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    //! Special types handling

    use super::spanned::Sp;

    use syn::{
        spanned::Spanned, GenericArgument, Path, PathArguments, PathArguments::AngleBracketed,
        PathSegment, Type, TypePath,
    };

    #[derive(Copy, Clone, PartialEq, Eq, Debug)]
    pub enum Ty {
        Unit,
        Vec,
        Option,
        OptionOption,
        OptionVec,
        Other,
    }

    impl Ty {
        pub fn from_syn_ty(ty: &syn::Type) -> Sp<Self> {
            use self::Ty::*;
            let t = |kind| Sp::new(kind, ty.span());

            if is_unit_ty(ty) {
                t(Unit)
            } else if is_generic_ty(ty, "Vec") {
                t(Vec)
            } else if let Some(subty) = subty_if_name(ty, "Option") {
                if is_generic_ty(subty, "Option") {
                    t(OptionOption)
                } else if is_generic_ty(subty, "Vec") {
                    t(OptionVec)
                } else {
                    t(Option)
                }
            } else {
                t(Other)
            }
        }

        pub fn as_str(&self) -> &'static str {
            match self {
                Self::Unit => "()",
                Self::Vec => "Vec<T>",
                Self::Option => "Option<T>",
                Self::OptionOption => "Option<Option<T>>",
                Self::OptionVec => "Option<Vec<T>>",
                Self::Other => "...other...",
            }
        }
    }

    pub fn inner_type(field_ty: &syn::Type) -> &syn::Type {
        let ty = Ty::from_syn_ty(field_ty);
        match *ty {
            Ty::Vec | Ty::Option => sub_type(field_ty).unwrap_or(field_ty),
            Ty::OptionOption | Ty::OptionVec => {
                sub_type(field_ty).and_then(sub_type).unwrap_or(field_ty)
            }
            _ => field_ty,
        }
    }

    pub fn sub_type(ty: &syn::Type) -> Option<&syn::Type> {
        subty_if(ty, |_| true)
    }

    fn only_last_segment(mut ty: &syn::Type) -> Option<&PathSegment> {
        while let syn::Type::Group(syn::TypeGroup { elem, .. }) = ty {
            ty = elem;
        }
        match ty {
            Type::Path(TypePath {
                qself: None,
                path:
                    Path {
                        leading_colon: None,
                        segments,
                    },
            }) => only_one(segments.iter()),

            _ => None,
        }
    }

    fn subty_if<F>(ty: &syn::Type, f: F) -> Option<&syn::Type>
    where
        F: FnOnce(&PathSegment) -> bool,
    {
        only_last_segment(ty)
            .filter(|segment| f(segment))
            .and_then(|segment| {
                if let AngleBracketed(args) = &segment.arguments {
                    only_one(args.args.iter()).and_then(|genneric| {
                        if let GenericArgument::Type(ty) = genneric {
                            Some(ty)
                        } else {
                            None
                        }
                    })
                } else {
                    None
                }
            })
    }

    pub fn subty_if_name<'a>(ty: &'a syn::Type, name: &str) -> Option<&'a syn::Type> {
        subty_if(ty, |seg| seg.ident == name)
    }

    pub fn is_simple_ty(ty: &syn::Type, name: &str) -> bool {
        only_last_segment(ty)
            .map(|segment| {
                if let PathArguments::None = segment.arguments {
                    segment.ident == name
                } else {
                    false
                }
            })
            .unwrap_or(false)
    }

    fn is_generic_ty(ty: &syn::Type, name: &str) -> bool {
        subty_if_name(ty, name).is_some()
    }

    fn is_unit_ty(ty: &syn::Type) -> bool {
        if let syn::Type::Tuple(tuple) = ty {
            tuple.elems.is_empty()
        } else {
            false
        }
    }

    fn only_one<I, T>(mut iter: I) -> Option<T>
    where
        I: Iterator<Item = T>,
    {
        iter.next().filter(|_| iter.next().is_none())
    }