use proc_macro2::{Span, TokenStream, TokenTree};
use quote::{quote, quote_spanned, ToTokens};
use syn::parse::{Parse, ParseStream, Parser};
use syn::{braced, Attribute, Ident, Path, Signature, Visibility};

// syn::AttributeArgs does not implement syn::Parse
type AttributeArgs = syn::punctuated::Punctuated<syn::Meta, syn::Token![,]>;

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

impl RuntimeFlavor {
    fn from_str(s: &str) -> Result<RuntimeFlavor, String> {
        match s {
            "current_thread" => Ok(RuntimeFlavor::CurrentThread),
            "multi_thread" => Ok(RuntimeFlavor::Threaded),
            "local" => Ok(RuntimeFlavor::Local),
            "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 `{s}`. The runtime flavors are `current_thread`, `local`, and `multi_thread`.")),
        }
    }
}

#[derive(Clone, Copy, PartialEq)]
enum UnhandledPanic {
    Ignore,
    ShutdownRuntime,
}

impl UnhandledPanic {
    fn from_str(s: &str) -> Result<UnhandledPanic, String> {
        match s {
            "ignore" => Ok(UnhandledPanic::Ignore),
            "shutdown_runtime" => Ok(UnhandledPanic::ShutdownRuntime),
            _ => Err(format!("No such unhandled panic behavior `{s}`. The unhandled panic behaviors are `ignore` and `shutdown_runtime`.")),
        }
    }

    fn into_tokens(self, crate_path: &TokenStream) -> TokenStream {
        match self {
            UnhandledPanic::Ignore => quote! { #crate_path::runtime::UnhandledPanic::Ignore },
            UnhandledPanic::ShutdownRuntime => {
                quote! { #crate_path::runtime::UnhandledPanic::ShutdownRuntime }
            }
        }
    }
}

struct FinalConfig {
    name: Option<String>,
    flavor: RuntimeFlavor,
    worker_threads: Option<usize>,
    start_paused: Option<bool>,
    crate_name: Option<Path>,
    unhandled_panic: Option<UnhandledPanic>,
}

/// Config used in case of the attribute not being able to build a valid config
const DEFAULT_ERROR_CONFIG: FinalConfig = FinalConfig {
    name: None,
    flavor: RuntimeFlavor::CurrentThread,
    worker_threads: None,
    start_paused: None,
    crate_name: None,
    unhandled_panic: None,
};

struct Configuration {
    name: Option<String>,
    rt_multi_thread_available: bool,
    default_flavor: RuntimeFlavor,
    flavor: Option<RuntimeFlavor>,
    worker_threads: Option<(usize, Span)>,
    start_paused: Option<(bool, Span)>,
    is_test: bool,
    crate_name: Option<Path>,
    unhandled_panic: Option<(UnhandledPanic, Span)>,
}

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

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

        let runtime_name = parse_string(name, span, "name")?;
        self.name = Some(runtime_name);
        Ok(())
    }

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

        let start_paused = parse_bool(start_paused, span, "start_paused")?;
        self.start_paused = Some((start_paused, span));
        Ok(())
    }

    fn set_crate_name(&mut self, name: syn::Lit, span: Span) -> Result<(), syn::Error> {
        if self.crate_name.is_some() {
            return Err(syn::Error::new(span, "`crate` set multiple times."));
        }
        let name_path = parse_path(name, span, "crate")?;
        self.crate_name = Some(name_path);
        Ok(())
    }

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

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

    fn macro_name(&self) -> &'static str {
        if self.is_test {
            "tokio::test"
        } else {
            "tokio::main"
        }
    }

    fn build(&self) -> Result<FinalConfig, syn::Error> {
        use RuntimeFlavor as F;

        let flavor = self.flavor.unwrap_or(self.default_flavor);

        let worker_threads = match (flavor, self.worker_threads) {
            (F::CurrentThread | F::Local, Some((_, worker_threads_span))) => {
                let msg = format!(
                    "The `worker_threads` option requires the `multi_thread` runtime flavor. Use `#[{}(flavor = \"multi_thread\")]`",
                    self.macro_name(),
                );
                return Err(syn::Error::new(worker_threads_span, msg));
            }
            (F::CurrentThread | F::Local, None) => None,
            (F::Threaded, worker_threads) if self.rt_multi_thread_available => {
                worker_threads.map(|(val, _span)| val)
            }
            (F::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."
                };
                return Err(syn::Error::new(Span::call_site(), msg));
            }
        };

