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    extern crate proc_macro;
    extern crate syn;

    #[macro_use] extern crate synstructure;
    #[macro_use] extern crate quote;

    decl_derive!([Fail, attributes(fail, cause)] => fail_derive);

    fn fail_derive(s: synstructure::Structure) -> quote::Tokens {
        let cause_body = s.each_variant(|v| {
            if let Some(cause) = v.bindings().iter().find(is_cause) {
                quote!(return Some(#cause))
            } else {
                quote!(return None)
            }
        });

        let bt_body = s.each_variant(|v| {
            if let Some(bi) = v.bindings().iter().find(is_backtrace) {
                quote!(return Some(#bi))
            } else {
                quote!(return None)
            }
        });

        #[cfg(feature = "std")]
        let fail = s.bound_impl("::failure::Fail", quote! {
            #[allow(unreachable_code)]
            fn cause(&self) -> ::std::option::Option<&::failure::Fail> {
                match *self { #cause_body }
                None
            }

            #[allow(unreachable_code)]
            fn backtrace(&self) -> ::std::option::Option<&::failure::Backtrace> {
                match *self { #bt_body }
                None
            }
        });

        #[cfg(not(feature = "std"))]
        let fail = s.bound_impl("::failure::Fail", quote! {
            #[allow(unreachable_code)]
            fn cause(&self) -> ::core::option::Option<&::failure::Fail> {
                match *self { #cause_body }
                None
            }

            #[allow(unreachable_code)]
            fn backtrace(&self) -> ::core::option::Option<&::failure::Backtrace> {
                match *self { #bt_body }
                None
            }
        });

        #[cfg(feature = "std")]
        let display = display_body(&s).map(|display_body| {
            s.bound_impl("::std::fmt::Display", quote! {
                #[allow(unreachable_code)]
                fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                    match *self { #display_body }
                    write!(f, "An error has occurred.")
                }
            })
        });

        #[cfg(not(feature = "std"))]
        let display = display_body(&s).map(|display_body| {
            s.bound_impl("::core::fmt::Display", quote! {
                #[allow(unreachable_code)]
                fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
                    match *self { #display_body }
                    write!(f, "An error has occurred.")
                }
            })
        });

        quote! {
            #fail
            #display
        }
    }

    fn display_body(s: &synstructure::Structure) -> Option<quote::Tokens> {
        let mut msgs = s.variants().iter().map(|v| find_error_msg(&v.ast().attrs));
        if msgs.all(|msg| msg.is_none()) { return None; }

        Some(s.each_variant(|v| {
            let msg = find_error_msg(&v.ast().attrs).expect("All variants must have display attribute.");
            if msg.is_empty() {
                panic!("Expected at least one argument to fail attribute");
            }

            let s = match msg[0] {
                syn::NestedMetaItem::MetaItem(syn::MetaItem::NameValue(ref i, ref lit)) if i == "display" => {
                    lit.clone()
                }
                _ => panic!("Fail attribute must begin `display = \"\"` to control the Display message."),
            };
            let args = msg[1..].iter().map(|arg| match *arg {
                syn::NestedMetaItem::Literal(syn::Lit::Int(i, _)) => {
                    let bi = &v.bindings()[i as usize];
                    quote!(#bi)
                }
                syn::NestedMetaItem::MetaItem(syn::MetaItem::Word(ref id)) => {
                    if id.as_ref().starts_with("_") {
                        if let Ok(idx) = id.as_ref()[1..].parse::<usize>() {
                            let bi = &v.bindings()[idx];
                            return quote!(#bi)
                        }
                    }
                    for bi in v.bindings() {
                        if bi.ast().ident.as_ref() == Some(id) {
                            return quote!(#bi);
                        }
                    }
                    panic!("Couldn't find a field with this name!");
                }
                _ => panic!("Invalid argument to fail attribute!"),
            });

            quote! {
                return write!(f, #s #(, #args)*)
            }
        }))
    }

    fn find_error_msg(attrs: &[syn::Attribute]) -> Option<&[syn::NestedMetaItem]> {
        let mut error_msg = None;
        for attr in attrs {
            if attr.name() == "fail" {
                if error_msg.is_some() {
                    panic!("Cannot have two display attributes")
                } else {
                    if let syn::MetaItem::List(_, ref list)  = attr.value {
                        error_msg = Some(&list[..]);
                    } else {
                        panic!("fail attribute must take a list in parantheses")
                    }
                }
            }
        }
        error_msg
    }

    fn is_backtrace(bi: &&synstructure::BindingInfo) -> bool {
            match bi.ast().ty {
                syn::Ty::Path(None, syn::Path { segments: ref path, .. }) => {
                    path.last().map_or(false, |s| s.ident == "Backtrace" && s.parameters.is_empty())
                }
                _ => false
            }
    }

    fn is_cause(bi: &&synstructure::BindingInfo) -> bool {
        bi.ast().attrs.iter().any(|attr| attr.name() == "cause")
    }

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