debug, Error)]
+  |                 ^^^^^
+  |
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/compile_fail/error/forward_no_source_variant.rs

@@ -0,0 +1,19 @@
+use derive_more::Error;
+
+#[derive(Debug, Error)]
+enum Foo {
+    #[error(forward)]
+    Bar,
+    #[error(forward)]
+    Baz {
+        source: Box<dyn Error + Send + 'static>,
+    },
+}
+
+impl ::core::fmt::Display for Foo {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+        write!(f, "")
+    }
+}
+
+fn main() {}

tests/compile_fail/error/forward_no_source_variant.stderr

@@ -0,0 +1,7 @@
+error: `#[error(forward)]` cannot be used when an error has no source
+ --> tests/compile_fail/error/forward_no_source_variant.rs:3:17
+  |
+3 | #[derive(Debug, Error)]
+  |                 ^^^^^
+  |
+  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)