crates/uniffi_macros/src/error.rs

use proc_macro2::{Ident, Span, TokenStream};
use quote::quote;
use syn::{
    parse::{Parse, ParseStream},
    Data, DataEnum, DeriveInput, Index,
};

use crate::{
    enum_::{rich_error_ffi_converter_impl, variant_metadata, EnumAttr},
    util::{
        chain, create_metadata_items, derive_ffi_traits, either_attribute_arg, extract_docstring,
        ident_to_string, kw, mod_path, parse_comma_separated, tagged_impl_header,
        try_metadata_value_from_usize, AttributeSliceExt, UniffiAttributeArgs,
    },
};

pub fn expand_error(
    input: DeriveInput,
    // Attributes from #[derive_error_for_udl()], if we are in udl mode
    attr_from_udl_mode: Option<ErrorAttr>,
    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 currently only supports enums",
            ));
        }
    };
    let ident = &input.ident;
    let docstring = extract_docstring(&input.attrs)?;
    let mut attr: ErrorAttr = 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 = error_ffi_converter_impl(ident, &enum_, &attr, udl_mode)?;
    let meta_static_var = (!udl_mode).then(|| {
        error_meta_static_var(ident, docstring, &enum_, &attr)
            .unwrap_or_else(syn::Error::into_compile_error)
    });

    let variant_errors: TokenStream = enum_
        .variants
        .iter()
        .flat_map(|variant| {
            chain(
                variant.attrs.uniffi_attr_args_not_allowed_here(),
                variant
                    .fields
                    .iter()
                    .flat_map(|field| field.attrs.uniffi_attr_args_not_allowed_here()),
            )
        })
        .map(syn::Error::into_compile_error)
        .collect();

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

fn error_ffi_converter_impl(
    ident: &Ident,
    enum_: &DataEnum,
    attr: &ErrorAttr,
    udl_mode: bool,
) -> syn::Result<TokenStream> {
    Ok(if attr.flat.is_some() {
        flat_error_ffi_converter_impl(ident, enum_, udl_mode, attr)
    } else {
        rich_error_ffi_converter_impl(ident, enum_, udl_mode, &attr.clone().try_into()?)
    })
}

// FfiConverters for "flat errors"
//
// These are errors where we only lower the to_string() value, rather than any associated data.
// We lower the to_string() value unconditionally, whether the enum has associated data or not.
fn flat_error_ffi_converter_impl(
    ident: &Ident,
    enum_: &DataEnum,
    udl_mode: bool,
    attr: &ErrorAttr,
) -> TokenStream {
    let name = ident_to_string(ident);
    let lower_impl_spec = tagged_impl_header("Lower", ident, udl_mode);
    let lift_impl_spec = tagged_impl_header("Lift", ident, udl_mode);
    let derive_ffi_traits = derive_ffi_traits(ident, udl_mode, &["ConvertError"]);
    let mod_path = match mod_path() {
        Ok(p) => p,
        Err(e) => return e.into_compile_error(),
    };

    let lower_impl = {
        let mut match_arms: Vec<_> = enum_.variants.iter().enumerate().map(|(i, v)| {
            let v_ident = &v.ident;
            let idx = Index::from(i + 1);

            quote! {
                Self::#v_ident { .. } => {
                    ::uniffi::deps::bytes::BufMut::put_i32(buf, #idx);
                    <::std::string::String as ::uniffi::Lower<crate::UniFfiTag>>::write(error_msg, buf);
                }
            }
        }).collect();
        if attr.non_exhaustive.is_some() {
            match_arms.push(quote! {
                _ => panic!("Unexpected variant in non-exhaustive enum"),
            })
        }

        quote! {
            #[automatically_derived]
            unsafe #lower_impl_spec {
                type FfiType = ::uniffi::RustBuffer;

                fn write(obj: Self, buf: &mut ::std::vec::Vec<u8>) {
                    let error_msg = ::std::string::ToString::to_string(&obj);
                    match obj { #(#match_arms)* }
                }

                fn lower(obj: Self) -> ::uniffi::RustBuffer {
                    <Self as ::uniffi::Lower<crate::UniFfiTag>>::lower_into_rust_buffer(obj)
                }