        let start_paused = match (flavor, self.start_paused) {
            (F::Threaded, Some((_, start_paused_span))) => {
                let msg = format!(
                    "The `start_paused` option requires the `current_thread` runtime flavor. Use `#[{}(flavor = \"current_thread\")]`",
                    self.macro_name(),
                );
                return Err(syn::Error::new(start_paused_span, msg));
            }
            (F::CurrentThread | F::Local, Some((start_paused, _))) => Some(start_paused),
            (_, None) => None,
        };

        let unhandled_panic = match (flavor, self.unhandled_panic) {
            (F::Threaded, Some((_, unhandled_panic_span))) => {
                let msg = format!(
                    "The `unhandled_panic` option requires the `current_thread` runtime flavor. Use `#[{}(flavor = \"current_thread\")]`",
                    self.macro_name(),
                );
                return Err(syn::Error::new(unhandled_panic_span, msg));
            }
            (F::CurrentThread | F::Local, Some((unhandled_panic, _))) => Some(unhandled_panic),
            (_, None) => None,
        };

        Ok(FinalConfig {
            name: self.name.clone(),
            crate_name: self.crate_name.clone(),
            flavor,
            worker_threads,
            start_paused,
            unhandled_panic,
        })
    }
}

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 value of `{field}` as integer: {e}"),
            )),
        },
        _ => Err(syn::Error::new(
            span,
            format!("Failed to parse value of `{field}` as integer."),
        )),
    }
}

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 value of `{field}` as string."),
        )),
    }
}

fn parse_path(lit: syn::Lit, span: Span, field: &str) -> Result<Path, syn::Error> {
    match lit {
        syn::Lit::Str(s) => {
            let err = syn::Error::new(
                span,
                format!(
                    "Failed to parse value of `{}` as path: \"{}\"",
                    field,
                    s.value()
                ),
            );
            s.parse::<syn::Path>().map_err(|_| err.clone())
        }
        _ => Err(syn::Error::new(
            span,
            format!("Failed to parse value of `{field}` as path."),
        )),
    }
}

fn parse_bool(bool: syn::Lit, span: Span, field: &str) -> Result<bool, syn::Error> {
    match bool {
        syn::Lit::Bool(b) => Ok(b.value),
        _ => Err(syn::Error::new(
            span,
            format!("Failed to parse value of `{field}` as bool."),
        )),
    }
}

fn contains_impl_trait(ty: &syn::Type) -> bool {
    match ty {
        syn::Type::ImplTrait(_) => true,
        syn::Type::Array(t) => contains_impl_trait(&t.elem),
        syn::Type::Ptr(t) => contains_impl_trait(&t.elem),
        syn::Type::Reference(t) => contains_impl_trait(&t.elem),
        syn::Type::Slice(t) => contains_impl_trait(&t.elem),
        syn::Type::Tuple(t) => t.elems.iter().any(contains_impl_trait),
        syn::Type::Paren(t) => contains_impl_trait(&t.elem),
        syn::Type::Group(t) => contains_impl_trait(&t.elem),
        syn::Type::Path(t) => match t.path.segments.last() {
            Some(segment) => match &segment.arguments {
                syn::PathArguments::AngleBracketed(args) => args.args.iter().any(|arg| match arg {
                    syn::GenericArgument::Type(t) => contains_impl_trait(t),
                    syn::GenericArgument::AssocType(t) => contains_impl_trait(&t.ty),
                    _ => false,
                }),
                syn::PathArguments::Parenthesized(args) => {
                    args.inputs.iter().any(contains_impl_trait)
                        || matches!(&args.output, syn::ReturnType::Type(_, t) if contains_impl_trait(t))
                }
                syn::PathArguments::None => false,
            },
            None => false,
        },
        _ => false,
    }
}

fn build_config(
    input: &ItemFn,
    args: AttributeArgs,
    is_test: bool,
    rt_multi_thread: bool,
) -> Result<FinalConfig, syn::Error> {
    if input.sig.asyncness.is_none() {
        let msg = "the `async` keyword is missing from the function declaration";
        return Err(syn::Error::new_spanned(input.sig.fn_token, msg));
    }

    let mut config = Configuration::new(is_test, rt_multi_thread);
    let macro_name = config.macro_name();

