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    use internals::ast::{Container, Data, Field, Style};
    use internals::attr::{Identifier, TagType};
    use internals::{ungroup, Ctxt, Derive};
    use syn::{Member, Type};

    /// Cross-cutting checks that require looking at more than a single attrs
    /// object. Simpler checks should happen when parsing and building the attrs.
    pub fn check(cx: &Ctxt, cont: &mut Container, derive: Derive) {
        check_remote_generic(cx, cont);
        check_getter(cx, cont);
        check_flatten(cx, cont);
        check_identifier(cx, cont);
        check_variant_skip_attrs(cx, cont);
        check_internal_tag_field_name_conflict(cx, cont);
        check_adjacent_tag_conflict(cx, cont);
        check_transparent(cx, cont, derive);
        check_from_and_try_from(cx, cont);
    }

    /// Remote derive definition type must have either all of the generics of the
    /// remote type:
    ///
    ///     #[serde(remote = "Generic")]
    ///     struct Generic<T> {…}
    ///
    /// or none of them, i.e. defining impls for one concrete instantiation of the
    /// remote type only:
    ///
    ///     #[serde(remote = "Generic<T>")]
    ///     struct ConcreteDef {…}
    ///
    fn check_remote_generic(cx: &Ctxt, cont: &Container) {
        if let Some(remote) = cont.attrs.remote() {
            let local_has_generic = !cont.generics.params.is_empty();
            let remote_has_generic = !remote.segments.last().unwrap().arguments.is_none();
            if local_has_generic && remote_has_generic {
                cx.error_spanned_by(remote, "remove generic parameters from this path");
            }
        }
    }

    /// Getters are only allowed inside structs (not enums) with the `remote`
    /// attribute.
    fn check_getter(cx: &Ctxt, cont: &Container) {
        match cont.data {
            Data::Enum(_) => {
                if cont.data.has_getter() {
                    cx.error_spanned_by(
                        cont.original,
                        "#[serde(getter = \"...\")] is not allowed in an enum",
                    );
                }
            }
            Data::Struct(_, _) => {
                if cont.data.has_getter() && cont.attrs.remote().is_none() {
                    cx.error_spanned_by(
                        cont.original,
                        "#[serde(getter = \"...\")] can only be used in structs that have #[serde(remote = \"...\")]",
                    );
                }
            }
        }
    }

    /// Flattening has some restrictions we can test.
    fn check_flatten(cx: &Ctxt, cont: &Container) {
        match &cont.data {
            Data::Enum(variants) => {
                for variant in variants {
                    for field in &variant.fields {
                        check_flatten_field(cx, variant.style, field);
                    }
                }
            }
            Data::Struct(style, fields) => {
                for field in fields {
                    check_flatten_field(cx, *style, field);
                }
            }
        }
    }

    fn check_flatten_field(cx: &Ctxt, style: Style, field: &Field) {
        if !field.attrs.flatten() {
            return;
        }
        match style {
            Style::Tuple => {
                cx.error_spanned_by(
                    field.original,
                    "#[serde(flatten)] cannot be used on tuple structs",
                );
            }
            Style::Newtype => {
                cx.error_spanned_by(
                    field.original,
                    "#[serde(flatten)] cannot be used on newtype structs",
                );
            }
            _ => {}
        }
    }

    /// The `other` attribute must be used at most once and it must be the last
    /// variant of an enum.
    ///
    /// Inside a `variant_identifier` all variants must be unit variants. Inside a
    /// `field_identifier` all but possibly one variant must be unit variants. The
    /// last variant may be a newtype variant which is an implicit "other" case.
    fn check_identifier(cx: &Ctxt, cont: &Container) {
        let variants = match &cont.data {
            Data::Enum(variants) => variants,
            Data::Struct(_, _) => {
                return;
            }
        };

        for (i, variant) in variants.iter().enumerate() {
            match (
                variant.style,
                cont.attrs.identifier(),
                variant.attrs.other(),
                cont.attrs.tag(),
            ) {
                // The `other` attribute may not be used in a variant_identifier.
                (_, Identifier::Variant, true, _) => {
                    cx.error_spanned_by(
                        variant.original,
                        "#[serde(other)] may not be used on a variant identifier",
                    );
                }

                // Variant with `other` attribute cannot appear in untagged enum
                (_, Identifier::No, true, &TagType::None) => {
                    cx.error_spanned_by(
                        variant.original,
                        "#[serde(other)] cannot appear on untagged enum",
                    );
                }

