bytemuck_derive/src/traits.rs

#![allow(unused_imports)]
use proc_macro2::{Ident, Span, TokenStream, TokenTree};
use quote::{quote, quote_spanned, ToTokens};
use syn::{
  parse::{Parse, ParseStream, Parser},
  punctuated::Punctuated,
  spanned::Spanned,
  Result, *,
};

macro_rules! bail {
  ($msg:expr $(,)?) => {
    return Err(Error::new(Span::call_site(), &$msg[..]))
  };

  ( $msg:expr => $span_to_blame:expr $(,)? ) => {
    return Err(Error::new_spanned(&$span_to_blame, $msg))
  };
}

pub trait Derivable {
  fn ident(input: &DeriveInput) -> Result<syn::Path>;
  fn implies_trait() -> Option<TokenStream> {
    None
  }
  fn asserts(_input: &DeriveInput) -> Result<TokenStream> {
    Ok(quote!())
  }
  fn check_attributes(_ty: &Data, _attributes: &[Attribute]) -> Result<()> {
    Ok(())
  }
  fn trait_impl(_input: &DeriveInput) -> Result<(TokenStream, TokenStream)> {
    Ok((quote!(), quote!()))
  }
  fn requires_where_clause() -> bool {
    true
  }
  fn explicit_bounds_attribute_name() -> Option<&'static str> {
    None
  }
}

pub struct Pod;

impl Derivable for Pod {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::Pod))
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    let repr = get_repr(&input.attrs)?;

    let completly_packed =
      repr.packed == Some(1) || repr.repr == Repr::Transparent;

    if !completly_packed && !input.generics.params.is_empty() {
      bail!("\
        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\
      " => input.generics.params.first().unwrap());
    }

    match &input.data {
      Data::Struct(_) => {
        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(input)?)?;

        Ok(quote!(
          #assert_no_padding
          #assert_fields_are_pod
        ))
      }
      Data::Enum(_) => bail!("Deriving Pod is not supported for enums"),
      Data::Union(_) => bail!("Deriving Pod is not supported for unions"),
    }
  }

  fn check_attributes(_ty: &Data, attributes: &[Attribute]) -> Result<()> {
    let repr = get_repr(attributes)?;
    match repr.repr {
      Repr::C => Ok(()),
      Repr::Transparent => Ok(()),
      _ => {
        bail!("Pod requires the type to be #[repr(C)] or #[repr(transparent)]")
      }
    }
  }
}

pub struct AnyBitPattern;

impl Derivable for AnyBitPattern {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::AnyBitPattern))
  }

  fn implies_trait() -> Option<TokenStream> {
    Some(quote!(::bytemuck::Zeroable))
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    match &input.data {
      Data::Union(_) => Ok(quote!()), // unions are always `AnyBitPattern`
      Data::Struct(_) => generate_fields_are_trait(input, Self::ident(input)?),
      Data::Enum(_) => {
        bail!("Deriving AnyBitPattern is not supported for enums")
      }
    }
  }
}

pub struct Zeroable;

impl Derivable for Zeroable {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::Zeroable))
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    match &input.data {
      Data::Union(_) => Ok(quote!()), // unions are always `Zeroable`
      Data::Struct(_) => generate_fields_are_trait(input, Self::ident(input)?),
      Data::Enum(_) => bail!("Deriving Zeroable is not supported for enums"),
    }
  }

  fn explicit_bounds_attribute_name() -> Option<&'static str> {
    Some("zeroable")
  }
}

pub struct NoUninit;

impl Derivable for NoUninit {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::NoUninit))
  }

  fn check_attributes(ty: &Data, attributes: &[Attribute]) -> Result<()> {
    let repr = get_repr(attributes)?;
    match ty {
      Data::Struct(_) => match repr.repr {
        Repr::C | Repr::Transparent => Ok(()),
        _ => bail!("NoUninit requires the struct to be #[repr(C)] or #[repr(transparent)]"),
      },
      Data::Enum(_) => if repr.repr.is_integer() {
        Ok(())
      } else {
        bail!("NoUninit requires the enum to be an explicit #[repr(Int)]")
      },
      Data::Union(_) => bail!("NoUninit can only be derived on enums and structs")
    }
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    if !input.generics.params.is_empty() {
      bail!("NoUninit cannot be derived for structs containing generic parameters because the padding requirements can't be verified for generic structs");
    }

    match &input.data {
      Data::Struct(DataStruct { .. }) => {
        let assert_no_padding = generate_assert_no_padding(&input)?;
        let assert_fields_are_no_padding =
          generate_fields_are_trait(&input, Self::ident(input)?)?;

