Allow repr(transparent) to be used generically in derive(Pod) (#139)

* Enabled transparent generics

* Move trait checks to implementation block

* Replace add_trait_marker impl

derive/src/lib.rs

@@ -9,7 +9,8 @@ use quote::quote;
 use syn::{parse_macro_input, DeriveInput, Result};
 
 use crate::traits::{
-  AnyBitPattern, Contiguous, Derivable, CheckedBitPattern, NoUninit, Pod, TransparentWrapper, Zeroable,
+  AnyBitPattern, CheckedBitPattern, Contiguous, Derivable, NoUninit, Pod,
+  TransparentWrapper, Zeroable,
 };
 
 /// Derive the `Pod` trait for a struct
@@ -56,8 +57,9 @@ pub fn derive_pod(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 pub fn derive_anybitpattern(
   input: proc_macro::TokenStream,
 ) -> proc_macro::TokenStream {
-  let expanded =
-    derive_marker_trait::<AnyBitPattern>(parse_macro_input!(input as DeriveInput));
+  let expanded = derive_marker_trait::<AnyBitPattern>(parse_macro_input!(
+    input as DeriveInput
+  ));
 
   proc_macro::TokenStream::from(expanded)
 }
@@ -99,8 +101,8 @@ pub fn derive_zeroable(
 /// for the `NoUninit` trait.
 ///
 /// The following constraints need to be satisfied for the macro to succeed
-/// (the rest of the constraints are guaranteed by the `NoUninit` subtrait bounds,
-/// i.e. the type must be `Sized + Copy + 'static`):
+/// (the rest of the constraints are guaranteed by the `NoUninit` subtrait
+/// bounds, i.e. the type must be `Sized + Copy + 'static`):
 ///
 /// If applied to a struct:
 /// - All fields in the struct must implement `NoUninit`
@@ -129,9 +131,9 @@ pub fn derive_no_uninit(
 /// definition and `is_valid_bit_pattern` method for the type automatically.
 ///
 /// The following constraints need to be satisfied for the macro to succeed
-/// (the rest of the constraints are guaranteed by the `CheckedBitPattern` subtrait bounds,
-/// i.e. are guaranteed by the requirements of the `NoUninit` trait which `CheckedBitPattern`
-/// is a subtrait of):
+/// (the rest of the constraints are guaranteed by the `CheckedBitPattern`
+/// subtrait bounds, i.e. are guaranteed by the requirements of the `NoUninit`
+/// trait which `CheckedBitPattern` is a subtrait of):
 ///
 /// If applied to a struct:
 /// - All fields must implement `CheckedBitPattern`
@@ -142,8 +144,9 @@ pub fn derive_no_uninit(
 pub fn derive_maybe_pod(
   input: proc_macro::TokenStream,
 ) -> proc_macro::TokenStream {
-  let expanded =
-    derive_marker_trait::<CheckedBitPattern>(parse_macro_input!(input as DeriveInput));
+  let expanded = derive_marker_trait::<CheckedBitPattern>(parse_macro_input!(
+    input as DeriveInput
+  ));
 
   proc_macro::TokenStream::from(expanded)
 }
@@ -228,17 +231,19 @@ fn derive_marker_trait<Trait: Derivable>(input: DeriveInput) -> TokenStream {
 }
 
 fn derive_marker_trait_inner<Trait: Derivable>(
-  input: DeriveInput,
+  mut input: DeriveInput,
 ) -> Result<TokenStream> {
+  // Enforce Pod on all generic fields.
+  let trait_ = Trait::ident(&input)?;
+  add_trait_marker(&mut input.generics, &trait_);
+
   let name = &input.ident;
 
   let (impl_generics, ty_generics, where_clause) =
     input.generics.split_for_impl();
 
-  let trait_ = Trait::ident();
   Trait::check_attributes(&input.data, &input.attrs)?;
   let asserts = Trait::asserts(&input)?;
-  let trait_params = Trait::generic_params(&input)?;
   let (trait_impl_extras, trait_impl) = Trait::trait_impl(&input)?;
 
   let implies_trait = if let Some(implies_trait) = Trait::implies_trait() {
@@ -252,10 +257,23 @@ fn derive_marker_trait_inner<Trait: Derivable>(
 
     #trait_impl_extras
 
-    unsafe impl #impl_generics #trait_ #trait_params for #name #ty_generics #where_clause {
+    unsafe impl #impl_generics #trait_ for #name #ty_generics #where_clause {
       #trait_impl
     }
 
     #implies_trait
   })
 }
+
+/// Add a trait marker to the generics if it is not already present
+fn add_trait_marker(generics: &mut syn::Generics, trait_name: &syn::Path) {
+  // Get each generic type parameter.
+  let type_params = generics
+    .type_params()
+    .map(|param| &param.ident)
+    .map(|param| syn::parse_quote!(
+      #param: #trait_name
+    )).collect::<Vec<syn::WherePredicate>>();
+
+  generics.make_where_clause().predicates.extend(type_params);
+}