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//! # proc-macro-error //! //! This crate aims to make error reporting in proc-macros simple and easy to use. //! Migrate from `panic!`-based errors for as little effort as possible! //! //! Also, there's ability to [append a dummy token stream][dummy] to your errors. //! //! ## Enticement //! //! Your errors look like this? //! ```text //! error: proc-macro derive panicked //! --> $DIR/bool_default_value.rs:11:10 //! | //! 11 | #[derive(StructOpt, Debug)] //! | ^^^^^^^^^ //! | //! = help: message: default_value is meaningless for bool //! ``` //! //! But you would like it to be like this! //! ```text //! error: default_value is meaningless for bool //! --> $DIR/bool_default_value.rs:14:24 //! | //! 14 | #[structopt(short, default_value = true)] //! | ^^^^^^^^^^^^^ //! ``` //! //! This is exactly what this crate is built for!!! //! //! ## Usage //! //! ### Panic-like usage //! //! ```rust //! # fn some_logic(_input: &DeriveInput) -> Result<(), Dummy> { unimplemented!() } //! # fn more_logic(_input: &DeriveInput) -> Result<(), Dummy> { unimplemented!() } //! # fn more_logic_for_logic_god(_input: &DeriveInput) -> bool { unimplemented!() } //! # struct Dummy { //! # span: proc_macro2::Span, //! # msg: String //! # } //! # impl Into<MacroError> for Dummy { //! # fn into(self) -> MacroError { unimplemented!() } //! # } //! use proc_macro_error::*; //! use proc_macro::TokenStream; //! use syn::{DeriveInput, parse_macro_input}; //! use quote::quote; //! //! # static _IGNORE: &str = " //! // This is your main entry point //! #[proc_macro] //! // this attribute *MUST* be placed on top of the #[proc_macro] function //! #[proc_macro_error] //! # "; //! pub fn make_answer(input: TokenStream) -> TokenStream { //! let input = parse_macro_input!(input as DeriveInput); //! //! if let Err(err) = some_logic(&input) { //! // we've got a span to blame, let's use it //! // This immediately aborts the proc-macro and shows the error //! abort!(err.span, "You made an error, go fix it: {}", err.msg); //! } //! //! // `Result` has some handy shortcuts if your error type implements //! // `Into<MacroError>`. `Option` has one unconditionally. //! more_logic(&input).expect_or_abort("What a careless user, behave!"); //! //! if !more_logic_for_logic_god(&input) { //! // We don't have an exact location this time, //! // so just highlight the proc-macro invocation itself //! abort_call_site!( //! "Bad, bad user! Now go stand in the corner and think about what you did!"); //! } //! //! // Now all the processing is done, return `proc_macro::TokenStream` //! quote!(/* stuff */).into() //! } //! ``` //! //! ### Multiple errors //! //! ```rust //! use proc_macro_error::*; //! use proc_macro::TokenStream; //! use syn::{spanned::Spanned, DeriveInput, ItemStruct, Fields, Attribute , parse_macro_input}; //! use quote::quote; //! //! # fn process_attr(_a: &Attribute) -> Result<Attribute, String> { unimplemented!() } //! fn process_attrs(attrs: &[Attribute]) -> Vec<Attribute> { //! attrs //! .iter() //! .filter_map(|attr| match process_attr(attr) { //! Ok(res) => Some(res), //! Err(msg) => { //! emit_error!(attr.span(), "Invalid attribute: {}", msg); //! None //! } //! }) //! .collect() //! } //! //! fn process_fields(_attrs: &Fields) -> Vec<TokenStream> { //! // processing fields in pretty much the same way as attributes //! unimplemented!() //! } //! //! # static _IGNORE: &str = " //! #[proc_macro] //! #[proc_macro_error] //! # "; //! pub fn make_answer(input: TokenStream) -> TokenStream { //! let input = parse_macro_input!(input as ItemStruct); //! let attrs = process_attrs(&input.attrs); //! //! // abort right now if some errors were encountered //! // at the attributes processing stage //! abort_if_dirty(); //! //! let fields = process_fields(&input.fields); //! //! // no need to think about emitted errors //! // #[proc_macro_error] will handle them for you //! // //! // just return a TokenStream as you normally would //! quote!(/* stuff */).into() //! } //! ``` //! //! ## Limitations //! //! - No support for warnings. //! - "help" suggestions cannot have their own span info. //! - If a panic occurs somewhere in your macro no errors will be displayed. //! //! ## Motivation //! //! Error handling in proc-macros sucks. It's not much of a choice today: //! you either "bubble up" the error up to the top-level of your macro and convert it to //! a [`compile_error!`][compl_err] invocation or just use a good old panic. Both these ways suck: //! //! - Former sucks because it's quite redundant to unroll a proper error handling //! just for critical errors that will crash the macro anyway so people mostly //! choose not to bother with it at all and use panic. Almost nobody does it, //! simple `.expect` is too tempting. //! //! - Later sucks because there's no way to carry out span info via `panic!