support deriving Pod for packed generic types. (#123)

* improve `#[repr]` parsing

* allow deriving `Pod` for packed generic types

* Revert "Update Cargo.toml"

This reverts commit 6632bcef2c.

Cargo.toml

@@ -28,9 +28,7 @@ unsound_ptr_pod_impl = []
 nightly_portable_simd = []
 
 [dependencies]
-# use the upper line for testing against bytemuck_derive changes, if any
+bytemuck_derive = { version = "1.1", path = "derive", optional = true }
-#bytemuck_derive = { path = "./derive", optional = true }
-bytemuck_derive = { version = "1.1", optional = true }
 
 [package.metadata.docs.rs]
 # Note(Lokathor): Don't use all-features or it would use `unsound_ptr_pod_impl` too.

derive/src/traits.rs

@@ -1,9 +1,9 @@
-use proc_macro2::{Ident, TokenStream, TokenTree};
+use proc_macro2::{Ident, Span, TokenStream, TokenTree};
 use quote::{quote, quote_spanned, ToTokens};
 use syn::{
   spanned::Spanned, AttrStyle, Attribute, Data, DataEnum, DataStruct,
-  DeriveInput, Expr, ExprLit, ExprUnary, Fields, Lit, LitInt, Type, UnOp,
+  DataUnion, DeriveInput, Expr, ExprLit, ExprUnary, Fields, Lit, LitInt, Meta,
-  Variant, DataUnion,
+  NestedMeta, Type, UnOp, Variant,
 };
 
 pub trait Derivable {
@@ -37,13 +37,21 @@ impl Derivable for Pod {
   }
 
   fn asserts(input: &DeriveInput) -> Result<TokenStream, &'static str> {
-    if !input.generics.params.is_empty() {
+    let repr = get_repr(&input.attrs);
-      return Err("Pod requires cannot be derived for types containing generic parameters because the padding requirements can't be verified for generic structs");
+
+    let completly_packed = repr.packed == Some(1);
+
+    if !completly_packed && !input.generics.params.is_empty() {
+      return Err("Pod requires cannot be derived for non-packed types containing generic parameters because the padding requirements can't be verified for generic non-packed structs");
     }
 
     match &input.data {
       Data::Struct(_) => {
-        let assert_no_padding = generate_assert_no_padding(input)?;
+        let assert_no_padding = if !completly_packed {
+          Some(generate_assert_no_padding(input)?)
+        } else {
+          None
+        };
         let assert_fields_are_pod =
           generate_fields_are_trait(input, Self::ident())?;
 
@@ -51,9 +59,9 @@ impl Derivable for Pod {
           #assert_no_padding
           #assert_fields_are_pod
         ))
-      },
+      }
       Data::Enum(_) => Err("Deriving Pod is not supported for enums"),
-      Data::Union(_) => Err("Deriving Pod is not supported for unions")
+      Data::Union(_) => Err("Deriving Pod is not supported for unions"),
     }
   }
 
