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use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::quote;
use syn::{
    parse, parse_macro_input, spanned::Spanned, Ident, ItemFn, ReturnType, Type, Visibility,
};

#[proc_macro_attribute]
pub fn entry(args: TokenStream, input: TokenStream) -> TokenStream {
    let f = parse_macro_input!(input as ItemFn);

    let valid_signature = f.sig.constness.is_none()
        && f.vis == Visibility::Inherited
        && f.sig.abi.is_none()
        && f.sig.inputs.is_empty()
        && f.sig.generics.params.is_empty()
        && f.sig.generics.where_clause.is_none()
        && f.sig.variadic.is_none()
        && match f.sig.output {
            ReturnType::Default => false,
            ReturnType::Type(_, ref ty) => matches! {**ty, Type::Never(_)},
        };

    if !valid_signature {
        return parse::Error::new(
            f.span(),
            "`#[entry]` function must have signature `[unsafe] fn() -> !`",
        )
        .to_compile_error()
        .into();
    }

    if !args.is_empty() {
        return parse::Error::new(Span::call_site(), "This attribute accepts no arguments")
            .to_compile_error()
            .into();
    }

    let tramp_ident = Ident::new(&format!("{}_trampoline", f.sig.ident), Span::call_site());
    let ident = &f.sig.ident;

    quote! {
        global_asm!(
            ".section   .head.text, \"ax\"",
            "_head:",
            "   b   stext",
            "   .long   0",
            ".global stext;",
            ".align 2",
            "stext:",
            "   mov x1, #3 << 20",
            "   msr cpacr_el1, x1   // Enable FP/ASIMD",
            "   adrp x4, __top_stack",
            "   mov x1, x4",
            "   mov sp, x1",
            "   bl  clear_bss",
            "   b   main",
            ".type stext, @function;",
            ".size stext, .-stext",
        );

        #[doc(hidden)]
        #[export_name = "main"]
        pub unsafe extern "C" fn #tramp_ident() {
            #ident()
        }

        #f
    }
    .into()
}