Error if attrs field is declared and forward_attrs is missing.

Previously, this would result in a cryptic error due to an expect(...) failure if neither `attributes` nor `forward_attrs` were declared.

This change is technically breaking, as it was previously possible to declare this field without using `forward_attrs`; if `attributes` was used the `attrs` field would be initialized to an empty array.
That behavior was more likely to mask issues than be useful, but it's possible someone declared the field in order to manually populate it later.

Fixes #317

core/src/options/outer_from.rs

@@ -1,12 +1,13 @@
 use syn::spanned::Spanned;
 use syn::{Field, Ident, Meta};
 
+use crate::ast::Data;
 use crate::codegen::ForwardAttrs;
 use crate::options::{
     Core, DefaultExpression, ForwardAttrsFilter, ForwardedField, ParseAttribute, ParseData,
 };
 use crate::util::PathList;
-use crate::{FromField, FromMeta, Result};
+use crate::{Error, FromField, FromMeta, Result};
 
 /// Reusable base for `FromDeriveInput`, `FromVariant`, `FromField`, and other top-level
 /// `From*` traits.
@@ -91,5 +92,20 @@ impl ParseData for OuterFrom {
 
     fn validate_body(&self, errors: &mut crate::error::Accumulator) {
         self.container.validate_body(errors);
+        if let Some(attrs) = &self.attrs {
+            if self.forward_attrs.is_none() {
+                let container_name = match &self.container.data {
+                    Data::Enum(_) => "enum",
+                    Data::Struct(_) => "struct",
+                };
+                errors.push(
+                    Error::custom(format!(
+                        "field will not be populated because `forward_attrs` is not set on the {}",
+                        container_name
+                    ))
+                    .with_span(&attrs.ident),
+                );
+            }
+        }
     }
 }

tests/compile-fail/attrs_without_forward_attrs.rs

@@ -0,0 +1,10 @@
+use darling::FromDeriveInput;
+use syn::{Attribute, Ident};
+
+#[derive(FromDeriveInput)]
+struct HelloArgs {
+    ident: Ident,
+    attrs: Vec<Attribute>,
+}
+
+fn main() {}

tests/compile-fail/attrs_without_forward_attrs.stderr

@@ -0,0 +1,5 @@
+error: field will not be populated because `forward_attrs` is not set on the struct
+ --> tests/compile-fail/attrs_without_forward_attrs.rs:7:5
+  |
+7 |     attrs: Vec<Attribute>,
+  |     ^^^^^