        Ok(quote!(
            #assert_no_padding
            #assert_fields_are_no_padding
        ))
      }
      Data::Enum(DataEnum { variants, .. }) => {
        if variants.iter().any(|variant| !variant.fields.is_empty()) {
          bail!("Only fieldless enums are supported for NoUninit")
        } else {
          Ok(quote!())
        }
      }
      Data::Union(_) => bail!("NoUninit cannot be derived for unions"), /* shouldn't be possible since we already error in attribute check for this case */
    }
  }

  fn trait_impl(_input: &DeriveInput) -> Result<(TokenStream, TokenStream)> {
    Ok((quote!(), quote!()))
  }
}

pub struct CheckedBitPattern;

impl Derivable for CheckedBitPattern {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::CheckedBitPattern))
  }

  fn check_attributes(ty: &Data, attributes: &[Attribute]) -> Result<()> {
    let repr = get_repr(attributes)?;
    match ty {
      Data::Struct(_) => match repr.repr {
        Repr::C | Repr::Transparent => Ok(()),
        _ => bail!("CheckedBitPattern derive requires the struct to be #[repr(C)] or #[repr(transparent)]"),
      },
      Data::Enum(_) => if repr.repr.is_integer() {
        Ok(())
      } else {
        bail!("CheckedBitPattern requires the enum to be an explicit #[repr(Int)]")
      },
      Data::Union(_) => bail!("CheckedBitPattern can only be derived on enums and structs")
    }
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    if !input.generics.params.is_empty() {
      bail!("CheckedBitPattern cannot be derived for structs containing generic parameters");
    }

    match &input.data {
      Data::Struct(DataStruct { .. }) => {
        let assert_fields_are_maybe_pod =
          generate_fields_are_trait(&input, Self::ident(input)?)?;

        Ok(assert_fields_are_maybe_pod)
      }
      Data::Enum(_) => Ok(quote!()), /* nothing needed, already guaranteed OK by NoUninit */
      Data::Union(_) => bail!("Internal error in CheckedBitPattern derive"), /* shouldn't be possible since we already error in attribute check for this case */
    }
  }

  fn trait_impl(input: &DeriveInput) -> Result<(TokenStream, TokenStream)> {
    match &input.data {
      Data::Struct(DataStruct { fields, .. }) => {
        generate_checked_bit_pattern_struct(&input.ident, fields, &input.attrs)
      }
      Data::Enum(_) => generate_checked_bit_pattern_enum(input),
      Data::Union(_) => bail!("Internal error in CheckedBitPattern derive"), /* shouldn't be possible since we already error in attribute check for this case */
    }
  }
}

pub struct TransparentWrapper;

impl TransparentWrapper {
  fn get_wrapper_type(
    attributes: &[Attribute], fields: &Fields,
  ) -> Option<TokenStream> {
    let transparent_param = get_simple_attr(attributes, "transparent");
    transparent_param.map(|ident| ident.to_token_stream()).or_else(|| {
      let mut types = get_field_types(&fields);
      let first_type = types.next();
      if let Some(_) = types.next() {
        // can't guess param type if there is more than one field
        return None;
      } else {
        first_type.map(|ty| ty.to_token_stream())
      }
    })
  }
}

impl Derivable for TransparentWrapper {
  fn ident(input: &DeriveInput) -> Result<syn::Path> {
    let fields = get_struct_fields(input)?;

    let ty = match Self::get_wrapper_type(&input.attrs, &fields) {
      Some(ty) => ty,
      None => bail!(
        "\
        when deriving TransparentWrapper for a struct with more than one field \
        you need to specify the transparent field using #[transparent(T)]\
      "
      ),
    };