    for arg in args {
        match arg {
            syn::Meta::NameValue(namevalue) => {
                let ident = namevalue
                    .path
                    .get_ident()
                    .ok_or_else(|| {
                        syn::Error::new_spanned(&namevalue, "Must have specified ident")
                    })?
                    .to_string()
                    .to_lowercase();
                let lit = match &namevalue.value {
                    syn::Expr::Lit(syn::ExprLit { lit, .. }) => lit,
                    expr => return Err(syn::Error::new_spanned(expr, "Must be a literal")),
                };
                match ident.as_str() {
                    "worker_threads" => {
                        config.set_worker_threads(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    "flavor" => {
                        config.set_flavor(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    "start_paused" => {
                        config.set_start_paused(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    "core_threads" => {
                        let msg = "Attribute `core_threads` is renamed to `worker_threads`";
                        return Err(syn::Error::new_spanned(namevalue, msg));
                    }
                    "crate" => {
                        config.set_crate_name(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    "unhandled_panic" => {
                        config
                            .set_unhandled_panic(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    "name" => {
                        config.set_name(lit.clone(), syn::spanned::Spanned::span(lit))?;
                    }
                    name => {
                        let msg = format!(
                            "Unknown attribute {name} is specified; expected one of: `flavor`, `worker_threads`, `start_paused`, `crate`, `unhandled_panic`, `name`.",
                        );
                        return Err(syn::Error::new_spanned(namevalue, msg));
                    }
                }
            }
            syn::Meta::Path(path) => {
                let name = path
                    .get_ident()
                    .ok_or_else(|| syn::Error::new_spanned(&path, "Must have specified ident"))?
                    .to_string()
                    .to_lowercase();
                let msg = match name.as_str() {
                    "threaded_scheduler" | "multi_thread" => {
                        format!(
                            "Set the runtime flavor with #[{macro_name}(flavor = \"multi_thread\")]."
                        )
                    }
                    "basic_scheduler" | "current_thread" | "single_threaded" => {
                        format!(
                            "Set the runtime flavor with #[{macro_name}(flavor = \"current_thread\")]."
                        )
                    }
                    "flavor" | "worker_threads" | "start_paused" | "crate" | "unhandled_panic"
                    | "name" => {
                        format!("The `{name}` attribute requires an argument.")
                    }
                    name => {
                        format!("Unknown attribute {name} is specified; expected one of: `flavor`, `worker_threads`, `start_paused`, `crate`, `unhandled_panic`, `name`.")
                    }
                };
                return Err(syn::Error::new_spanned(path, msg));
            }
            other => {
                return Err(syn::Error::new_spanned(
                    other,
                    "Unknown attribute inside the macro",
                ));
            }
        }
    }

    config.build()
}

fn parse_knobs(mut input: ItemFn, is_test: bool, config: FinalConfig) -> TokenStream {
    input.sig.asyncness = None;

    // If type mismatch occurs, the current rustc points to the last statement.
    let (last_stmt_start_span, last_stmt_end_span) = {
        let mut last_stmt = input.stmts.last().cloned().unwrap_or_default().into_iter();

        // `Span` on stable Rust has a limitation that only points to the first
        // token, not the whole tokens. We can work around this limitation by
        // using the first/last span of the tokens like
        // `syn::Error::new_spanned` does.
        let start = last_stmt.next().map_or_else(Span::call_site, |t| t.span());
        let end = last_stmt.last().map_or(start, |t| t.span());
        (start, end)
    };

    let crate_path = config
        .crate_name
        .map(ToTokens::into_token_stream)
        .unwrap_or_else(|| {
            Ident::new("tokio", Span::call_site().located_at(last_stmt_start_span))
                .into_token_stream()
        });

    let use_builder = quote_spanned! {Span::call_site().located_at(last_stmt_start_span)=>
        use #crate_path::runtime::Builder;
    };

    let mut rt = match config.flavor {
        RuntimeFlavor::CurrentThread | RuntimeFlavor::Local => {
            quote_spanned! {last_stmt_start_span=>
                Builder::new_current_thread()
            }
        }
        RuntimeFlavor::Threaded => quote_spanned! {last_stmt_start_span=>
            Builder::new_multi_thread()
        },
    };

    let build = if let RuntimeFlavor::Local = config.flavor {
        quote_spanned! {last_stmt_start_span=> build_local(Default::default())}
    } else {
        quote_spanned! {last_stmt_start_span=> build()}
    };

    if let Some(v) = config.worker_threads {
        rt = quote_spanned! {last_stmt_start_span=> #rt.worker_threads(#v) };
    }
    if let Some(v) = config.start_paused {
        rt = quote_spanned! {last_stmt_start_span=> #rt.start_paused(#v) };
    }
    if let Some(v) = config.unhandled_panic {
        let unhandled_panic = v.into_tokens(&crate_path);
        rt = quote_spanned! {last_stmt_start_span=> #rt.unhandled_panic(#unhandled_panic) };
    }
    if let Some(v) = config.name {
        rt = quote_spanned! {last_stmt_start_span=> #rt.name(#v) };
    }

