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/// A type for which the layout is determined by its C++ definition.
///
/// This trait serves the following two related purposes.
///
/// <br>
///
/// ## Safely unifying occurrences of the same extern type
///
/// `ExternType` makes it possible for CXX to safely share a consistent Rust
/// type across multiple #\[cxx::bridge\] invocations that refer to a common
/// extern C++ type.
///
/// In the following snippet, two #\[cxx::bridge\] invocations in different
/// files (possibly different crates) both contain function signatures involving
/// the same C++ type `example::Demo`. If both were written just containing
/// `type Demo;`, then both macro expansions would produce their own separate
/// Rust type called `Demo` and thus the compiler wouldn't allow us to take the
/// `Demo` returned by `file1::ffi::create_demo` and pass it as the `Demo`
/// argument accepted by `file2::ffi::take_ref_demo`. Instead, one of the two
/// `Demo`s has been defined as an extern type alias of the other, making them
/// the same type in Rust. The CXX code generator will use an automatically
/// generated `ExternType` impl emitted in file1 to statically verify that in
/// file2 `crate::file1::ffi::Demo` really does refer to the C++ type
/// `example::Demo` as expected in file2.
///
/// ```no_run
/// // file1.rs
/// # mod file1 {
/// #[cxx::bridge(namespace = example)]
/// pub mod ffi {
///     extern "C" {
///         type Demo;
///
///         fn create_demo() -> UniquePtr<Demo>;
///     }
/// }
/// # }
///
/// // file2.rs
/// #[cxx::bridge(namespace = example)]
/// pub mod ffi {
///     extern "C" {
///         type Demo = crate::file1::ffi::Demo;
///
///         fn take_ref_demo(demo: &Demo);
///     }
/// }
/// #
/// # fn main() {}
/// ```
///
/// <br><br>
///
/// ## Integrating with bindgen-generated types
///
/// Handwritten `ExternType` impls make it possible to plug in a data structure
/// emitted by bindgen as the definition of an opaque C++ type emitted by CXX.
///
/// By writing the unsafe `ExternType` impl, the programmer asserts that the C++
/// namespace and type name given in the type id refers to a C++ type that is
/// equivalent to Rust type that is the `Self` type of the impl.
///
/// ```no_run
/// # const _: &str = stringify! {
/// mod folly_sys;  // the bindgen-generated bindings
/// # };
/// # mod folly_sys {
/// #     #[repr(transparent)]
/// #     pub struct StringPiece([usize; 2]);
/// # }
///
/// use cxx::{type_id, ExternType};
///
/// unsafe impl ExternType for folly_sys::StringPiece {
///     type Id = type_id!("folly::StringPiece");
/// }
///
/// #[cxx::bridge(namespace = folly)]
/// pub mod ffi {
///     extern "C" {
///         include!("rust_cxx_bindings.h");
///
///         type StringPiece = crate::folly_sys::StringPiece;
///
///         fn print_string_piece(s: &StringPiece);
///     }
/// }
///
/// // Now if we construct a StringPiece or obtain one through one
/// // of the bindgen-generated signatures, we are able to pass it
/// // along to ffi::print_string_piece.
/// #
/// # fn main() {}
/// ```
pub unsafe trait ExternType {
    /// A type-level representation of the type's C++ namespace and type name.
    ///
    /// This will always be defined using `type_id!` in the following form:
    ///
    /// ```
    /// # struct TypeName;
    /// # unsafe impl cxx::ExternType for TypeName {
    /// type Id = cxx::type_id!("name::space::of::TypeName");
    /// # }
    /// ```
    type Id;
}

#[doc(hidden)]
pub fn verify_extern_type<T: ExternType<Id = Id>, Id>() {}