    Ok(syn::parse_quote!(::bytemuck::TransparentWrapper<#ty>))
  }

  fn asserts(input: &DeriveInput) -> Result<TokenStream> {
    let (impl_generics, _ty_generics, where_clause) =
      input.generics.split_for_impl();
    let fields = get_struct_fields(input)?;
    let wrapped_type = match Self::get_wrapper_type(&input.attrs, &fields) {
      Some(wrapped_type) => wrapped_type.to_string(),
      None => unreachable!(), /* other code will already reject this derive */
    };
    let mut wrapped_field_ty = None;
    let mut nonwrapped_field_tys = vec![];
    for field in fields.iter() {
      let field_ty = &field.ty;
      if field_ty.to_token_stream().to_string() == wrapped_type {
        if wrapped_field_ty.is_some() {
          bail!(
            "TransparentWrapper can only have one field of the wrapped type"
          );
        }
        wrapped_field_ty = Some(field_ty);
      } else {
        nonwrapped_field_tys.push(field_ty);
      }
    }
    if let Some(wrapped_field_ty) = wrapped_field_ty {
      Ok(quote!(
        const _: () = {
          #[repr(transparent)]
          struct AssertWrappedIsWrapped #impl_generics((u8, ::core::marker::PhantomData<#wrapped_field_ty>), #(#nonwrapped_field_tys),*) #where_clause;
          fn assert_zeroable<Z: ::bytemuck::Zeroable>() {}
          fn check #impl_generics () #where_clause {
            #(
              assert_zeroable::<#nonwrapped_field_tys>();
            )*
          }
        };
      ))
    } else {
      bail!("TransparentWrapper must have one field of the wrapped type")
    }
  }

  fn check_attributes(_ty: &Data, attributes: &[Attribute]) -> Result<()> {
    let repr = get_repr(attributes)?;

    match repr.repr {
      Repr::Transparent => Ok(()),
      _ => {
        bail!(
          "TransparentWrapper requires the struct to be #[repr(transparent)]"
        )
      }
    }
  }

  fn requires_where_clause() -> bool {
    false
  }
}

pub struct Contiguous;

impl Derivable for Contiguous {
  fn ident(_: &DeriveInput) -> Result<syn::Path> {
    Ok(syn::parse_quote!(::bytemuck::Contiguous))
  }

  fn trait_impl(input: &DeriveInput) -> Result<(TokenStream, TokenStream)> {
    let repr = get_repr(&input.attrs)?;

    let integer_ty = if let Some(integer_ty) = repr.repr.as_integer_type() {
      integer_ty
    } else {
      bail!("Contiguous requires the enum to be #[repr(Int)]");
    };

    let variants = get_enum_variants(input)?;
    let mut variants_with_discriminator =
      VariantDiscriminantIterator::new(variants);

    let (min, max, count) = variants_with_discriminator.try_fold(
      (i64::max_value(), i64::min_value(), 0),
      |(min, max, count), res| {
        let discriminator = res?;
        Ok::<_, Error>((
          i64::min(min, discriminator),
          i64::max(max, discriminator),
          count + 1,
        ))
      },
    )?;

    if max - min != count - 1 {
      bail! {
        "Contiguous requires the enum discriminants to be contiguous",
      }
    }

    let min_lit = LitInt::new(&format!("{}", min), input.span());
    let max_lit = LitInt::new(&format!("{}", max), input.span());

    // `from_integer` and `into_integer` are usually provided by the trait's default implementation.
    // We override this implementation because it goes through `transmute_copy`, which can lead to
    // inefficient assembly as seen in https://github.com/Lokathor/bytemuck/issues/175 .

    Ok((
      quote!(),
      quote! {
          type Int = #integer_ty;
          const MIN_VALUE: #integer_ty = #min_lit;
          const MAX_VALUE: #integer_ty = #max_lit;

          #[inline]
          fn from_integer(value: Self::Int) -> Option<Self> {
            #[allow(clippy::manual_range_contains)]
            if Self::MIN_VALUE <= value && value <= Self::MAX_VALUE {
              Some(unsafe { ::core::mem::transmute(value) })
            } else {
              None
            }
          }