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

    let body_ident = quote! { body };
    // This explicit `return` is intentional. See tokio-rs/tokio#4636
    let last_block = quote_spanned! {last_stmt_end_span=>

        #[allow(clippy::expect_used, clippy::diverging_sub_expression, clippy::needless_return, clippy::unwrap_in_result)]
        {
            #use_builder

            return #rt
                .enable_all()
                .#build
                .expect("Failed building the Runtime")
                .block_on(#body_ident);
        }

    };

    let body = input.body();

    // For test functions pin the body to the stack and use `Pin<&mut dyn
    // Future>` to reduce the amount of `Runtime::block_on` (and related
    // functions) copies we generate during compilation due to the generic
    // parameter `F` (the future to block on). This could have an impact on
    // performance, but because it's only for testing it's unlikely to be very
    // large.
    //
    // We don't do this for the main function as it should only be used once so
    // there will be no benefit.
    let output_type = match &input.sig.output {
        // For functions with no return value syn doesn't print anything,
        // but that doesn't work as `Output` for our boxed `Future`, so
        // default to `()` (the same type as the function output).
        syn::ReturnType::Default => quote! { () },
        syn::ReturnType::Type(_, ret_type) => quote! { #ret_type },
    };

    let body = if is_test {
        quote! {
            let body = async #body;
            #crate_path::pin!(body);
            let body: ::core::pin::Pin<&mut dyn ::core::future::Future<Output = #output_type>> = body;
        }
    } else {
        // force typecheck without runtime overhead
        let check_block = match &input.sig.output {
            syn::ReturnType::Type(_, t)
                if matches!(**t, syn::Type::Never(_)) || contains_impl_trait(t) =>
            {
                quote! {}
            }
            _ => quote! {
                if false {
                    let _: &dyn ::core::future::Future<Output = #output_type> = &body;
                }
            },
        };

        quote! {
            let body = async #body;
            // Compile-time assertion that the future's output matches the return type.
            let body = {
                #check_block
                body
            };
        }
    };

    input.into_tokens(generated_attrs, body, last_block)
}

fn token_stream_with_error(mut tokens: TokenStream, error: syn::Error) -> TokenStream {
    tokens.extend(error.into_compile_error());
    tokens
}

pub(crate) fn main(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
    // If any of the steps for this macro fail, we still want to expand to an item that is as close
    // to the expected output as possible. This helps out IDEs such that completions and other
    // related features keep working.
    let input: ItemFn = match syn::parse2(item.clone()) {
        Ok(it) => it,
        Err(e) => return token_stream_with_error(item, e),
    };

    let config = if input.sig.ident == "main" && !input.sig.inputs.is_empty() {
        let msg = "the main function cannot accept arguments";
        Err(syn::Error::new_spanned(&input.sig.ident, msg))
    } else {
        AttributeArgs::parse_terminated
            .parse2(args)
            .and_then(|args| build_config(&input, args, false, rt_multi_thread))
    };

    match config {
        Ok(config) => parse_knobs(input, false, config),
        Err(e) => token_stream_with_error(parse_knobs(input, false, DEFAULT_ERROR_CONFIG), e),
    }
}

// Check whether given attribute is a test attribute of forms:
// * `#[test]`
// * `#[core::prelude::*::test]` or `#[::core::prelude::*::test]`
// * `#[std::prelude::*::test]` or `#[::std::prelude::*::test]`
fn is_test_attribute(attr: &Attribute) -> bool {
    let path = match &attr.meta {
        syn::Meta::Path(path) => path,
        _ => return false,
    };
    let candidates = [
        ["core", "prelude", "*", "test"],
        ["std", "prelude", "*", "test"],
    ];
    if path.leading_colon.is_none()
        && path.segments.len() == 1
        && path.segments[0].arguments.is_none()
        && path.segments[0].ident == "test"
    {
        return true;
    } else if path.segments.len() != candidates[0].len() {
        return false;
    }
    candidates.into_iter().any(|segments| {
        path.segments.iter().zip(segments).all(|(segment, path)| {
            segment.arguments.is_none() && (path == "*" || segment.ident == path)
        })
    })
}