`. `rustc` will highlight //! the whole invocation itself but not some specific token inside it. //! Furthermore, panics aren't for error-reporting at all; panics are for bug-detecting //! (like unwrapping on `None` or out-of range indexing) or for early development stages //! when you need a prototype ASAP and error handling can wait. Mixing these usages only //! messes things up. //! //! - There is [`proc_macro::Diagnostics`] which is awesome but it has been experimental //! for more than a year and is unlikely to be stabilized any time soon. //! //! This crate will be deprecated once `Diagnostics` is stable. //! //! That said, we need a solution, but this solution must meet these conditions: //! //! - It must be better than `panic!`. The main point: it must offer a way to carry span information //! over to user. //! - It must require as little effort as possible to migrate from `panic!`. Ideally, a new //! macro with the same semantics plus ability to carry out span info. A support for //! emitting multiple errors would be great too. //! - **It must be usable on stable**. //! //! This crate aims to provide such a mechanism. All you have to do is annotate your top-level //! `#[proc_macro]` function with `#[proc_macro_errors]` attribute and change panics to //! [`abort!`]/[`abort_call_site!`] where appropriate, see [**Usage**](#usage). //! //! ## Disclaimer //! Please note that **this crate is not intended to be used in any other way //! than a proc-macro error reporting**, use `Result` and `?` for anything else. //! //! [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html //! [`proc_macro::Diagnostics`]: https://doc.rust-lang.org/proc_macro/struct.Diagnostic.html // reexports for use in macros pub extern crate proc_macro; pub extern crate proc_macro2; pub mod dummy; pub mod multi; pub mod single; pub use self::dummy::set_dummy; pub use self::multi::abort_if_dirty; pub use self::single::MacroError; pub use proc_macro_error_attr::proc_macro_error; use quote::quote; use std::panic::{catch_unwind, resume_unwind, UnwindSafe}; use std::sync::atomic::{AtomicBool, Ordering}; /// This traits expands [`Result<T, Into<MacroError>>`](std::result::Result) with some handy shortcuts. pub trait ResultExt { type Ok; /// Behaves like [`Result::unwrap`]: if self is `Ok` yield the contained value, /// otherwise abort macro execution via [`abort!`]. fn unwrap_or_abort(self) -> Self::Ok; /// Behaves like [`Result::expect`]: if self is `Ok` yield the contained value, /// otherwise abort macro execution via [`abort!`]. /// If it aborts then resulting error message will be preceded with `message`. fn expect_or_abort(self, msg: &str) -> Self::Ok; } /// This traits expands [`Option<T>`][std::option::Option] with some handy shortcuts. pub trait OptionExt { type Some; /// Behaves like [`Option::expect`]: if self is `Some` yield the contained value, /// otherwise abort macro execution via [`abort_call_site!`]. /// If it aborts the `message` will be used for [`compile_error!`][compl_err] invocation. /// /// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html fn expect_or_abort(self, msg: &str) -> Self::Some; } impl<T> OptionExt for Option<T> { type Some = T; fn expect_or_abort(self, message: &str) -> T { match self { Some(res) => res, None => abort_call_site!(message), } } } /// This is the entry point for your proc-macro. It is **must** to be /// used on the top level of the proc-macro (a function annotated with /// `#[proc_macro*] attribute). /// /// Typically, you use `#[proc_macro_error]` instead, see [module level docs][self]. pub fn entry_point<F>(f: F) -> proc_macro::TokenStream where F: FnOnce() -> proc_macro::TokenStream + UnwindSafe, { ENTERED_ENTRY_POINT.with(|flag| flag.store(true, Ordering::SeqCst)); let caught = catch_unwind(f); let dummy = dummy::cleanup(); let err_storage = multi::cleanup(); ENTERED_ENTRY_POINT.with(|flag| flag.store(false, Ordering::SeqCst)); match caught { Ok(ts) => { if err_storage.is_empty() { ts } else { quote!( #(#err_storage)* #dummy ).into() } } Err(boxed) => match boxed.downcast::<AbortNow>() { Ok(_) => { assert!(!err_storage.is_empty()); quote!( #(#err_storage)* #dummy ).into() } Err(boxed) => resume_unwind(boxed), }, } } thread_local! { static ENTERED_ENTRY_POINT: AtomicBool = AtomicBool::new(false); } struct AbortNow; fn check_correctness() { if !ENTERED_ENTRY_POINT.with(|flag| flag.load(Ordering::SeqCst)) { panic!("proc-macro-error API cannot be used outside of `entry_point` invocation. Perhaps you forgot to annotate your #[proc_macro] function with `#[proc_macro_error]"); } }