Better macro errors for get_struct_fields (#17639)

# Objective

- Currently, the error span for `get_struct_field` when encountering an
enum or union points to the macro invocation, rather than the `enum` or
`union` token. It also doesn't mention which macro reported the error.

## Solution

- Report the correct error span
- Add parameter for passing in the name of the macro invocation

## Testing

Bevy compiles fine with this change

## Migration Guide

```rs
// before
let fields = get_struct_fields(&ast.data);

// after
let fields = get_struct_fields(&ast.data, "derive(Bundle)");
```

---------

Co-authored-by: Chris Russell <8494645+chescock@users.noreply.github.com>

crates/bevy_ecs/macros/src/lib.rs

@@ -34,7 +34,7 @@ pub fn derive_bundle(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
     let ecs_path = bevy_ecs_path();
 
-    let named_fields = match get_struct_fields(&ast.data) {
+    let named_fields = match get_struct_fields(&ast.data, "derive(Bundle)") {
         Ok(fields) => fields,
         Err(e) => return e.into_compile_error().into(),
     };
@@ -191,12 +191,14 @@ pub fn derive_bundle(input: TokenStream) -> TokenStream {
 pub fn derive_map_entities(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);
     let ecs_path = bevy_ecs_path();
+
     let map_entities_impl = map_entities(
         &ast.data,
         Ident::new("self", Span::call_site()),
         false,
         false,
     );
+
     let struct_name = &ast.ident;
     let (impl_generics, type_generics, where_clause) = &ast.generics.split_for_impl();
     TokenStream::from(quote! {

crates/bevy_macro_utils/src/shape.rs

@@ -1,24 +1,29 @@
-use proc_macro::Span;
-use syn::{punctuated::Punctuated, token::Comma, Data, DataStruct, Error, Field, Fields};
+use syn::{
+    punctuated::Punctuated, spanned::Spanned, token::Comma, Data, DataEnum, DataUnion, Error,
+    Field, Fields,
+};
 
 /// Get the fields of a data structure if that structure is a struct with named fields;
 /// otherwise, return a compile error that points to the site of the macro invocation.
-pub fn get_struct_fields(data: &Data) -> syn::Result<&Punctuated<Field, Comma>> {
+///
+/// `meta` should be the name of the macro calling this function.
+pub fn get_struct_fields<'a>(
+    data: &'a Data,
+    meta: &str,
+) -> syn::Result<&'a Punctuated<Field, Comma>> {
     match data {
-        Data::Struct(DataStruct {
-            fields: Fields::Named(fields),
-            ..
-        }) => Ok(&fields.named),
-        Data::Struct(DataStruct {
-            fields: Fields::Unnamed(fields),
-            ..
-        }) => Ok(&fields.unnamed),
-        _ => Err(Error::new(
-            // This deliberately points to the call site rather than the structure
-            // body; marking the entire body as the source of the error makes it
-            // impossible to figure out which `derive` has a problem.
-            Span::call_site().into(),
-            "Only structs are supported",
+        Data::Struct(data_struct) => match &data_struct.fields {
+            Fields::Named(fields_named) => Ok(&fields_named.named),
+            Fields::Unnamed(fields_unnamed) => Ok(&fields_unnamed.unnamed),
+            Fields::Unit => Ok(const { &Punctuated::new() }),
+        },
+        Data::Enum(DataEnum { enum_token, .. }) => Err(Error::new(
+            enum_token.span(),
+            format!("#[{meta}] only supports structs, not enums"),
+        )),
+        Data::Union(DataUnion { union_token, .. }) => Err(Error::new(
+            union_token.span(),
+            format!("#[{meta}] only supports structs, not unions"),
         )),
     }
 }