          #[inline]
          fn into_integer(self) -> Self::Int {
              self as #integer_ty
          }
      },
    ))
  }
}

fn get_struct_fields(input: &DeriveInput) -> Result<&Fields> {
  if let Data::Struct(DataStruct { fields, .. }) = &input.data {
    Ok(fields)
  } else {
    bail!("deriving this trait is only supported for structs")
  }
}

fn get_fields(input: &DeriveInput) -> Result<Fields> {
  match &input.data {
    Data::Struct(DataStruct { fields, .. }) => Ok(fields.clone()),
    Data::Union(DataUnion { fields, .. }) => Ok(Fields::Named(fields.clone())),
    Data::Enum(_) => bail!("deriving this trait is not supported for enums"),
  }
}

fn get_enum_variants<'a>(
  input: &'a DeriveInput,
) -> Result<impl Iterator<Item = &'a Variant> + 'a> {
  if let Data::Enum(DataEnum { variants, .. }) = &input.data {
    Ok(variants.iter())
  } else {
    bail!("deriving this trait is only supported for enums")
  }
}

fn get_field_types<'a>(
  fields: &'a Fields,
) -> impl Iterator<Item = &'a Type> + 'a {
  fields.iter().map(|field| &field.ty)
}

fn generate_checked_bit_pattern_struct(
  input_ident: &Ident, fields: &Fields, attrs: &[Attribute],
) -> Result<(TokenStream, TokenStream)> {
  let bits_ty = Ident::new(&format!("{}Bits", input_ident), input_ident.span());

  let repr = get_repr(attrs)?;

  let field_names = fields
    .iter()
    .enumerate()
    .map(|(i, field)| {
      field.ident.clone().unwrap_or_else(|| {
        Ident::new(&format!("field{}", i), input_ident.span())
      })
    })
    .collect::<Vec<_>>();
  let field_tys = fields.iter().map(|field| &field.ty).collect::<Vec<_>>();

  let field_name = &field_names[..];
  let field_ty = &field_tys[..];

  let derive_dbg =
    quote!(#[cfg_attr(not(target_arch = "spirv"), derive(Debug))]);

  Ok((
    quote! {
        #repr
        #[derive(Clone, Copy, ::bytemuck::AnyBitPattern)]
        #derive_dbg
        pub struct #bits_ty {
            #(#field_name: <#field_ty as ::bytemuck::CheckedBitPattern>::Bits,)*
        }
    },
    quote! {
        type Bits = #bits_ty;

        #[inline]
        #[allow(clippy::double_comparisons)]
        fn is_valid_bit_pattern(bits: &#bits_ty) -> bool {
            #(<#field_ty as ::bytemuck::CheckedBitPattern>::is_valid_bit_pattern(&{ bits.#field_name }) && )* true
        }
    },
  ))
}

fn generate_checked_bit_pattern_enum(
  input: &DeriveInput,
) -> Result<(TokenStream, TokenStream)> {
  let span = input.span();
  let mut variants_with_discriminant =
    VariantDiscriminantIterator::new(get_enum_variants(input)?);

  let (min, max, count) = variants_with_discriminant.try_fold(
    (i64::max_value(), i64::min_value(), 0),
    |(min, max, count), res| {
      let discriminant = res?;
      Ok::<_, Error>((
        i64::min(min, discriminant),
        i64::max(max, discriminant),
        count + 1,
      ))
    },
  )?;

  let check = if count == 0 {
    quote_spanned!(span => false)
  } else if max - min == count - 1 {
    // contiguous range
    let min_lit = LitInt::new(&format!("{}", min), span);
    let max_lit = LitInt::new(&format!("{}", max), span);

    quote!(*bits >= #min_lit && *bits <= #max_lit)
  } else {
    // not contiguous range, check for each
    let variant_lits =
      VariantDiscriminantIterator::new(get_enum_variants(input)?)
        .map(|res| {
          let variant = res?;
          Ok(LitInt::new(&format!("{}", variant), span))
        })
        .collect::<Result<Vec<_>>>()?;

    // count is at least 1
    let first = &variant_lits[0];
    let rest = &variant_lits[1..];

    quote!(matches!(*bits, #first #(| #rest )*))
  };

  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 = #integer_ty;

        #[inline]
        #[allow(clippy::double_comparisons)]
        fn is_valid_bit_pattern(bits: &Self::Bits) -> bool {
            #check
        }
    },
  ))
}

