denoland/deno · error · syn::Error

cppgc inheritance requires #[repr(C)] on the type

Error message

cppgc inheritance requires #[repr(C)] on the type

What it means

The CppgcInherits derive emits compile-time offset assertions that rely on a stable C layout, so the target type must be #[repr(C)]. ensure_repr_c scans the type's attributes and only accepts a bare #[repr(C)] or a repr list containing C (e.g. #[repr(C, packed)]); anything else — no repr, #[repr(u8)], #[repr(Rust)] — yields this compile error.

Source

Thrown at libs/ops/cppgc.rs:161

}

fn ensure_repr_c(attrs: &[Attribute], span: proc_macro2::Span) -> Result<()> {
  for attr in attrs {
    if !attr.path().is_ident("repr") {
      continue;
    }
    if let Meta::List(list) = &attr.meta {
      let nested: Punctuated<Meta, Token![,]> =
        list.parse_args_with(Punctuated::parse_terminated)?;
      if nested
        .iter()
        .any(|meta| matches!(meta, Meta::Path(path) if path.is_ident("C")))
      {
        return Ok(());
      }
    }
  }
  Err(Error::new(
    span,
    "cppgc inheritance requires #[repr(C)] on the type",
  ))
}

fn parse_base_attr(attrs: &[Attribute]) -> Result<Type> {
  let mut found = None;
  for attr in attrs {
    if !attr.path().is_ident("cppgc_inherits_from") {
      continue;
    }
    if found.is_some() {
      return Err(Error::new(
        attr.span(),
        "cppgc_inherits_from specified more than once",
      ));
    }
    let base = attr.parse_args::<Type>()?;

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Add #[repr(C)] directly above the struct: #[derive(CppgcInherits)] #[cppgc_inherits_from(Base)] #[repr(C)] struct Derived { base: Base, ... }.
2. In multi-level chains, verify every type in the chain (base and derived) carries #[repr(C)], as the offset assertions walk the whole chain.

Example fix

// before
#[derive(CppgcInherits)]
#[cppgc_inherits_from(Shape)]
pub struct Rectangle { // error: cppgc inheritance requires #[repr(C)] on the type
  base: Shape,
}

// after
#[derive(CppgcInherits)]
#[cppgc_inherits_from(Shape)]
#[repr(C)]
pub struct Rectangle {
  base: Shape,
}

When it happens

Trigger: Declaring a derived cppgc class without a repr attribute, or with a non-C repr like #[repr(u8)] or #[repr(transparent)], under #[derive(CppgcInherits)] #[cppgc_inherits_from(...)].

Common situations: Converting an existing repr-Rust struct into a cppgc base/derived class and forgetting the layout attribute; enum-like reprs copy-pasted onto the struct; multi-level inheritance where an intermediate type was never marked.

Related errors

• cppgc inheritance requires at least one field
• first field must be the base type for cppgc inheritance
• FromV8 enum derive currently supports only unit and single-e
• Unions are not supported
• Unit fields are currently not supported

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/a59514ff136aac74. Report an issue: GitHub.