[derive] Overhaul repr parsing

Represent the result of parsing all `#[repr(...)]` attributes on a type
as a high-level type which is only capable of representing valid
combinations of `#[repr(...)]` attributes and processes them into a
concise representation that's easier for high-level code to work with.

This prepares us to more easily fix #1748

zerocopy-derive/src/lib.rs

@@ -33,6 +33,7 @@ mod ext;
 #[cfg(test)]
 mod output_tests;
 mod repr;
+mod repr2;
 
 use proc_macro2::{TokenStream, TokenTree};
 use quote::ToTokens;
@@ -790,6 +791,8 @@ fn derive_from_bytes_union(ast: &DeriveInput, unn: &DataUnion) -> proc_macro2::T
 }
 
 fn derive_into_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> proc_macro2::TokenStream {
+ let _repr = try_or_print!(repr2::StructUnionRepr::from_attrs(&ast.attrs).map_err(|e| vec![e]));
+
  let reprs = try_or_print!(STRUCT_UNION_INTO_BYTES_CFG.validate_reprs(ast));
  let is_transparent = reprs.contains(&StructRepr::Transparent);
  let is_packed = reprs.contains(&StructRepr::Packed);