/// Check that a struct has no padding by asserting that the size of the struct
/// is equal to the sum of the size of it's fields
fn generate_assert_no_padding(input: &DeriveInput) -> Result<TokenStream> {
  let struct_type = &input.ident;
  let span = input.ident.span();
  let fields = get_fields(input)?;

  let mut field_types = get_field_types(&fields);
  let size_sum = if let Some(first) = field_types.next() {
    let size_first = quote_spanned!(span => ::core::mem::size_of::<#first>());
    let size_rest =
      quote_spanned!(span => #( + ::core::mem::size_of::<#field_types>() )*);

    quote_spanned!(span => #size_first #size_rest)
  } else {
    quote_spanned!(span => 0)
  };

  Ok(quote_spanned! {span => const _: fn() = || {
    #[doc(hidden)]
    struct TypeWithoutPadding([u8; #size_sum]);
    let _ = ::core::mem::transmute::<#struct_type, TypeWithoutPadding>;
  };})
}

/// Check that all fields implement a given trait
fn generate_fields_are_trait(
  input: &DeriveInput, trait_: syn::Path,
) -> Result<TokenStream> {
  let (impl_generics, _ty_generics, where_clause) =
    input.generics.split_for_impl();
  let fields = get_fields(input)?;
  let span = input.span();
  let field_types = get_field_types(&fields);
  Ok(quote_spanned! {span => #(const _: fn() = || {
      #[allow(clippy::missing_const_for_fn)]
      #[doc(hidden)]
      fn check #impl_generics () #where_clause {
        fn assert_impl<T: #trait_>() {}
        assert_impl::<#field_types>();
      }
    };)*
  })
}

fn get_ident_from_stream(tokens: TokenStream) -> Option<Ident> {
  match tokens.into_iter().next() {
    Some(TokenTree::Group(group)) => get_ident_from_stream(group.stream()),
    Some(TokenTree::Ident(ident)) => Some(ident),
    _ => None,
  }
}

/// get a simple #[foo(bar)] attribute, returning "bar"
fn get_simple_attr(attributes: &[Attribute], attr_name: &str) -> Option<Ident> {
  for attr in attributes {
    if let (AttrStyle::Outer, Meta::List(list)) = (&attr.style, &attr.meta) {
      if list.path.is_ident(attr_name) {
        if let Some(ident) = get_ident_from_stream(list.tokens.clone()) {
          return Some(ident);
        }
      }
    }
  }

  None
}

fn get_repr(attributes: &[Attribute]) -> Result<Representation> {
  attributes
    .iter()
    .filter_map(|attr| {
      if attr.path().is_ident("repr") {
        Some(attr.parse_args::<Representation>())
      } else {
        None
      }
    })
    .try_fold(Representation::default(), |a, b| {
      let b = b?;
      Ok(Representation {
        repr: match (a.repr, b.repr) {
          (a, Repr::Rust) => a,
          (Repr::Rust, b) => b,
          _ => bail!("conflicting representation hints"),
        },
        packed: match (a.packed, b.packed) {
          (a, None) => a,
          (None, b) => b,
          _ => bail!("conflicting representation hints"),
        },
        align: match (a.align, b.align) {
          (a, None) => a,
          (None, b) => b,
          _ => bail!("conflicting representation hints"),
        },
      })
    })
}

mk_repr! {
  U8 => u8,
  I8 => i8,
  U16 => u16,
  I16 => i16,
  U32 => u32,
  I32 => i32,
  U64 => u64,
  I64 => i64,
  I128 => i128,
  U128 => u128,
  Usize => usize,
  Isize => isize,
}
// where
macro_rules! mk_repr {(
  $(
    $Xn:ident => $xn:ident
  ),* $(,)?
) => (
  #[derive(Clone, Copy, PartialEq)]
  enum Repr {
    Rust,
    C,
    Transparent,
    $($Xn),*
  }

  impl Repr {
    fn is_integer(self) -> bool {
      match self {
        Repr::Rust | Repr::C | Repr::Transparent => false,
        _ => true,
      }
    }

    fn as_integer_type(self) -> Option<TokenStream> {
      match self {
        Repr::Rust | Repr::C | Repr::Transparent => None,
        $(
          Repr::$Xn => Some(quote! { ::core::primitive::$xn }),
        )*
      }
    }
  }