@@ -61,9 +69,9 @@ impl Derivable for Pod {
     _ty: &Data, attributes: &[Attribute],
   ) -> Result<(), &'static str> {
     let repr = get_repr(attributes);
-    match repr.as_ref().map(|repr| repr.as_str()) {
+    match repr.repr {
-      Some("C") => Ok(()),
+      Repr::C => Ok(()),
-      Some("transparent") => Ok(()),
+      Repr::Transparent => Ok(()),
       _ => {
         Err("Pod requires the type to be #[repr(C)] or #[repr(transparent)]")
       }
@@ -119,11 +127,11 @@ impl Derivable for NoUninit {
   ) -> Result<(), &'static str> {
     let repr = get_repr(attributes);
     match ty {
-      Data::Struct(_) => match repr.as_deref() {
+      Data::Struct(_) => match repr.repr {
-        Some("C" | "transparent") => Ok(()),
+        Repr::C | Repr::Transparent => Ok(()),
         _ => Err("NoUninit requires the struct to be #[repr(C)] or #[repr(transparent)]"),
       },
-      Data::Enum(_) => if repr.map(|repr| repr.starts_with('u') || repr.starts_with('i')) == Some(true) {
+      Data::Enum(_) => if repr.repr.is_integer() {
         Ok(())
       } else {
         Err("NoUninit requires the enum to be an explicit #[repr(Int)]")
@@ -178,11 +186,11 @@ impl Derivable for CheckedBitPattern {
   ) -> Result<(), &'static str> {
     let repr = get_repr(attributes);
     match ty {
-      Data::Struct(_) => match repr.as_deref() {
+      Data::Struct(_) => match repr.repr {
-        Some("C" | "transparent") => Ok(()),
+        Repr::C | Repr::Transparent => Ok(()),
         _ => Err("CheckedBitPattern derive requires the struct to be #[repr(C)] or #[repr(transparent)]"),
       },
-      Data::Enum(_) => if repr.map(|repr| repr.starts_with('u') || repr.starts_with('i')) == Some(true) {
+      Data::Enum(_) => if repr.repr.is_integer() {
         Ok(())
       } else {
         Err("CheckedBitPattern requires the enum to be an explicit #[repr(Int)]")
@@ -212,9 +220,9 @@ impl Derivable for CheckedBitPattern {
     input: &DeriveInput,
   ) -> Result<(TokenStream, TokenStream), &'static str> {
     match &input.data {
-      Data::Struct(DataStruct { fields, .. }) => {
+      Data::Struct(DataStruct { fields, .. }) => Ok(
-        Ok(generate_checked_bit_pattern_struct(&input.ident, fields, &input.attrs))
+        generate_checked_bit_pattern_struct(&input.ident, fields, &input.attrs),
-      }
+      ),
       Data::Enum(_) => generate_checked_bit_pattern_enum(input),
       Data::Union(_) => Err("Internal error in CheckedBitPattern derive"), // shouldn't be possible since we already error in attribute check for this case
     }
@@ -277,8 +285,8 @@ impl Derivable for TransparentWrapper {
   ) -> Result<(), &'static str> {
     let repr = get_repr(attributes);
 
-    match repr.as_ref().map(|repr| repr.as_str()) {
+    match repr.repr {
-      Some("transparent") => Ok(()),
+      Repr::Transparent => Ok(()),
       _ => {
         Err("TransparentWrapper requires the struct to be #[repr(transparent)]")
       }
@@ -296,12 +304,13 @@ impl Derivable for Contiguous {
   fn trait_impl(
     input: &DeriveInput,
   ) -> Result<(TokenStream, TokenStream), &'static str> {
-    let repr = get_repr(&input.attrs)
+    let repr = get_repr(&input.attrs);
-      .ok_or("Contiguous requires the enum to be #[repr(Int)]")?;
 
-    if !repr.starts_with('u') && !repr.starts_with('i') {
+    let integer_ty = if let Some(integer_ty) = repr.repr.as_integer_type() {
+      integer_ty
+    } else {
       return Err("Contiguous requires the enum to be #[repr(Int)]");
-    }
+    };
 
     let variants = get_enum_variants(input)?;
     let mut variants_with_discriminator =
@@ -325,16 +334,15 @@ impl Derivable for Contiguous {
       );
     }
 
-    let repr_ident = Ident::new(&repr, input.span());
     let min_lit = LitInt::new(&format!("{}", min), input.span());
     let max_lit = LitInt::new(&format!("{}", max), input.span());
 
     Ok((
       quote!(),
       quote! {
-          type Int = #repr_ident;
+          type Int = #integer_ty;
-          const MIN_VALUE: #repr_ident = #min_lit;
+          const MIN_VALUE: #integer_ty = #min_lit;
-          const MAX_VALUE: #repr_ident = #max_lit;
+          const MAX_VALUE: #integer_ty = #max_lit;
       },
     ))
   }
@@ -352,7 +360,7 @@ fn get_fields(input: &DeriveInput) -> Result<Fields, &'static str> {
   match &input.data {
     Data::Struct(DataStruct { fields, .. }) => Ok(fields.clone()),
     Data::Union(DataUnion { fields, .. }) => Ok(Fields::Named(fields.clone())),
-    Data::Enum(_) => Err("deriving this trait is not supported for enums")
+    Data::Enum(_) => Err("deriving this trait is not supported for enums"),
   }
 }
 
@@ -377,7 +385,7 @@ fn generate_checked_bit_pattern_struct(
 ) -> (TokenStream, TokenStream) {
   let bits_ty = Ident::new(&format!("{}Bits", input_ident), input_ident.span());
 
