crates/uniffi_macros/src/enum_.rs

use proc_macro2::{Ident, Span, TokenStream};
use quote::quote;
use syn::{
    parse::{Parse, ParseStream},
    spanned::Spanned,
    Attribute, Data, DataEnum, DeriveInput, Expr, Index, Lit, Variant,
};

use crate::util::{
    create_metadata_items, derive_all_ffi_traits, either_attribute_arg, extract_docstring,
    ident_to_string, kw, mod_path, parse_comma_separated, tagged_impl_header,
    try_metadata_value_from_usize, try_read_field, AttributeSliceExt, UniffiAttributeArgs,
};

fn extract_repr(attrs: &[Attribute]) -> syn::Result<Option<Ident>> {
    let mut result = None;
    for attr in attrs {
        if attr.path().is_ident("repr") {
            attr.parse_nested_meta(|meta| {
                result = match meta.path.get_ident() {
                    Some(i) => {
                        let s = i.to_string();
                        match s.as_str() {
                            "u8" | "u16" | "u32" | "u64" | "usize" | "i8" | "i16" | "i32"
                            | "i64" | "isize" => Some(i.clone()),
                            // while the default repr for an enum is `isize` we don't apply that default here.
                            _ => None,
                        }
                    }
                    _ => None,
                };
                Ok(())
            })?
        }
    }
    Ok(result)
}

pub fn expand_enum(
    input: DeriveInput,
    // Attributes from #[derive_error_for_udl()], if we are in udl mode
    attr_from_udl_mode: Option<EnumAttr>,
    udl_mode: bool,
) -> syn::Result<TokenStream> {
    let enum_ = match input.data {
        Data::Enum(e) => e,
        _ => {
            return Err(syn::Error::new(
                Span::call_site(),
                "This derive must only be used on enums",
            ))
        }
    };
    let ident = &input.ident;
    let docstring = extract_docstring(&input.attrs)?;
    let discr_type = extract_repr(&input.attrs)?;
    let mut attr: EnumAttr = input.attrs.parse_uniffi_attr_args()?;
    if let Some(attr_from_udl_mode) = attr_from_udl_mode {
        attr = attr.merge(attr_from_udl_mode)?;
    }
    let ffi_converter_impl = enum_ffi_converter_impl(ident, &enum_, udl_mode, &attr);

    let meta_static_var = (!udl_mode).then(|| {
        enum_meta_static_var(ident, docstring, discr_type, &enum_, &attr)
            .unwrap_or_else(syn::Error::into_compile_error)
    });

    Ok(quote! {
        #ffi_converter_impl
        #meta_static_var
    })
}

pub(crate) fn enum_ffi_converter_impl(
    ident: &Ident,
    enum_: &DataEnum,
    udl_mode: bool,
    attr: &EnumAttr,
) -> TokenStream {
    enum_or_error_ffi_converter_impl(
        ident,
        enum_,
        udl_mode,
        attr,
        quote! { ::uniffi::metadata::codes::TYPE_ENUM },
    )
}

pub(crate) fn rich_error_ffi_converter_impl(
    ident: &Ident,
    enum_: &DataEnum,
    udl_mode: bool,
    attr: &EnumAttr,
) -> TokenStream {
    enum_or_error_ffi_converter_impl(
        ident,
        enum_,
        udl_mode,
        attr,
        quote! { ::uniffi::metadata::codes::TYPE_ENUM },
    )
}

