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extern crate proc_macro; use proc_macro::TokenStream; use quote::{quote, quote_spanned}; use syn::parse_macro_input; use syn::spanned::Spanned; macro_rules! err { ($meta: expr) => { err!($meta, r#"expected `def = "..."`"#) }; ($meta: expr, $err: expr) => { Err(syn::Error::new_spanned($meta, $err)) }; } #[proc_macro_derive(Defaults, attributes(def))] pub fn derive(input: TokenStream) -> TokenStream { let ast = parse_macro_input!(input as syn::DeriveInput); let name = &ast.ident; let tts = match &ast.data { syn::Data::Struct(s) => derive_struct(name, &s.fields), syn::Data::Enum(_) => match derive_enum(name, &ast.attrs) { Ok(tts) => tts, Err(e) => e.to_compile_error(), }, _ => syn::Error::new_spanned( name, "`Defaults` cannot be derived for unions, only structs and enums", ) .to_compile_error(), }; tts.into() } fn derive_struct(name: &syn::Ident, fields: &syn::Fields) -> proc_macro2::TokenStream { let tts = match fields { syn::Fields::Named(fields) => struct_named_impl(name, fields), syn::Fields::Unnamed(fields) => struct_unnamed_impl(name, fields), syn::Fields::Unit => struct_unit_impl(name), }; match tts { Ok(tts) => tts, Err(e) => e.to_compile_error(), } } fn derive_enum( name: &proc_macro2::Ident, attrs: &[syn::Attribute], ) -> syn::Result<proc_macro2::TokenStream> { match def_attr(attrs)? { Some(attr) => { let def_val = def_val_of(attr)?; Ok(quote_spanned! { def_val.span() => impl std::default::Default for #name { fn default() -> Self { Self::#def_val } } }) } None => err!( name, "Explicit default is required for enums.\nhelp: Add `#[def = \"...\"]` to the enum definition" ), } } fn def_attr(attrs: &[syn::Attribute]) -> syn::Result<Option<&syn::Attribute>> { let mut out = None; for attr in attrs { if attr.path.segments.len() == 1 && attr.path.segments[0].ident == "def" { if out.is_none() { out = Some(attr); } else { return err!(attr, r#"multiple definitions of `def` found"#); } } } Ok(out) } fn def_val_of(attr: &syn::Attribute) -> syn::Result<syn::Expr> { let nv = match attr.parse_meta()? { syn::Meta::NameValue(nv) => nv, meta => return err!(meta), }; match &nv.lit { syn::Lit::Str(s) => match s.parse::<syn::Expr>() { Ok(expr) => Ok(expr), _ => err!(s, "Not a valid expression"), }, _ => err!(nv), } } fn struct_unit_impl(name: &proc_macro2::Ident) -> syn::Result<proc_macro2::TokenStream> { let val = quote! { impl std::default::Default for #name { fn default() -> Self { Self } } }; Ok(val) } fn struct_named_impl( name: &proc_macro2::Ident, fields: &syn::FieldsNamed, ) -> syn::Result<proc_macro2::TokenStream> { let mut mapped = vec![]; for f in &fields.named { let name = &f.ident; let mapped_field = match def_attr(&f.attrs)? { Some(attr) => { let def_val = def_val_of(attr)?; quote_spanned! {f.span() => #name: #def_val } } None => quote_spanned! {f.span() => #name: Default::default() }, }; mapped.push(mapped_field); } let out = quote! { impl std::default::Default for #name { fn default() -> Self { Self { #(#mapped,)* } } } }; Ok(out) } fn struct_unnamed_impl( name: &proc_macro2::Ident, fields: &syn::FieldsUnnamed, ) -> syn::Result<proc_macro2::TokenStream> { let mut mapped = vec![]; for f in &fields.unnamed { let f = match def_attr(&f.attrs)? { Some(attr) => { let def_val = def_val_of(attr)?; quote_spanned! {f.span() => #def_val } } None => quote_spanned! {f.span() => Default::default() }, }; mapped.push(f); } let out = quote! { impl std::default::Default for #name { fn default() -> Self { Self ( #(#mapped,)* ) } } }; Ok(out) }