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//! Struct fields that can be read and written from within the same module, but //! from outside the module can only be read. //! //! # Syntax //! //! Place `#[readonly::make]` on a braced struct or tuple struct. This will make //! all fields of the struct publicly readable according to their individual //! visibility specifiers, but not writable from other modules. //! //! ``` //! mod m { //! #[readonly::make] //! pub struct S { //! pub n: i32, //! } //! //! impl S { //! pub fn demo(&mut self) { //! // Can read and write from inside the same module. //! println!("{}", self.n); //! self.n += 1; //! } //! } //! } //! //! fn demo(s: &mut m::S) { //! // From outside the module, can only read. //! println!("{}", s.n); //! //! // Does not compile: //! //s.n += 1; //! } //! ``` //! //! The error appears as follows. //! //! ```console //! error[E0594]: cannot assign to data in a `&` reference //! --> readme.rs:21:5 //! | //! 21 | s.n += 1; //! | ^^^^^^^^ cannot assign //! ``` //! //! Optionally, place `#[readonly]` on individual struct fields to make just //! those fields publicly readable, without affecting other fields of the //! struct. //! //! ``` //! # mod m { //! #[readonly::make] //! pub struct S { //! // This field can be read (but not written) by super. //! #[readonly] //! pub(super) readable: i32, //! //! // This field can be neither read nor written by other modules. //! private: i32, //! } //! # } //! ``` extern crate proc_macro; mod args; mod expand; use crate::args::Args; use proc_macro::TokenStream; use quote::quote; use syn::{parse_macro_input, DeriveInput}; #[proc_macro_attribute] pub fn make(args: TokenStream, tokens: TokenStream) -> TokenStream { let original = tokens.clone(); let args = parse_macro_input!(args as Args); let input = parse_macro_input!(tokens as DeriveInput); expand::readonly(args, input) .unwrap_or_else(|e| { let original = proc_macro2::TokenStream::from(original); let compile_error = e.to_compile_error(); quote! { #original #compile_error } }) .into() }