ref-cast-impl 1.0.0

extern crate proc_macro;

use proc_macro::TokenStream;
use proc_macro2::{Span, TokenStream as TokenStream2};
use quote::quote;
use syn::parse::Nothing;
use syn::punctuated::Punctuated;
use syn::{
    parse_macro_input, Data, DeriveInput, Error, Field, Meta, NestedMeta, Result, Token, Type,
};

#[proc_macro_derive(RefCast, attributes(trivial))]
pub fn derive_ref_cast(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    expand(input)
        .unwrap_or_else(|err| err.to_compile_error())
        .into()
}

fn expand(input: DeriveInput) -> Result<TokenStream2> {
    if !has_repr_c(&input) {
        return Err(Error::new(
            Span::call_site(),
            "RefCast trait requires #[repr(C)] or #[repr(transparent)]",
        ));
    }

    let name = &input.ident;
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

    let fields = fields(&input)?;
    let from = only_field_ty(fields)?;
    let trivial = trivial_fields(fields)?;

    let assert_trivial_fields = if !trivial.is_empty() {
        Some(quote! {
            if false {
                #(
                    ::ref_cast::private::assert_trivial::<#trivial>();
                )*
            }
        })
    } else {
        None
    };

    Ok(quote! {
        impl #impl_generics ::ref_cast::RefCast for #name #ty_generics #where_clause {
            type From = #from;

            #[inline]
            fn ref_cast(_from: &Self::From) -> &Self {
                // TODO: assert that `Self::From` and `Self` have the same size
                // and alignment.
                //
                // Cannot do this because `Self::From` may be a generic type
                // parameter of `Self` where `transmute` is not allowed:
                //
                //     #[allow(unused)]
                //     unsafe fn assert_same_size #impl_generics #where_clause () {
                //         _core::mem::forget(
                //             _core::mem::transmute::<#from, #name #ty_generics>(
                //                 _core::mem::uninitialized()));
                //     }
                //
                // Cannot do this because `Self::From` may not be sized:
                //
                //     debug_assert_eq!(_core::mem::size_of::<Self::From>(),
                //                      _core::mem::size_of::<Self>());
                //     debug_assert_eq!(_core::mem::align_of::<Self::From>(),
                //                      _core::mem::align_of::<Self>());

                #assert_trivial_fields
                unsafe {
                    &*(_from as *const Self::From as *const Self)
                }
            }

            #[inline]
            fn ref_cast_mut(_from: &mut Self::From) -> &mut Self {
                unsafe {
                    &mut *(_from as *mut Self::From as *mut Self)
                }
            }
        }
    })
}

fn has_repr_c(input: &DeriveInput) -> bool {
    for attr in &input.attrs {
        if let Ok(Meta::List(meta)) = attr.parse_meta() {
            if meta.path.is_ident("repr") && meta.nested.len() == 1 {
                if let NestedMeta::Meta(Meta::Path(path)) = &meta.nested[0] {
                    if path.is_ident("C") || path.is_ident("transparent") {
                        return true;
                    }
                }
            }
        }
    }
    false
}

type Fields = Punctuated<Field, Token![,]>;

fn fields(input: &DeriveInput) -> Result<&Fields> {
    use syn::Fields;

    match &input.data {
        Data::Struct(data) => match &data.fields {
            Fields::Named(fields) => Ok(&fields.named),
            Fields::Unnamed(fields) => Ok(&fields.unnamed),
            Fields::Unit => Err(Error::new(
                Span::call_site(),
                "RefCast does not support unit structs",
            )),
        },
        Data::Enum(_) => Err(Error::new(
            Span::call_site(),
            "RefCast does not support enums",
        )),
        Data::Union(_) => Err(Error::new(
            Span::call_site(),
            "RefCast does not support unions",
        )),
    }
}

fn only_field_ty(fields: &Fields) -> Result<&Type> {
    let is_trivial = decide_trivial(fields)?;
    let mut only_field = None;

    for field in fields {
        if !is_trivial(field)? {
            if only_field.take().is_some() {
                break;
            }
            only_field = Some(&field.ty);
        }
    }

    only_field.ok_or_else(|| {
        Error::new(
            Span::call_site(),
            "RefCast requires a struct with a single field",
        )
    })
}

fn trivial_fields(fields: &Fields) -> Result<Vec<&Type>> {
    let is_trivial = decide_trivial(fields)?;
    let mut trivial = Vec::new();

    for field in fields {
        if is_trivial(field)? {
            trivial.push(&field.ty);
        }
    }

    Ok(trivial)
}

fn decide_trivial(fields: &Fields) -> Result<fn(&Field) -> Result<bool>> {
    for field in fields {
        if is_explicit_trivial(field)? {
            return Ok(is_explicit_trivial);
        }
    }
    Ok(is_implicit_trivial)
}

fn is_implicit_trivial(field: &Field) -> Result<bool> {
    match &field.ty {
        Type::Tuple(ty) => Ok(ty.elems.is_empty()),
        Type::Path(ty) => Ok(ty.path.segments.last().unwrap().ident == "PhantomData"),
        _ => Ok(false),
    }
}

fn is_explicit_trivial(field: &Field) -> Result<bool> {
    for attr in &field.attrs {
        if attr.path.is_ident("trivial") {
            syn::parse2::<Nothing>(attr.tokens.clone())?;
            return Ok(true);
        }
    }
    Ok(false)
}