                // Variant with `other` attribute must be the last one.
                (Style::Unit, Identifier::Field, true, _) | (Style::Unit, Identifier::No, true, _) => {
                    if i < variants.len() - 1 {
                        cx.error_spanned_by(
                            variant.original,
                            "#[serde(other)] must be on the last variant",
                        );
                    }
                }

                // Variant with `other` attribute must be a unit variant.
                (_, Identifier::Field, true, _) | (_, Identifier::No, true, _) => {
                    cx.error_spanned_by(
                        variant.original,
                        "#[serde(other)] must be on a unit variant",
                    );
                }

                // Any sort of variant is allowed if this is not an identifier.
                (_, Identifier::No, false, _) => {}

                // Unit variant without `other` attribute is always fine.
                (Style::Unit, _, false, _) => {}

                // The last field is allowed to be a newtype catch-all.
                (Style::Newtype, Identifier::Field, false, _) => {
                    if i < variants.len() - 1 {
                        cx.error_spanned_by(
                            variant.original,
                            format!("`{}` must be the last variant", variant.ident),
                        );
                    }
                }

                (_, Identifier::Field, false, _) => {
                    cx.error_spanned_by(
                        variant.original,
                        "#[serde(field_identifier)] may only contain unit variants",
                    );
                }

                (_, Identifier::Variant, false, _) => {
                    cx.error_spanned_by(
                        variant.original,
                        "#[serde(variant_identifier)] may only contain unit variants",
                    );
                }
            }
        }
    }

    /// Skip-(de)serializing attributes are not allowed on variants marked
    /// (de)serialize_with.
    fn check_variant_skip_attrs(cx: &Ctxt, cont: &Container) {
        let variants = match &cont.data {
            Data::Enum(variants) => variants,
            Data::Struct(_, _) => {
                return;
            }
        };

        for variant in variants.iter() {
            if variant.attrs.serialize_with().is_some() {
                if variant.attrs.skip_serializing() {
                    cx.error_spanned_by(
                        variant.original,
                        format!(
                            "variant `{}` cannot have both #[serde(serialize_with)] and #[serde(skip_serializing)]",
                            variant.ident
                        ),
                    );
                }

                for field in &variant.fields {
                    let member = member_message(&field.member);

                    if field.attrs.skip_serializing() {
                        cx.error_spanned_by(
                            variant.original,
                            format!(
                                "variant `{}` cannot have both #[serde(serialize_with)] and a field {} marked with #[serde(skip_serializing)]",
                                variant.ident, member
                            ),
                        );
                    }

                    if field.attrs.skip_serializing_if().is_some() {
                        cx.error_spanned_by(
                            variant.original,
                            format!(
                                "variant `{}` cannot have both #[serde(serialize_with)] and a field {} marked with #[serde(skip_serializing_if)]",
                                variant.ident, member
                            ),
                        );
                    }
                }
            }

            if variant.attrs.deserialize_with().is_some() {
                if variant.attrs.skip_deserializing() {
                    cx.error_spanned_by(
                        variant.original,
                        format!(
                            "variant `{}` cannot have both #[serde(deserialize_with)] and #[serde(skip_deserializing)]",
                            variant.ident
                        ),
                    );
                }

                for field in &variant.fields {
                    if field.attrs.skip_deserializing() {
                        let member = member_message(&field.member);

                        cx.error_spanned_by(
                            variant.original,
                            format!(
                                "variant `{}` cannot have both #[serde(deserialize_with)] and a field {} marked with #[serde(skip_deserializing)]",
                                variant.ident, member
                            ),
                        );
                    }
                }
            }
        }
    }

    /// The tag of an internally-tagged struct variant must not be
    /// the same as either one of its fields, as this would result in
    /// duplicate keys in the serialized output and/or ambiguity in
    /// the to-be-deserialized input.
    fn check_internal_tag_field_name_conflict(cx: &Ctxt, cont: &Container) {
        let variants = match &cont.data {
            Data::Enum(variants) => variants,
            Data::Struct(_, _) => return,
        };

        let tag = match cont.attrs.tag() {
            TagType::Internal { tag } => tag.as_str(),
            TagType::External | TagType::Adjacent { .. } | TagType::None => return,
        };

        let diagnose_conflict = || {
            cx.error_spanned_by(
                cont.original,
                format!("variant field name `{}` conflicts with internal tag", tag),
            );
        };

        for variant in variants {
            match variant.style {
                Style::Struct => {
                    for field in &variant.fields {
                        let check_ser = !field.attrs.skip_serializing();
                        let check_de = !field.attrs.skip_deserializing();
                        let name = field.attrs.name();
                        let ser_name = name.serialize_name();

