embassy/embassy-executor-macros/src/macros/main.rs

use std::str::FromStr;

use darling::export::NestedMeta;
use darling::FromMeta;
use proc_macro2::TokenStream;
use quote::quote;
use syn::{ReturnType, Type};

use crate::util::*;

enum Flavor {
    Standard,
    Wasm,
}

pub(crate) struct Arch {
    default_entry: Option<&'static str>,
    flavor: Flavor,
    executor_required: bool,
}

pub static ARCH_AVR: Arch = Arch {
    default_entry: Some("avr_device::entry"),
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_RISCV: Arch = Arch {
    default_entry: Some("riscv_rt::entry"),
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_CORTEX_M: Arch = Arch {
    default_entry: Some("cortex_m_rt::entry"),
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_CORTEX_AR: Arch = Arch {
    default_entry: None,
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_SPIN: Arch = Arch {
    default_entry: None,
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_STD: Arch = Arch {
    default_entry: None,
    flavor: Flavor::Standard,
    executor_required: false,
};

pub static ARCH_WASM: Arch = Arch {
    default_entry: Some("wasm_bindgen::prelude::wasm_bindgen(start)"),
    flavor: Flavor::Wasm,
    executor_required: false,
};

pub static ARCH_UNSPECIFIED: Arch = Arch {
    default_entry: None,
    flavor: Flavor::Standard,
    executor_required: true,
};

#[derive(Debug, FromMeta, Default)]
struct Args {
    #[darling(default)]
    entry: Option<String>,
    #[darling(default)]
    executor: Option<String>,
}

pub fn run(args: TokenStream, item: TokenStream, arch: &Arch) -> TokenStream {
    let mut errors = TokenStream::new();

    // 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 f: ItemFn = match syn::parse2(item.clone()) {
        Ok(x) => x,
        Err(e) => return token_stream_with_error(item, e),
    };

    let args = match NestedMeta::parse_meta_list(args) {
        Ok(x) => x,
        Err(e) => return token_stream_with_error(item, e),
    };

    let args = match Args::from_list(&args) {
        Ok(x) => x,
        Err(e) => {
            errors.extend(e.write_errors());
            Args::default()
        }
    };

    let fargs = f.sig.inputs.clone();

    if f.sig.asyncness.is_none() {
        error(&mut errors, &f.sig, "main function must be async");
    }
    if !f.sig.generics.params.is_empty() {
        error(&mut errors, &f.sig, "main function must not be generic");
    }
    if !f.sig.generics.where_clause.is_none() {
        error(&mut errors, &f.sig, "main function must not have `where` clauses");
    }
    if !f.sig.abi.is_none() {
        error(&mut errors, &f.sig, "main function must not have an ABI qualifier");
    }
    if !f.sig.variadic.is_none() {
        error(&mut errors, &f.sig, "main function must not be variadic");
    }
    match &f.sig.output {
        ReturnType::Default => {}
        ReturnType::Type(_, ty) => match &**ty {
            Type::Tuple(tuple) if tuple.elems.is_empty() => {}
            Type::Never(_) => {}
            _ => error(
                &mut errors,
                &f.sig,
                "main function must either not return a value, return `()` or return `!`",
            ),
        },
    }

    if fargs.len() != 1 {
        error(&mut errors, &f.sig, "main function must have 1 argument: the spawner.");
    }

    let entry = match (args.entry.as_deref(), arch.default_entry.as_deref()) {
        (None, None) => TokenStream::new(),
        (Some(x), _) | (None, Some(x)) if x == "" => TokenStream::new(),
        (Some(x), _) | (None, Some(x)) => match TokenStream::from_str(x) {
            Ok(x) => quote!(#[#x]),
            Err(e) => {
                error(&mut errors, &f.sig, e);
                TokenStream::new()
            }
        },
    };

    let executor = match (args.executor.as_deref(), arch.executor_required) {
        (None, true) => {
            error(
                &mut errors,
                &f.sig,
                "\
No architecture selected for embassy-executor. Make sure you've enabled one of the `arch-*` features in your Cargo.toml.

Alternatively, if you would like to use a custom executor implementation, specify it with the `executor` argument.
For example: `#[embassy_executor::main(entry = ..., executor = \"some_crate::Executor\")]",
            );
            ""
        }
        (Some(x), _) => x,
        (None, _) => "::embassy_executor::Executor",
    };

    let executor = TokenStream::from_str(executor).unwrap_or_else(|e| {
        error(&mut errors, &f.sig, e);
        TokenStream::new()
    });

    let f_body = f.body;
    let out = &f.sig.output;

    let (main_ret, mut main_body) = match arch.flavor {
        Flavor::Standard => (
            quote!(!),
            quote! {
                unsafe fn __make_static<T>(t: &mut T) -> &'static mut T {
                    ::core::mem::transmute(t)
                }

                let mut executor = #executor::new();
                let executor = unsafe { __make_static(&mut executor) };
                executor.run(|spawner| {
                    spawner.must_spawn(__embassy_main(spawner));
                })
            },
        ),
        Flavor::Wasm => (
            quote!(Result<(), wasm_bindgen::JsValue>),
            quote! {
                let executor = ::std::boxed::Box::leak(::std::boxed::Box::new(#executor::new()));

                executor.start(|spawner| {
                    spawner.must_spawn(__embassy_main(spawner));
                });

                Ok(())
            },
        ),
    };

    let mut main_attrs = TokenStream::new();
    for attr in f.attrs {
        main_attrs.extend(quote!(#attr));
    }

    if !errors.is_empty() {
        main_body = quote! {loop{}};
    }

    let result = quote! {
        #[::embassy_executor::task()]
        #[allow(clippy::future_not_send)]
        async fn __embassy_main(#fargs) #out {
            #f_body
        }

        #entry
        #main_attrs
        fn main() -> #main_ret {
            #main_body
        }

        #errors
    };

    result
}