Crate synthez

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 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 )* }
    }
}

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.

Re-exports

pub use synthez_core::proc_macro2;

pub use synthez_core::quote;

pub use synthez_core::syn;

Modules

ext

Extensions for syn types.

field

Batteries for working with struct fields.

has

Abstraction over syn types containing something.

parse

Batteries for syn::parse.

spanned

Batteries for Span and syn::spanned.

Structs

Required

Container requiring a field to have a value mandatory.

Spanning

Wrapper for non-Spanned types to hold their Span.

Traits

DataExt

Extension of a syn::Data.

IdentExt

Extension of a syn::Ident.

ParseAttrs

Parseing of syn::Attributes into a custom defined struct.

ParseBufferExt

Extension of a syn::parse::ParseBuffer providing common function widely used by this crate for parsing.

ToTokens

Types that can be interpolated inside a quote! invocation.

Derive Macros

ParseAttrs

Deriving of synthez::ParseAttrs along with a [syn::parse::Parse] implementation to parse [syn::Attribute]s into a custom defined struct.

ToTokens

Deriving of a quote::ToTokens implementation.