pub(crate) fn test(args: TokenStream, item: TokenStream, rt_multi_thread: bool) -> TokenStream {
    // If any of the steps for this macro fail, we still want to expand to an item that is as close
    // to the expected output as possible. This helps out IDEs such that completions and other
    // related features keep working.
    let input: ItemFn = match syn::parse2(item.clone()) {
        Ok(it) => it,
        Err(e) => return token_stream_with_error(item, e),
    };
    let config = if let Some(attr) = input.attrs().find(|attr| is_test_attribute(attr)) {
        let msg = "second test attribute is supplied, consider removing or changing the order of your test attributes";
        Err(syn::Error::new_spanned(attr, msg))
    } else {
        AttributeArgs::parse_terminated
            .parse2(args)
            .and_then(|args| build_config(&input, args, true, rt_multi_thread))
    };

    match config {
        Ok(config) => parse_knobs(input, true, config),
        Err(e) => token_stream_with_error(parse_knobs(input, true, DEFAULT_ERROR_CONFIG), e),
    }
}

struct ItemFn {
    outer_attrs: Vec<Attribute>,
    vis: Visibility,
    sig: Signature,
    brace_token: syn::token::Brace,
    inner_attrs: Vec<Attribute>,
    stmts: Vec<proc_macro2::TokenStream>,
}

impl ItemFn {
    /// Access all attributes of the function item.
    fn attrs(&self) -> impl Iterator<Item = &Attribute> {
        self.outer_attrs.iter().chain(self.inner_attrs.iter())
    }

    /// Get the body of the function item in a manner so that it can be
    /// conveniently used with the `quote!` macro.
    fn body(&self) -> Body<'_> {
        Body {
            brace_token: self.brace_token,
            stmts: &self.stmts,
        }
    }

    /// Convert our local function item into a token stream.
    fn into_tokens(
        self,
        generated_attrs: proc_macro2::TokenStream,
        body: proc_macro2::TokenStream,
        last_block: proc_macro2::TokenStream,
    ) -> TokenStream {
        let mut tokens = proc_macro2::TokenStream::new();
        // Outer attributes are simply streamed as-is.
        for attr in self.outer_attrs {
            attr.to_tokens(&mut tokens);
        }

        // Inner attributes require extra care, since they're not supported on
        // blocks (which is what we're expanded into) we instead lift them
        // outside of the function. This matches the behavior of `syn`.
        for mut attr in self.inner_attrs {
            attr.style = syn::AttrStyle::Outer;
            attr.to_tokens(&mut tokens);
        }

        // Add generated macros at the end, so macros processed later are aware of them.
        generated_attrs.to_tokens(&mut tokens);

        self.vis.to_tokens(&mut tokens);
        self.sig.to_tokens(&mut tokens);

        self.brace_token.surround(&mut tokens, |tokens| {
            body.to_tokens(tokens);
            last_block.to_tokens(tokens);
        });

        tokens
    }
}

impl Parse for ItemFn {
    #[inline]
    fn parse(input: ParseStream<'_>) -> syn::Result<Self> {
        // This parse implementation has been largely lifted from `syn`, with
        // the exception of:
        // * We don't have access to the plumbing necessary to parse inner
        //   attributes in-place.
        // * We do our own statements parsing to avoid recursively parsing
        //   entire statements and only look for the parts we're interested in.

        let outer_attrs = input.call(Attribute::parse_outer)?;
        let vis: Visibility = input.parse()?;
        let sig: Signature = input.parse()?;

        let content;
        let brace_token = braced!(content in input);
        let inner_attrs = Attribute::parse_inner(&content)?;

        let mut buf = proc_macro2::TokenStream::new();
        let mut stmts = Vec::new();

        while !content.is_empty() {
            if let Some(semi) = content.parse::<Option<syn::Token![;]>>()? {
                semi.to_tokens(&mut buf);
                stmts.push(buf);
                buf = proc_macro2::TokenStream::new();
                continue;
            }

            // Parse a single token tree and extend our current buffer with it.
            // This avoids parsing the entire content of the sub-tree.
            buf.extend([content.parse::<TokenTree>()?]);
        }

        if !buf.is_empty() {
            stmts.push(buf);
        }

        Ok(Self {
            outer_attrs,
            vis,
            sig,
            brace_token,
            inner_attrs,
            stmts,
        })
    }
}

struct Body<'a> {
    brace_token: syn::token::Brace,
    // Statements, with terminating `;`.
    stmts: &'a [TokenStream],
}

impl ToTokens for Body<'_> {
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
        self.brace_token.surround(tokens, |tokens| {
            for stmt in self.stmts {
                stmt.to_tokens(tokens);
            }
        });
    }
}