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

    let lift_impl = if attr.with_try_read.is_some() {
        let match_arms = enum_.variants.iter().enumerate().map(|(i, v)| {
            let v_ident = &v.ident;
            let idx = Index::from(i + 1);

            quote! {
                #idx => Self::#v_ident,
            }
        });
        quote! {
            #[automatically_derived]
            unsafe #lift_impl_spec {
                type FfiType = ::uniffi::RustBuffer;

                fn try_read(buf: &mut &[::std::primitive::u8]) -> ::uniffi::deps::anyhow::Result<Self> {
                    Ok(match ::uniffi::deps::bytes::Buf::get_i32(buf) {
                        #(#match_arms)*
                        v => ::uniffi::deps::anyhow::bail!("Invalid #ident enum value: {}", v),
                    })
                }

                fn try_lift(v: ::uniffi::RustBuffer) -> ::uniffi::deps::anyhow::Result<Self> {
                    <Self as ::uniffi::Lift<crate::UniFfiTag>>::try_lift_from_rust_buffer(v)
                }

                const TYPE_ID_META: ::uniffi::MetadataBuffer = <Self as ::uniffi::Lower<crate::UniFfiTag>>::TYPE_ID_META;
            }

        }
    } else {
        quote! {
            // Lifting flat errors is not currently supported, but we still define the trait so
            // that dicts containing flat errors don't cause compile errors (see ReturnOnlyDict in
            // coverall.rs).
            //
            // Note: it would be better to not derive `Lift` for dictionaries containing flat
            // errors, but getting the trait bounds and derived impls there would be much harder.
            // For now, we just fail at runtime.
            #[automatically_derived]
            unsafe #lift_impl_spec {
                type FfiType = ::uniffi::RustBuffer;

                fn try_read(buf: &mut &[::std::primitive::u8]) -> ::uniffi::deps::anyhow::Result<Self> {
                    panic!("Can't lift flat errors")
                }

                fn try_lift(v: ::uniffi::RustBuffer) -> ::uniffi::deps::anyhow::Result<Self> {
                    panic!("Can't lift flat errors")
                }

                const TYPE_ID_META: ::uniffi::MetadataBuffer = <Self as ::uniffi::Lower<crate::UniFfiTag>>::TYPE_ID_META;
            }
        }
    };

    quote! {
        #lower_impl
        #lift_impl
        #derive_ffi_traits
    }
}

pub(crate) fn error_meta_static_var(
    ident: &Ident,
    docstring: String,
    enum_: &DataEnum,
    attr: &ErrorAttr,
) -> syn::Result<TokenStream> {
    let name = ident_to_string(ident);
    let module_path = mod_path()?;
    let flat = attr.flat.is_some();
    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(Some(#flat))
                .concat_bool(false) // discr_type: None
    };
    if flat {
        metadata_expr.extend(flat_error_variant_metadata(enum_)?)
    } else {
        metadata_expr.extend(variant_metadata(enum_)?);
    }
    metadata_expr.extend(quote! {
        .concat_bool(#non_exhaustive)
        .concat_long_str(#docstring)
    });
    Ok(create_metadata_items("error", &name, metadata_expr, None))
}

pub fn flat_error_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 name = ident_to_string(&v.ident);
            let docstring = extract_docstring(&v.attrs)?;
            Ok(quote! {
                .concat_str(#name)
                .concat_long_str(#docstring)
            })
        }))
        .collect()
}

#[derive(Clone, Default)]
pub struct ErrorAttr {
    pub flat: Option<kw::flat_error>,
    pub with_try_read: Option<kw::with_try_read>,
    pub non_exhaustive: Option<kw::non_exhaustive>,
}

impl UniffiAttributeArgs for ErrorAttr {
    fn parse_one(input: ParseStream<'_>) -> syn::Result<Self> {
        let lookahead = input.lookahead1();
        if lookahead.peek(kw::flat_error) {
            Ok(Self {
                flat: input.parse()?,
                ..Self::default()
            })
        } else if lookahead.peek(kw::with_try_read) {
            Ok(Self {
                with_try_read: input.parse()?,
                ..Self::default()
            })
        } else if lookahead.peek(kw::non_exhaustive) {
            Ok(Self {
                non_exhaustive: input.parse()?,
                ..Self::default()
            })
        } else if lookahead.peek(kw::handle_unknown_callback_error) {
            // Not used anymore, but still allowed
            Ok(Self::default())
        } else {
            Err(lookahead.error())
        }
    }

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

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

impl TryFrom<ErrorAttr> for EnumAttr {
    type Error = syn::Error;

    fn try_from(error_attr: ErrorAttr) -> Result<Self, Self::Error> {
        if error_attr.flat.is_some() {
            Err(syn::Error::new(
                Span::call_site(),
                "flat attribute not valid for rich enum errors",
            ))
        } else if error_attr.with_try_read.is_some() {
            Err(syn::Error::new(
                Span::call_site(),
                "with_try_read attribute not valid for rich enum errors",
            ))
        } else {
            Ok(EnumAttr {
                non_exhaustive: error_attr.non_exhaustive,
            })
        }
    }
}