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    use proc_macro::TokenStream;
    use proc_macro2::Span;
    use quote::quote;
    use syn::spanned::Spanned;

    #[derive(Clone, Copy, PartialEq)]
    enum RuntimeFlavor {
        CurrentThread,
        Threaded,
    }

    impl RuntimeFlavor {
        fn from_str(s: &str) -> Result<RuntimeFlavor, String> {
            match s {
                "current_thread" => Ok(RuntimeFlavor::CurrentThread),
                "multi_thread" => Ok(RuntimeFlavor::Threaded),
                "single_thread" => Err("The single threaded runtime flavor is called `current_thread`.".to_string()),
                "basic_scheduler" => Err("The `basic_scheduler` runtime flavor has been renamed to `current_thread`.".to_string()),
                "threaded_scheduler" => Err("The `threaded_scheduler` runtime flavor has been renamed to `multi_thread`.".to_string()),
                _ => Err(format!("No such runtime flavor `{}`. The runtime flavors are `current_thread` and `multi_thread`.", s)),
            }
        }
    }

    struct FinalConfig {
        flavor: RuntimeFlavor,
        worker_threads: Option<usize>,
    }

    struct Configuration {
        rt_multi_thread_available: bool,
        default_flavor: RuntimeFlavor,
        flavor: Option<RuntimeFlavor>,
        worker_threads: Option<(usize, Span)>,
    }

    impl Configuration {
        fn new(is_test: bool, rt_multi_thread: bool) -> Self {
            Configuration {
                rt_multi_thread_available: rt_multi_thread,
                default_flavor: match is_test {
                    true => RuntimeFlavor::CurrentThread,
                    false => RuntimeFlavor::Threaded,
                },
                flavor: None,
                worker_threads: None,
            }
        }

        fn set_flavor(&mut self, runtime: syn::Lit, span: Span) -> Result<(), syn::Error> {
            if self.flavor.is_some() {
                return Err(syn::Error::new(span, "`flavor` set multiple times."));
            }

            let runtime_str = parse_string(runtime, span, "flavor")?;
            let runtime =
                RuntimeFlavor::from_str(&runtime_str).map_err(|err| syn::Error::new(span, err))?;
            self.flavor = Some(runtime);
            Ok(())
        }

        fn set_worker_threads(
            &mut self,
            worker_threads: syn::Lit,
            span: Span,
        ) -> Result<(), syn::Error> {
            if self.worker_threads.is_some() {
                return Err(syn::Error::new(
                    span,
                    "`worker_threads` set multiple times.",
                ));
            }

            let worker_threads = parse_int(worker_threads, span, "worker_threads")?;
            if worker_threads == 0 {
                return Err(syn::Error::new(span, "`worker_threads` may not be 0."));
            }
            self.worker_threads = Some((worker_threads, span));
            Ok(())
        }

        fn build(&self) -> Result<FinalConfig, syn::Error> {
            let flavor = self.flavor.unwrap_or(self.default_flavor);
            use RuntimeFlavor::*;
            match (flavor, self.worker_threads) {
                (CurrentThread, Some((_, worker_threads_span))) => Err(syn::Error::new(
                    worker_threads_span,
                    "The `worker_threads` option requires the `multi_thread` runtime flavor.",
                )),
                (CurrentThread, None) => Ok(FinalConfig {
                    flavor,
                    worker_threads: None,
                }),
                (Threaded, worker_threads) if self.rt_multi_thread_available => Ok(FinalConfig {
                    flavor,
                    worker_threads: worker_threads.map(|(val, _span)| val),
                }),
                (Threaded, _) => {
                    let msg = if self.flavor.is_none() {
                        "The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled."
                    } else {
                        "The runtime flavor `multi_thread` requires the `rt-multi-thread` feature."
                    };
                    Err(syn::Error::new(Span::call_site(), msg))
                }
            }
        }
    }

    fn parse_int(int: syn::Lit, span: Span, field: &str) -> Result<usize, syn::Error> {
        match int {
            syn::Lit::Int(lit) => match lit.base10_parse::<usize>() {
                Ok(value) => Ok(value),
                Err(e) => Err(syn::Error::new(
                    span,
                    format!("Failed to parse {} as integer: {}", field, e),
                )),
            },
            _ => Err(syn::Error::new(
                span,
                format!("Failed to parse {} as integer.", field),
            )),
        }
    }

    fn parse_string(int: syn::Lit, span: Span, field: &str) -> Result<String, syn::Error> {
        match int {
            syn::Lit::Str(s) => Ok(s.value()),
            syn::Lit::Verbatim(s) => Ok(s.to_string()),
            _ => Err(syn::Error::new(
                span,
                format!("Failed to parse {} as string.", field),
            )),
        }
    }