                        if check_ser && ser_name == tag {
                            diagnose_conflict();
                            return;
                        }

                        for de_name in field.attrs.aliases() {
                            if check_de && de_name == tag {
                                diagnose_conflict();
                                return;
                            }
                        }
                    }
                }
                Style::Unit | Style::Newtype | Style::Tuple => {}
            }
        }
    }

    /// In the case of adjacently-tagged enums, the type and the
    /// contents tag must differ, for the same reason.
    fn check_adjacent_tag_conflict(cx: &Ctxt, cont: &Container) {
        let (type_tag, content_tag) = match cont.attrs.tag() {
            TagType::Adjacent { tag, content } => (tag, content),
            TagType::Internal { .. } | TagType::External | TagType::None => return,
        };

        if type_tag == content_tag {
            cx.error_spanned_by(
                cont.original,
                format!(
                    "enum tags `{}` for type and content conflict with each other",
                    type_tag
                ),
            );
        }
    }

    /// Enums and unit structs cannot be transparent.
    fn check_transparent(cx: &Ctxt, cont: &mut Container, derive: Derive) {
        if !cont.attrs.transparent() {
            return;
        }

        if cont.attrs.type_from().is_some() {
            cx.error_spanned_by(
                cont.original,
                "#[serde(transparent)] is not allowed with #[serde(from = \"...\")]",
            );
        }

        if cont.attrs.type_try_from().is_some() {
            cx.error_spanned_by(
                cont.original,
                "#[serde(transparent)] is not allowed with #[serde(try_from = \"...\")]",
            );
        }

        if cont.attrs.type_into().is_some() {
            cx.error_spanned_by(
                cont.original,
                "#[serde(transparent)] is not allowed with #[serde(into = \"...\")]",
            );
        }

        let fields = match &mut cont.data {
            Data::Enum(_) => {
                cx.error_spanned_by(
                    cont.original,
                    "#[serde(transparent)] is not allowed on an enum",
                );
                return;
            }
            Data::Struct(Style::Unit, _) => {
                cx.error_spanned_by(
                    cont.original,
                    "#[serde(transparent)] is not allowed on a unit struct",
                );
                return;
            }
            Data::Struct(_, fields) => fields,
        };

        let mut transparent_field = None;

        for field in fields {
            if allow_transparent(field, derive) {
                if transparent_field.is_some() {
                    cx.error_spanned_by(
                        cont.original,
                        "#[serde(transparent)] requires struct to have at most one transparent field",
                    );
                    return;
                }
                transparent_field = Some(field);
            }
        }

        match transparent_field {
            Some(transparent_field) => transparent_field.attrs.mark_transparent(),
            None => match derive {
                Derive::Serialize => {
                    cx.error_spanned_by(
                        cont.original,
                        "#[serde(transparent)] requires at least one field that is not skipped",
                    );
                }
                Derive::Deserialize => {
                    cx.error_spanned_by(
                        cont.original,
                        "#[serde(transparent)] requires at least one field that is neither skipped nor has a default",
                    );
                }
            },
        }
    }

    fn member_message(member: &Member) -> String {
        match member {
            Member::Named(ident) => format!("`{}`", ident),
            Member::Unnamed(i) => format!("#{}", i.index),
        }
    }

    fn allow_transparent(field: &Field, derive: Derive) -> bool {
        if let Type::Path(ty) = ungroup(field.ty) {
            if let Some(seg) = ty.path.segments.last() {
                if seg.ident == "PhantomData" {
                    return false;
                }
            }
        }

        match derive {
            Derive::Serialize => !field.attrs.skip_serializing(),
            Derive::Deserialize => !field.attrs.skip_deserializing() && field.attrs.default().is_none(),
        }
    }

    fn check_from_and_try_from(cx: &Ctxt, cont: &mut Container) {
        if cont.attrs.type_from().is_some() && cont.attrs.type_try_from().is_some() {
            cx.error_spanned_by(
                cont.original,
                "#[serde(from = \"...\")] and #[serde(try_from = \"...\")] conflict with each other",
            );
        }
    }