fn enum_or_error_ffi_converter_impl(
    ident: &Ident,
    enum_: &DataEnum,
    udl_mode: bool,
    attr: &EnumAttr,
    metadata_type_code: TokenStream,
) -> TokenStream {
    let name = ident_to_string(ident);
    let impl_spec = tagged_impl_header("FfiConverter", ident, udl_mode);
    let derive_ffi_traits = derive_all_ffi_traits(ident, udl_mode);
    let mod_path = match mod_path() {
        Ok(p) => p,
        Err(e) => return e.into_compile_error(),
    };
    let mut write_match_arms: Vec<_> = enum_
        .variants
        .iter()
        .enumerate()
        .map(|(i, v)| {
            let v_ident = &v.ident;
            let field_idents = v
                .fields
                .iter()
                .enumerate()
                .map(|(i, f)| {
                    f.ident
                        .clone()
                        .unwrap_or_else(|| Ident::new(&format!("e{i}"), f.span()))
                })
                .collect::<Vec<Ident>>();
            let idx = Index::from(i + 1);
            let write_fields =
                std::iter::zip(v.fields.iter(), field_idents.iter()).map(|(f, ident)| {
                    let ty = &f.ty;
                    quote! {
                        <#ty as ::uniffi::Lower<crate::UniFfiTag>>::write(#ident, buf);
                    }
                });
            let is_tuple = v.fields.iter().any(|f| f.ident.is_none());
            let fields = if is_tuple {
                quote! { ( #(#field_idents),* ) }
            } else {
                quote! { { #(#field_idents),* } }
            };

            quote! {
                Self::#v_ident #fields => {
                    ::uniffi::deps::bytes::BufMut::put_i32(buf, #idx);
                    #(#write_fields)*
                }
            }
        })
        .collect();
    if attr.non_exhaustive.is_some() {
        write_match_arms.push(quote! {
            _ => panic!("Unexpected variant in non-exhaustive enum"),
        })
    }
    let write_impl = quote! {
        match obj { #(#write_match_arms)* }
    };

    let try_read_match_arms = enum_.variants.iter().enumerate().map(|(i, v)| {
        let idx = Index::from(i + 1);
        let v_ident = &v.ident;
        let is_tuple = v.fields.iter().any(|f| f.ident.is_none());
        let try_read_fields = v.fields.iter().map(try_read_field);

        if is_tuple {
            quote! {
                #idx => Self::#v_ident ( #(#try_read_fields)* ),
            }
        } else {
            quote! {
                #idx => Self::#v_ident { #(#try_read_fields)* },
            }
        }
    });
    let error_format_string = format!("Invalid {ident} enum value: {{}}");
    let try_read_impl = quote! {
        ::uniffi::check_remaining(buf, 4)?;

        Ok(match ::uniffi::deps::bytes::Buf::get_i32(buf) {
            #(#try_read_match_arms)*
            v => ::uniffi::deps::anyhow::bail!(#error_format_string, v),
        })
    };

    quote! {
        #[automatically_derived]
        unsafe #impl_spec {
            ::uniffi::ffi_converter_rust_buffer_lift_and_lower!(crate::UniFfiTag);

            fn write(obj: Self, buf: &mut ::std::vec::Vec<u8>) {
                #write_impl
            }

            fn try_read(buf: &mut &[::std::primitive::u8]) -> ::uniffi::deps::anyhow::Result<Self> {
                #try_read_impl
            }

            const TYPE_ID_META: ::uniffi::MetadataBuffer = ::uniffi::MetadataBuffer::from_code(#metadata_type_code)
                .concat_str(#mod_path)
                .concat_str(#name);
        }

        #derive_ffi_traits
    }
}

pub(crate) fn enum_meta_static_var(
    ident: &Ident,
    docstring: String,
    discr_type: Option<Ident>,
    enum_: &DataEnum,
    attr: &EnumAttr,
) -> syn::Result<TokenStream> {
    let name = ident_to_string(ident);
    let module_path = mod_path()?;
    let non_exhaustive = attr.non_exhaustive.is_some();

    let mut metadata_expr = quote! {
        ::uniffi::MetadataBuffer::from_code(::uniffi::metadata::codes::ENUM)
            .concat_str(#module_path)
            .concat_str(#name)
            .concat_option_bool(None) // forced_flatness
    };
    metadata_expr.extend(match discr_type {
        None => quote! { .concat_bool(false) },
        Some(t) => quote! { .concat_bool(true).concat(<#t as ::uniffi::Lower<crate::UniFfiTag>>::TYPE_ID_META) }
    });
    metadata_expr.extend(variant_metadata(enum_)?);
    metadata_expr.extend(quote! {
        .concat_bool(#non_exhaustive)
        .concat_long_str(#docstring)
    });
    Ok(create_metadata_items("enum", &name, metadata_expr, None))
}

fn variant_value(v: &Variant) -> syn::Result<TokenStream> {
    let Some((_, e)) = &v.discriminant else {
        return Ok(quote! { .concat_bool(false) });
    };
    // Attempting to expose an enum value which we don't understand is a hard-error
    // rather than silently ignoring it. If we had the ability to emit a warning that
    // might make more sense.