-  let repr = get_simple_attr(attrs, "repr").unwrap(); // should be checked in attr check already
+  let repr = get_repr(attrs);
 
   let field_names = fields
     .iter()
@@ -400,7 +408,7 @@ fn generate_checked_bit_pattern_struct(
 
   (
     quote! {
-        #[repr(#repr)]
+        #repr
         #[derive(Clone, Copy, ::bytemuck::AnyBitPattern)]
         #derive_dbg
         pub struct #bits_ty {
@@ -459,11 +467,12 @@ fn generate_checked_bit_pattern_enum(
     quote!(matches!(*bits, #first #(| #rest )*))
   };
 
-  let repr = get_simple_attr(&input.attrs, "repr").unwrap(); // should be checked in attr check already
+  let repr = get_repr(&input.attrs);
+  let integer_ty = repr.repr.as_integer_type().unwrap(); // should be checked in attr check already
   Ok((
     quote!(),
     quote! {
-        type Bits = #repr;
+        type Bits = #integer_ty;
 
         #[inline]
         #[allow(clippy::double_comparisons)]
@@ -544,8 +553,199 @@ fn get_simple_attr(attributes: &[Attribute], attr_name: &str) -> Option<Ident> {
   None
 }
 
-fn get_repr(attributes: &[Attribute]) -> Option<String> {
+#[derive(Clone, Copy)]
-  get_simple_attr(attributes, "repr").map(|ident| ident.to_string())
+struct Representation {
+  packed: Option<u32>,
+  repr: Repr,
+}
+
+impl ToTokens for Representation {
+  fn to_tokens(&self, tokens: &mut TokenStream) {
+    let repr = match self.repr {
+      Repr::Rust => None,
+      Repr::C => Some("C"),
+      Repr::Transparent => Some("transparent"),
+      Repr::U8 => Some("u8"),
+      Repr::I8 => Some("i8"),
+      Repr::U16 => Some("u16"),
+      Repr::I16 => Some("i16"),
+      Repr::U32 => Some("u32"),
+      Repr::I32 => Some("i32"),
+      Repr::U64 => Some("u64"),
+      Repr::I64 => Some("i64"),
+      Repr::I128 => Some("i128"),
+      Repr::U128 => Some("u128"),
+    };
+    if let Some(repr) = repr {
+      let ident = Ident::new(repr, Span::call_site());
+      tokens.extend(quote! {
+        #[repr(#ident)]
+      });
+    }
+
+    if let Some(packed) = self.packed {
+      let lit = LitInt::new(&packed.to_string(), Span::call_site());
+      tokens.extend(quote! {
+        #[repr(packed(#lit))]
+      });
+    }
+  }
+}
+
+#[derive(Clone, Copy)]
+enum Repr {
+  Rust,
+  C,
+  Transparent,
+  U8,
+  I8,
+  U16,
+  I16,
+  U32,
+  I32,
+  U64,
+  I64,
+  I128,
+  U128,
+}
+
+impl Repr {
+  fn is_integer(&self) -> bool {
+    match *self {
+      Repr::Rust | Repr::C | Repr::Transparent => false,
+      Repr::U8
+      | Repr::I8
+      | Repr::U16
+      | Repr::I16
+      | Repr::U32
+      | Repr::I32
+      | Repr::U64
+      | Repr::I64
+      | Repr::I128
+      | Repr::U128 => true,
+    }
+  }
+
+  fn as_integer_type(&self) -> Option<TokenStream> {
+    match self {
+      Repr::Rust | Repr::C | Repr::Transparent => None,
+      Repr::U8 => Some(quote! { ::core::primitive::u8 }),
+      Repr::I8 => Some(quote! { ::core::primitive::i8 }),
+      Repr::U16 => Some(quote! { ::core::primitive::u16 }),
+      Repr::I16 => Some(quote! { ::core::primitive::i16 }),
+      Repr::U32 => Some(quote! { ::core::primitive::u32 }),
+      Repr::I32 => Some(quote! { ::core::primitive::i32 }),
+      Repr::U64 => Some(quote! { ::core::primitive::u64 }),
+      Repr::I64 => Some(quote! { ::core::primitive::i64 }),
+      Repr::I128 => Some(quote! { ::core::primitive::u128 }),
+      Repr::U128 => Some(quote! { ::core::primitive::i128 }),
+    }
+  }
+}
+
+fn get_repr(attributes: &[Attribute]) -> Representation {
+  let mut repr = Representation { packed: None, repr: Repr::Rust };
+
+  for attr in attributes {
+    let meta = if let Ok(meta) = attr.parse_meta() { meta } else { continue };
