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use proc_macro::Span;
use syn::{Data, DataStruct, Error, Fields, FieldsNamed};
/// 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_named_struct_fields(data: &syn::Data) -> syn::Result<&FieldsNamed> {
match data {
Data::Struct(DataStruct {
fields: Fields::Named(fields),
..
}) => Ok(fields),
_ => 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 with named fields are supported",
)),
}
}