synthez

API Docs | Changelog

Steroids for syn, quote and proc_macro2 crates.

Cargo features

full

Same as full feature of syn crate.

Enables support of data structures for representing the syntax tree of all valid Rust source code, including items and expressions.

Example of writing proc_macro_derive

This is an example of how this library can be used to write a simplified proc_macro_derive for deriving a From implementations.

# use std::collections::HashMap;
#
# use synthez::{proc_macro2::{Span, TokenStream}, quote::quote, syn};
use synthez::{DataExt as _, ParseAttrs, ToTokens};

pub fn derive(input: syn::DeriveInput) -> syn::Result<TokenStream> {
    let attrs = Attrs::parse_attrs("from", &input)?;
    match (attrs.forward.is_some(), !attrs.custom.is_empty()) {
        (true, true) => Err(syn::Error::new_spanned(
            input,
            "`forward` and `on` arguments are mutually exclusive",
        )),
        (false, false) => Err(syn::Error::new_spanned(
            input,
            "either `forward` or `on` argument is expected",
        )),

        // #[from(forward)]
        (true, _) => {
            if !matches!(&input.data, syn::Data::Struct(_)) {
                return Err(syn::Error::new_spanned(
                    input,
                    "only tuple structs can forward-derive From",
                ));
            }
            let fields = input.data.unnamed_fields()?;
            if fields.len() > 1 {
                return Err(syn::Error::new_spanned(
                    fields,
                    "only single-field tuple structs can forward-derive \
                     From",
                ));
            }
            let definition = ForwardDefinition {
                ty: input.ident,
                inner_ty: fields.into_iter().last().unwrap().ty,
            };
            Ok(quote! {
                #definition
            })
        }

        // #[from(on <type> = <func>)]
        (_, true) => {
            let definitions =
                CustomDefinitions { ty: input.ident, funcs: attrs.custom };
            Ok(quote! {
                #definitions
            })
        }
    }
}

#[derive(Default, ParseAttrs)]
struct Attrs {
    #[parse(ident)]
    forward: Option<syn::Ident>,
    #[parse(map, arg = on)]
    custom: HashMap<syn::Type, syn::Expr>,
}

#[derive(ToTokens)]
#[to_tokens(append(impl_from))]
struct ForwardDefinition {
    ty: syn::Ident,
    inner_ty: syn::Type,
}

impl ForwardDefinition {
    fn impl_from(&self) -> TokenStream {
        let (ty, inner_ty) = (&self.ty, &self.inner_ty);
        quote! {
            impl<T> From<T> for #ty where #inner_ty: From<T> {
                fn from(v: T) -> Self {
                    Self(v.into())
                }
            }
        }
    }
}

#[derive(ToTokens)]
#[to_tokens(append(impl_froms))]
struct CustomDefinitions {
    ty: syn::Ident,
    funcs: HashMap<syn::Type, syn::Expr>,
}

impl CustomDefinitions {
    fn impl_froms(&self) -> TokenStream {
        let ty = &self.ty;
        // We sort here for tests below not failing due to undetermined
        // order only. Real implementation may omit this.
        let mut sorted = self.funcs.iter().collect::<Vec<_>>();
        sorted.sort_unstable_by(|(ty1, _), (ty2, _)| {
            quote!(#ty1).to_string().cmp(&quote!(#ty2).to_string())
        });
        let impls = sorted.into_iter().map(move |(from_ty, func)| {
            quote! {
                impl From<#from_ty> for #ty {
                    fn from(v: #from_ty) -> Self {
                        #func(v)
                    }
                }
            }
        });
        quote! { #( #impls )* }
    }
}

# fn main() {
let input = syn::parse_quote! {
    #[derive(From)]
    #[from(forward)]
    struct Id(u64);
};
let output = quote! {
    impl<T> From<T> for Id where u64: From<T> {
        fn from(v: T) -> Self {
            Self(v.into())
        }
    }
};
assert_eq!(derive(input).unwrap().to_string(), output.to_string());

let input = syn::parse_quote! {
    #[derive(From)]
    #[from(on bool = Self::parse_bool)]
    #[from(on u8 = from_u8_to_maybe)]
    enum Maybe {
        Yes,
        No,
    }
};
let output = quote! {
    impl From<bool> for Maybe {
        fn from(v: bool) -> Self {
            Self::parse_bool(v)
        }
    }
    impl From<u8> for Maybe {
        fn from(v: u8) -> Self {
            from_u8_to_maybe(v)
        }
    }
};
assert_eq!(derive(input).unwrap().to_string(), output.to_string());
# }

License

This software is subject to the terms of the Blue Oak Model License 1.0.0. If a copy of the BlueOak-1.0.0 license was not distributed with this file, You can obtain one at https://blueoakcouncil.org/license/1.0.0.