tailcall-impl 2.0.3

The procedural macro implementation for the tailcall crate
Documentation 
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use proc_macro2::TokenStream;
use quote::quote;
use syn::{Error, ImplItemMethod, ItemFn, Signature};

use crate::{
    rewrite::TailPositionRewriter,
    signature::{function_argument_exprs, helper_signature, method_helper_signature},
};

pub fn apply_fn_tailcall_transform(item_fn: ItemFn) -> TokenStream {
    match TailcallTransform::new(item_fn).expand() {
        Ok(output) => output,
        Err(error) => error.to_compile_error(),
    }
}

pub fn apply_method_tailcall_transform(method: ImplItemMethod) -> TokenStream {
    match TailcallMethodTransform::new(method).expand() {
        Ok(output) => output,
        Err(error) => error.to_compile_error(),
    }
}

struct TailcallTransform {
    item_fn: ItemFn,
}

struct TailcallMethodTransform {
    method: ImplItemMethod,
}

impl TailcallMethodTransform {
    fn new(method: ImplItemMethod) -> Self {
        Self { method }
    }

    fn expand(self) -> Result<TokenStream, Error> {
        let ImplItemMethod {
            attrs,
            vis,
            defaultness,
            sig,
            block,
        } = self.method;

        reject_unsupported_signature(&sig)?;

        let helper_sig = method_helper_signature(&sig)?;
        let helper_fn_ident = &helper_sig.ident;
        let helper_args = function_argument_exprs(&sig)?;
        let helper_block = TailPositionRewriter::rewrite(block)?;

        Ok(quote! {
            #(#attrs)*
            #defaultness #vis #sig {
                Self::#helper_fn_ident(#(#helper_args),*).call()
            }

            #[doc(hidden)]
            #[inline(always)]
            #helper_sig {
                tailcall::Thunk::bounce(move || #helper_block)
            }
        })
    }
}

impl TailcallTransform {
    fn new(item_fn: ItemFn) -> Self {
        Self { item_fn }
    }

    fn expand(self) -> Result<TokenStream, Error> {
        let ItemFn {
            attrs,
            vis,
            sig,
            block,
        } = self.item_fn;

        reject_unsupported_signature(&sig)?;

        let helper_sig = helper_signature(&sig);
        let helper_fn_ident = &helper_sig.ident;
        let helper_args = function_argument_exprs(&sig)?;
        let helper_block = TailPositionRewriter::rewrite(*block)?;

        Ok(quote! {
            #(#attrs)*
            #vis #sig {
                #helper_fn_ident(#(#helper_args),*).call()
            }

            #[doc(hidden)]
            #[inline(always)]
            #helper_sig {
                tailcall::Thunk::bounce(move || #helper_block)
            }
        })
    }
}

fn reject_unsupported_signature(sig: &Signature) -> Result<(), Error> {
    if sig.constness.is_some() {
        return Err(Error::new_spanned(
            sig.constness,
            "#[tailcall] does not support const functions",
        ));
    }

    if sig.asyncness.is_some() {
        return Err(Error::new_spanned(
            sig.asyncness,
            "#[tailcall] does not support async functions",
        ));
    }

    Ok(())
}