Optionally transform Extend fields to add items one by one

src/field_info.rs

@@ -36,6 +36,20 @@ impl<'a> FieldInfo<'a> {
  syn::GenericParam::Type(self.generic_ident.clone().into())
  }
 
+ pub fn item_name(&self) -> String {
+ self.builder_attr
+ .setter
+ .extend
+ .as_ref()
+ .expect("Tried to retrieve item_name() on FieldInfo without extend.")
+ .item_name
+ .as_ref()
+ .map_or_else(
+ || self.name.to_string().trim_start().trim_start_matches("r#").to_string() + "_item",
+ |item_name| item_name.to_string(),
+ )
+ }
+
  pub fn type_ident(&self) -> syn::Type {
  ident_to_type(self.generic_ident.clone())
  }

src/struct_info.rs

@@ -1,5 +1,6 @@
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, quote_spanned, ToTokens};
+use std::fmt::Write;
 use syn::{parse::Error, spanned::Spanned};
 
 use crate::field_info::{Configured, ExtendField, FieldBuilderAttr, FieldInfo, SetterSettings};
@@ -287,7 +288,7 @@ impl<'a> StructInfo<'a> {
  keyword_span,
  from_first,
  from_iter,
- item_name,
+ item_name: _,
  }) = &field.builder_attr.setter.extend
  {
  // Changing the builder field type entirely here (instead of just the argument) means there
@@ -299,12 +300,7 @@ impl<'a> StructInfo<'a> {
  field_type
  };
 
- let item_name = item_name.clone().unwrap_or_else(|| {
- syn::Ident::new(
- (field.name.to_string() + "_item").trim_start().trim_start_matches("r#"),
- field.name.span(),
- )
- });
+ let item_name = syn::Ident::new(&field.item_name(), field.name.span());
 
  let descructuring = descructuring.collect::<Vec<_>>();
  let reconstructing = reconstructing.collect::<Vec<_>>();
@@ -609,7 +605,10 @@ impl<'a> StructInfo<'a> {
  ),
  proc_macro2::Span::call_site(),
  );
- let early_build_error_message = format!("Missing required field {}", field_name);
+ let mut early_build_error_message = format!("Missing required field {}", field_name);
+ if field.builder_attr.setter.extend.is_some() {
+ write!(&mut early_build_error_message, " (single item setter: {})", field.item_name()).unwrap();
+ }
 
  Ok(quote! {
  #[doc(hidden)]