  #[derive(Clone, Copy)]
  struct Representation {
    packed: Option<u32>,
    align: Option<u32>,
    repr: Repr,
  }

  impl Default for Representation {
    fn default() -> Self {
      Self { packed: None, align: None, repr: Repr::Rust }
    }
  }

  impl Parse for Representation {
    fn parse(input: ParseStream<'_>) -> Result<Representation> {
      let mut ret = Representation::default();
      while !input.is_empty() {
        let keyword = input.parse::<Ident>()?;
        // preëmptively call `.to_string()` *once* (rather than on `is_ident()`)
        let keyword_str = keyword.to_string();
        let new_repr = match keyword_str.as_str() {
          "C" => Repr::C,
          "transparent" => Repr::Transparent,
          "packed" => {
            ret.packed = Some(if input.peek(token::Paren) {
              let contents; parenthesized!(contents in input);
              LitInt::base10_parse::<u32>(&contents.parse()?)?
            } else {
              1
            });
            let _: Option<Token![,]> = input.parse()?;
            continue;
          },
          "align" => {
            let contents; parenthesized!(contents in input);
            ret.align = Some(LitInt::base10_parse::<u32>(&contents.parse()?)?);
            let _: Option<Token![,]> = input.parse()?;
            continue;
          },
        $(
          stringify!($xn) => Repr::$Xn,
        )*
          _ => return Err(input.error("unrecognized representation hint"))
        };
        if ::core::mem::replace(&mut ret.repr, new_repr) != Repr::Rust {
          input.error("duplicate representation hint");
        }
        let _: Option<Token![,]> = input.parse()?;
      }
      Ok(ret)
    }
  }

  impl ToTokens for Representation {
    fn to_tokens(&self, tokens: &mut TokenStream) {
      let repr = match self.repr {
        Repr::Rust => None,
        Repr::C => Some(quote!(C)),
        Repr::Transparent => Some(quote!(transparent)),
        $(
          Repr::$Xn => Some(quote!($xn)),
        )*
      };
      let packed = self.packed.map(|p| {
        let lit = LitInt::new(&p.to_string(), Span::call_site());
        quote!(packed(#lit))
      });
      let comma = if packed.is_some() && repr.is_some() {
        Some(quote!(,))
      } else {
        None
      };
      tokens.extend(quote!(
        #[repr( #repr #comma #packed )]
      ));
    }
  }
)}
use mk_repr;

struct VariantDiscriminantIterator<'a, I: Iterator<Item = &'a Variant> + 'a> {
  inner: I,
  last_value: i64,
}

impl<'a, I: Iterator<Item = &'a Variant> + 'a>
  VariantDiscriminantIterator<'a, I>
{
  fn new(inner: I) -> Self {
    VariantDiscriminantIterator { inner, last_value: -1 }
  }
}

impl<'a, I: Iterator<Item = &'a Variant> + 'a> Iterator
  for VariantDiscriminantIterator<'a, I>
{
  type Item = Result<i64>;

  fn next(&mut self) -> Option<Self::Item> {
    let variant = self.inner.next()?;
    if !variant.fields.is_empty() {
      return Some(Err(Error::new_spanned(
        &variant.fields,
        "Only fieldless enums are supported",
      )));
    }

    if let Some((_, discriminant)) = &variant.discriminant {
      let discriminant_value = match parse_int_expr(discriminant) {
        Ok(value) => value,
        Err(e) => return Some(Err(e)),
      };
      self.last_value = discriminant_value;
    } else {
      self.last_value += 1;
    }

    Some(Ok(self.last_value))
  }
}

fn parse_int_expr(expr: &Expr) -> Result<i64> {
  match expr {
    Expr::Unary(ExprUnary { op: UnOp::Neg(_), expr, .. }) => {
      parse_int_expr(expr).map(|int| -int)
    }
    Expr::Lit(ExprLit { lit: Lit::Int(int), .. }) => int.base10_parse(),
    Expr::Lit(ExprLit { lit: Lit::Byte(byte), .. }) => Ok(byte.value().into()),
    _ => bail!("Not an integer expression"),
  }
}