+    if !meta.path().is_ident("repr") {
+      continue;
+    }
+    let list = if let Meta::List(list) = meta {
+      list
+    } else {
+      // The other `Meta` variants are illegal for `repr`.
+      continue;
+    };
+
+    for item in list.nested {
+      let meta = if let NestedMeta::Meta(meta) = item {
+        meta
+      } else {
+        // Other nested items are illegal for `repr`.
+        continue;
+      };
+
+      match meta.path() {
+        path if path.is_ident("C") => {
+          repr.repr = Repr::C;
+        }
+        path if path.is_ident("transparent") => {
+          repr.repr = Repr::Transparent;
+        }
+        path if path.is_ident("u8") => {
+          repr.repr = Repr::U8;
+        }
+        path if path.is_ident("i8") => {
+          repr.repr = Repr::I8;
+        }
+        path if path.is_ident("u16") => {
+          repr.repr = Repr::U16;
+        }
+        path if path.is_ident("i16") => {
+          repr.repr = Repr::I16;
+        }
+        path if path.is_ident("u32") => {
+          repr.repr = Repr::U32;
+        }
+        path if path.is_ident("i32") => {
+          repr.repr = Repr::I32;
+        }
+        path if path.is_ident("u64") => {
+          repr.repr = Repr::U64;
+        }
+        path if path.is_ident("i64") => {
+          repr.repr = Repr::I64;
+        }
+        path if path.is_ident("u128") => {
+          repr.repr = Repr::U128;
+        }
+        path if path.is_ident("i128") => {
+          repr.repr = Repr::I128;
+        }
+        path if path.is_ident("packed") => {
+          let packed_alignment = match meta {
+            Meta::Path(_) => 1,
+            Meta::List(list) => {
+              if list.nested.len() != 1 {
+                // `repr(packed(n))` must have exactly one nested item.
+                continue;
+              }
+
+              let nested = &list.nested[0];
+              let int_lit = if let NestedMeta::Lit(Lit::Int(int_lit)) = nested {
+                int_lit
+              } else {
+                // The nested item must be an integer literal.
+                continue;
+              };
+
+              let value = if let Ok(value) = int_lit.base10_parse::<u32>() {
+                value
+              } else {
+                // The literal must be positive and less than 2^29.
+                continue;
+              };
+              value
+            }
+            Meta::NameValue(_) => {
+              // `repr(packed)` doesn't support name value syntax.
+              continue;
+            }
+          };
+
+          let new_packed_alignment = match repr.packed {
+            Some(prev) => u32::min(prev, packed_alignment),
+            None => packed_alignment,
+          };
+          repr.packed = Some(new_packed_alignment);
+        }
+        _ => {}
+      }
+    }
+  }
+
+  repr
 }
 
 struct VariantDiscriminantIterator<'a, I: Iterator<Item = &'a Variant> + 'a> {

derive/tests/basic.rs

@@ -12,6 +12,38 @@ struct Test {
   b: u16,
 }
 
+#[derive(Pod, Zeroable)]
+#[repr(C, packed)]
+struct GenericPackedStruct<T: Pod> {
+  a: u32,
+  b: T,
+  c: u32,
+}
+
+impl<T: Pod> Clone for GenericPackedStruct<T> {
+  fn clone(&self) -> Self {
+    *self
+  }
+}
+
+impl<T: Pod> Copy for GenericPackedStruct<T> {}
+
+#[derive(Pod, Zeroable)]
+#[repr(C, packed(1))]
+struct GenericPackedStructExplicitPackedAlignment<T: Pod> {
+  a: u32,
+  b: T,
+  c: u32,
+}
+
+impl<T: Pod> Clone for GenericPackedStructExplicitPackedAlignment<T> {
+  fn clone(&self) -> Self {
+    *self
+  }
+}
+
+impl<T: Pod> Copy for GenericPackedStructExplicitPackedAlignment<T> {}
+
 #[derive(Zeroable)]
 struct ZeroGeneric<T: bytemuck::Zeroable> {
   a: T,