    fn parse_knobs(
        mut input: syn::ItemFn,
        args: syn::AttributeArgs,
        is_test: bool,
        rt_multi_thread: bool,
    ) -> Result<TokenStream, syn::Error> {
        let sig = &mut input.sig;
        let body = &input.block;
        let attrs = &input.attrs;
        let vis = input.vis;

        if sig.asyncness.is_none() {
            let msg = "the async keyword is missing from the function declaration";
            return Err(syn::Error::new_spanned(sig.fn_token, msg));
        }

        sig.asyncness = None;

        let macro_name = if is_test {
            "tokio::test"
        } else {
            "tokio::main"
        };
        let mut config = Configuration::new(is_test, rt_multi_thread);

        for arg in args {
            match arg {
                syn::NestedMeta::Meta(syn::Meta::NameValue(namevalue)) => {
                    let ident = namevalue.path.get_ident();
                    if ident.is_none() {
                        let msg = "Must have specified ident";
                        return Err(syn::Error::new_spanned(namevalue, msg));
                    }
                    match ident.unwrap().to_string().to_lowercase().as_str() {
                        "worker_threads" => {
                            config.set_worker_threads(namevalue.lit.clone(), namevalue.span())?;
                        }
                        "flavor" => {
                            config.set_flavor(namevalue.lit.clone(), namevalue.span())?;
                        }
                        "core_threads" => {
                            let msg = "Attribute `core_threads` is renamed to `worker_threads`";
                            return Err(syn::Error::new_spanned(namevalue, msg));
                        }
                        name => {
                            let msg = format!("Unknown attribute {} is specified; expected one of: `flavor`, `worker_threads`", name);
                            return Err(syn::Error::new_spanned(namevalue, msg));
                        }
                    }
                }
                syn::NestedMeta::Meta(syn::Meta::Path(path)) => {
                    let ident = path.get_ident();
                    if ident.is_none() {
                        let msg = "Must have specified ident";
                        return Err(syn::Error::new_spanned(path, msg));
                    }
                    let name = ident.unwrap().to_string().to_lowercase();
                    let msg = match name.as_str() {
                        "threaded_scheduler" | "multi_thread" => {
                            format!(
                                "Set the runtime flavor with #[{}(flavor = \"multi_thread\")].",
                                macro_name
                            )
                        }
                        "basic_scheduler" | "current_thread" | "single_threaded" => {
                            format!(
                                "Set the runtime flavor with #[{}(flavor = \"current_thread\")].",
                                macro_name
                            )
                        }
                        "flavor" | "worker_threads" => {
                            format!("The `{}` attribute requires an argument.", name)
                        }
                        name => {
                            format!("Unknown attribute {} is specified; expected one of: `flavor`, `worker_threads`", name)
                        }
                    };
                    return Err(syn::Error::new_spanned(path, msg));
                }
                other => {
                    return Err(syn::Error::new_spanned(
                        other,
                        "Unknown attribute inside the macro",
                    ));
                }
            }
        }

        let config = config.build()?;

        let mut rt = match config.flavor {
            RuntimeFlavor::CurrentThread => quote! {
                tokio::runtime::Builder::new_current_thread()
            },
            RuntimeFlavor::Threaded => quote! {
                tokio::runtime::Builder::new_multi_thread()
            },
        };
        if let Some(v) = config.worker_threads {
            rt = quote! { #rt.worker_threads(#v) };
        }

        let header = {
            if is_test {
                quote! {
                    #[::core::prelude::v1::test]
                }
            } else {
                quote! {}
            }
        };

        let result = quote! {
            #header
            #(#attrs)*
            #vis #sig {
                #rt
                    .enable_all()
                    .build()
                    .unwrap()
                    .block_on(async { #body })
            }
        };

        Ok(result.into())
    }

    #[cfg(not(test))] // Work around for rust-lang/rust#62127
    pub(crate) fn main(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
        let input = syn::parse_macro_input!(item as syn::ItemFn);
        let args = syn::parse_macro_input!(args as syn::AttributeArgs);

        if input.sig.ident == "main" && !input.sig.inputs.is_empty() {
            let msg = "the main function cannot accept arguments";
            return syn::Error::new_spanned(&input.sig.ident, msg)
                .to_compile_error()
                .into();
        }

        parse_knobs(input, args, false, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
    }

    pub(crate) fn test(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
        let input = syn::parse_macro_input!(item as syn::ItemFn);
        let args = syn::parse_macro_input!(args as syn::AttributeArgs);

        for attr in &input.attrs {
            if attr.path.is_ident("test") {
                let msg = "second test attribute is supplied";
                return syn::Error::new_spanned(&attr, msg)
                    .to_compile_error()
                    .into();
            }
        }

        if !input.sig.inputs.is_empty() {
            let msg = "the test function cannot accept arguments";
            return syn::Error::new_spanned(&input.sig.inputs, msg)
                .to_compile_error()
                .into();
        }

        parse_knobs(input, args, true, rt_multi_thread).unwrap_or_else(|e| e.to_compile_error().into())
    }