Better macro errors for get_struct_fields

[ecoskey]

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

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

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

[mockersf]

wouldn't change much, but meta could be only the name of the trait being derived instead of derive{trait}

[ecoskey]

This is meant to accomodate attribute macros, which don't fit the derive() pattern

[chescock]

Tested it, and it works!

#[derive(VisitEntities, Bundle)]
struct UnitStruct;

#[derive(VisitEntities, Bundle)]
enum Enum {}

#[derive(VisitEntities, Bundle)]
union Union {
    x: (),
}

fails with

error: #[derive(VisitEntities)] only supports structs with more than zero fields
  --> crates\bevy_ecs\src\entity\visit_entities.rs:60:10
   |
60 | #[derive(VisitEntities, Bundle)]
   |          ^^^^^^^^^^^^^
   |
   = note: this error originates in the derive macro `VisitEntities` (in Nightly builds, run with -Z macro-backtrace for more info)

error: #[derive(Bundle)] only supports structs with more than zero fields
  --> crates\bevy_ecs\src\entity\visit_entities.rs:60:25
   |
60 | #[derive(VisitEntities, Bundle)]
   |                         ^^^^^^
   |
   = note: this error originates in the derive macro `Bundle` (in Nightly builds, run with -Z macro-backtrace for more info)

error: #[derive(VisitEntities)] only supports structs, not enums
  --> crates\bevy_ecs\src\entity\visit_entities.rs:64:1
   |
64 | enum Enum {}
   | ^^^^

error: #[derive(Bundle)] only supports structs, not enums
  --> crates\bevy_ecs\src\entity\visit_entities.rs:64:1
   |
64 | enum Enum {}
   | ^^^^

error: #[derive(VisitEntities)] only supports structs, not unions
  --> crates\bevy_ecs\src\entity\visit_entities.rs:67:1
   |
67 | union Union {
   | ^^^^^

error: #[derive(Bundle)] only supports structs, not unions
  --> crates\bevy_ecs\src\entity\visit_entities.rs:67:1
   |
67 | union Union {
   | ^^^^^

[chescock]

I realize this is how the code worked before, but would it make sense to support these derives on unit structs by returning an empty list of fields? (Although derive_visit_entities_base raises a separate error that we'd have to remove to fully support that.)

bevy/crates/bevy_ecs/macros/src/lib.rs

Line 231 in edba54a

if field.is_empty() {

Suggested change

	Fields::Unit => Err(Error::new(
	data_struct.fields.span(),
	format!("#[{meta}] only supports structs with more than zero fields"),
	)),
	Fields::Unit => Ok(const { &Punctuated::new() }),