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    use proc_macro2::{Delimiter, Group, Span, TokenStream, TokenTree};
    use quote::{format_ident, quote, ToTokens};
    use std::collections::BTreeSet as Set;
    use std::iter::FromIterator;
    use syn::parse::{Nothing, ParseStream};
    use syn::{
        braced, bracketed, parenthesized, token, Attribute, Error, Ident, Index, LitInt, LitStr,
        Result, Token,
    };

    pub struct Attrs<'a> {
        pub display: Option<Display<'a>>,
        pub source: Option<&'a Attribute>,
        pub backtrace: Option<&'a Attribute>,
        pub from: Option<&'a Attribute>,
        pub transparent: Option<Transparent<'a>>,
    }

    #[derive(Clone)]
    pub struct Display<'a> {
        pub original: &'a Attribute,
        pub fmt: LitStr,
        pub args: TokenStream,
        pub has_bonus_display: bool,
        pub implied_bounds: Set<(usize, Trait)>,
    }

    #[derive(Copy, Clone)]
    pub struct Transparent<'a> {
        pub original: &'a Attribute,
        pub span: Span,
    }

    #[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Debug)]
    pub enum Trait {
        Debug,
        Display,
        Octal,
        LowerHex,
        UpperHex,
        Pointer,
        Binary,
        LowerExp,
        UpperExp,
    }

    pub fn get(input: &[Attribute]) -> Result<Attrs> {
        let mut attrs = Attrs {
            display: None,
            source: None,
            backtrace: None,
            from: None,
            transparent: None,
        };

        for attr in input {
            if attr.path.is_ident("error") {
                parse_error_attribute(&mut attrs, attr)?;
            } else if attr.path.is_ident("source") {
                require_empty_attribute(attr)?;
                if attrs.source.is_some() {
                    return Err(Error::new_spanned(attr, "duplicate #[source] attribute"));
                }
                attrs.source = Some(attr);
            } else if attr.path.is_ident("backtrace") {
                require_empty_attribute(attr)?;
                if attrs.backtrace.is_some() {
                    return Err(Error::new_spanned(attr, "duplicate #[backtrace] attribute"));
                }
                attrs.backtrace = Some(attr);
            } else if attr.path.is_ident("from") {
                if !attr.tokens.is_empty() {
                    // Assume this is meant for derive_more crate or something.
                    continue;
                }
                if attrs.from.is_some() {
                    return Err(Error::new_spanned(attr, "duplicate #[from] attribute"));
                }
                attrs.from = Some(attr);
            }
        }

        Ok(attrs)
    }

    fn parse_error_attribute<'a>(attrs: &mut Attrs<'a>, attr: &'a Attribute) -> Result<()> {
        syn::custom_keyword!(transparent);

        attr.parse_args_with(|input: ParseStream| {
            if let Some(kw) = input.parse::<Option<transparent>>()? {
                if attrs.transparent.is_some() {
                    return Err(Error::new_spanned(
                        attr,
                        "duplicate #[error(transparent)] attribute",
                    ));
                }
                attrs.transparent = Some(Transparent {
                    original: attr,
                    span: kw.span,
                });
                return Ok(());
            }

            let display = Display {
                original: attr,
                fmt: input.parse()?,
                args: parse_token_expr(input, false)?,
                has_bonus_display: false,
                implied_bounds: Set::new(),
            };
            if attrs.display.is_some() {
                return Err(Error::new_spanned(
                    attr,
                    "only one #[error(...)] attribute is allowed",
                ));
            }
            attrs.display = Some(display);
            Ok(())
        })
    }

    fn parse_token_expr(input: ParseStream, mut begin_expr: bool) -> Result<TokenStream> {
        let mut tokens = Vec::new();
        while !input.is_empty() {
            if begin_expr && input.peek(Token![.]) {
                if input.peek2(Ident) {
                    input.parse::<Token![.]>()?;
                    begin_expr = false;
                    continue;
                }
                if input.peek2(LitInt) {
                    input.parse::<Token![.]>()?;
                    let int: Index = input.parse()?;
                    let ident = format_ident!("_{}", int.index, span = int.span);
                    tokens.push(TokenTree::Ident(ident));
                    begin_expr = false;
                    continue;
                }
            }

            begin_expr = input.peek(Token![break])
                || input.peek(Token![continue])
                || input.peek(Token![if])
                || input.peek(Token![in])
                || input.peek(Token![match])
                || input.peek(Token![mut])
                || input.peek(Token![return])
                || input.peek(Token![while])
                || input.peek(Token![+])
                || input.peek(Token![&])
                || input.peek(Token![!])
                || input.peek(Token![^])
                || input.peek(Token![,])
                || input.peek(Token![/])
                || input.peek(Token![=])
                || input.peek(Token![>])
                || input.peek(Token![<])
                || input.peek(Token![|])
                || input.peek(Token![%])
                || input.peek(Token![;])
                || input.peek(Token![*])
                || input.peek(Token![-]);

            let token: TokenTree = if input.peek(token::Paren) {
                let content;
                let delimiter = parenthesized!(content in input);
                let nested = parse_token_expr(&content, true)?;
                let mut group = Group::new(Delimiter::Parenthesis, nested);
                group.set_span(delimiter.span);
                TokenTree::Group(group)
            } else if input.peek(token::Brace) {
                let content;
                let delimiter = braced!(content in input);
                let nested = parse_token_expr(&content, true)?;
                let mut group = Group::new(Delimiter::Brace, nested);
                group.set_span(delimiter.span);
                TokenTree::Group(group)
            } else if input.peek(token::Bracket) {
                let content;
                let delimiter = bracketed!(content in input);
                let nested = parse_token_expr(&content, true)?;
                let mut group = Group::new(Delimiter::Bracket, nested);
                group.set_span(delimiter.span);
                TokenTree::Group(group)
            } else {
                input.parse()?
            };
            tokens.push(token);
        }
        Ok(TokenStream::from_iter(tokens))
    }

    fn require_empty_attribute(attr: &Attribute) -> Result<()> {
        syn::parse2::<Nothing>(attr.tokens.clone())?;
        Ok(())
    }

    impl ToTokens for Display<'_> {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            let fmt = &self.fmt;
            let args = &self.args;
            tokens.extend(quote! {
                write!(__formatter, #fmt #args)
            });
        }
    }

    impl ToTokens for Trait {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            let trait_name = format_ident!("{}", format!("{:?}", self));
            tokens.extend(quote!(std::fmt::#trait_name));
        }
    }