    // We can't sanely handle most expressions other than literals, but we can handle
    // negative literals.
    let mut negate = false;
    let lit = match e {
        Expr::Lit(lit) => lit,
        Expr::Unary(expr_unary) if matches!(expr_unary.op, syn::UnOp::Neg(_)) => {
            negate = true;
            match *expr_unary.expr {
                Expr::Lit(ref lit) => lit,
                _ => {
                    return Err(syn::Error::new_spanned(
                        e,
                        "UniFFI disciminant values must be a literal",
                    ));
                }
            }
        }
        _ => {
            return Err(syn::Error::new_spanned(
                e,
                "UniFFI disciminant values must be a literal",
            ));
        }
    };
    let Lit::Int(ref intlit) = lit.lit else {
        return Err(syn::Error::new_spanned(
            v,
            "UniFFI disciminant values must be a literal integer",
        ));
    };
    if !intlit.suffix().is_empty() {
        return Err(syn::Error::new_spanned(
            intlit,
            "integer literals with suffix not supported by UniFFI here",
        ));
    }
    let digits = if negate {
        format!("-{}", intlit.base10_digits())
    } else {
        intlit.base10_digits().to_string()
    };
    Ok(quote! {
        .concat_bool(true)
        .concat_value(::uniffi::metadata::codes::LIT_INT)
        .concat_str(#digits)
    })
}

pub fn variant_metadata(enum_: &DataEnum) -> syn::Result<Vec<TokenStream>> {
    let variants_len =
        try_metadata_value_from_usize(enum_.variants.len(), "UniFFI limits enums to 256 variants")?;
    std::iter::once(Ok(quote! { .concat_value(#variants_len) }))
        .chain(enum_.variants.iter().map(|v| {
            let fields_len = try_metadata_value_from_usize(
                v.fields.len(),
                "UniFFI limits enum variants to 256 fields",
            )?;

            let field_names = v
                .fields
                .iter()
                .map(|f| f.ident.as_ref().map(ident_to_string).unwrap_or_default())
                .collect::<Vec<_>>();

            let name = ident_to_string(&v.ident);
            let value_tokens = variant_value(v)?;
            let docstring = extract_docstring(&v.attrs)?;
            let field_types = v.fields.iter().map(|f| &f.ty);
            let field_docstrings = v
                .fields
                .iter()
                .map(|f| extract_docstring(&f.attrs))
                .collect::<syn::Result<Vec<_>>>()?;

            Ok(quote! {
                .concat_str(#name)
                #value_tokens
                .concat_value(#fields_len)
                    #(
                        .concat_str(#field_names)
                        .concat(<#field_types as ::uniffi::Lower<crate::UniFfiTag>>::TYPE_ID_META)
                        // field defaults not yet supported for enums
                        .concat_bool(false)
                        .concat_long_str(#field_docstrings)
                    )*
                .concat_long_str(#docstring)
            })
        }))
        .collect()
}

#[derive(Default)]
pub struct EnumAttr {
    pub non_exhaustive: Option<kw::non_exhaustive>,
}

// So ErrorAttr can be used with `parse_macro_input!`
impl Parse for EnumAttr {
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        parse_comma_separated(input)
    }
}

impl UniffiAttributeArgs for EnumAttr {
    fn parse_one(input: ParseStream<'_>) -> syn::Result<Self> {
        let lookahead = input.lookahead1();
        if lookahead.peek(kw::non_exhaustive) {
            Ok(Self {
                non_exhaustive: input.parse()?,
            })
        } else {
            Err(lookahead.error())
        }
    }

    fn merge(self, other: Self) -> syn::Result<Self> {
        Ok(Self {
            non_exhaustive: either_attribute_arg(self.non_exhaustive, other.non_exhaustive)?,
        })
    }
}