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//! Drop-in replacement to panics in proc-macros. //! //! # Motivation //! //! Error handling in proc-macros sucks. It's not much of a choice today: //! you either "bubble up" the error up to top-level of you 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. I have yet to see a crate > 700 //! lines of code that 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. //! //! # Solution //! //! That said, we need a solution, but this solution must meet these conditions: //! //! - It must be better than panics. 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. //! //! This crate aims to provide such a mechanism. All you have to do is enclose all //! the code inside your top-level `#[proc_macro]` function in [`filter_macro_errors!`] //! invocation and change panics to [`span_error!`]/[`call_site_error!`] where appropriate: //! //! ```rust, ignore //! // This is your main entry point //! #[proc_macro] //! pub fn make_answer(input: TokenStream) -> TokenStream { //! // This macro **must** be placed at the top level. //! // No need to touch the code inside though. //! filter_macro_errors! { //! // `parse_macro_input!` and friends work just fine inside this macro //! let input = parse_macro_input!(input as MyParser); //! //! if let Err(err) = some_logic(&input) { //! // we've got a span to blame, let's use it //! let span = err.span_should_be_highlighted(); //! let msg = err.message(); //! // This call jumps directly to the end of `filter_macro_errors!` invocation //! span_error!(span, "You made an error, go fix it: {}", msg); //! } //! //! // `Result` gets some handy shortcuts if your error type implements //! // `Into<``MacroError``>`. `Option` has some unconditionally //! use proc_macro_error::ResultExt; //! more_logic(&input).expect_or_exit("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 //! call_site_error!( //! "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() //! } //! //! // At this point we have a new shining `proc_macro::TokenStream`! //! } //! ``` //! //! # How it works //! I must confess: I used panics as a try/catch mechanism. I've committed this //! sin so others may live in peace and prosperity, god save my soul. //! //! Essentially, the [`filter_macro_errors!`] macro is a //! ```C++ //! try { //! /* your code */ //! } catch (MacroError) { //! /* conversion to compile_error! */ //! } //! ``` //! //! [`span_error!`] and co are //! ```C++ //! throw MacroError::new(span, format!(msg...)); //! ``` //! //! By calling [`span_error!`] you trigger panic //! that will be caught by [`filter_macro_errors!`] and converted to [`compile_error!`][compl_err] invocation. //! All the panics that wasn't triggered by [`span_error!`] and co but any other reason will be resumed as is. //! //! Panic catching is indeed *slow* but the macro is about to abort anyway so speed is not //! a concern here. Please note that this crate is not intended to be used in any other way //! than a proc-macro error reporting, use [`Result`] and `?` instead. //! //! # Testing //! TODO: fork https://github.com/laumann/compiletest-rs and make it understand explicit line numbers. //! //! [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html //! // reexports for use in macros pub extern crate proc_macro; pub extern crate proc_macro2; extern crate quote; extern crate syn; use proc_macro2::{Span, TokenStream}; use quote::quote_spanned; /// Makes a [`MacroError`] instance from provided arguments (`panic!`-like) /// and triggers panic in hope it will be caught by [`filter_macro_errors!`]. /// /// # Syntax /// /// This macro is meant to be a `panic!` drop-in replacement so its syntax is very similar to `panic!`, /// but it has three forms instead of two: /// 1. "panic-format-like" form: span, formatting [`str`] literal, comma-separated list of args. /// First argument is a span, all the rest gets passed to [`format!`] to build the error message. /// 2. "panic-single-arg-like" form: span, expr, no comma at the end. /// First argument is a span, the second is our error message, it must implement [`ToString`]. /// 3. "trigger_error-like" form: single expr. /// Literally `trigger_error(arg)`. It's here just for convenience so [`span_error!`] can be used /// with instances of [`syn::Error`], [`MacroError`], [`&str`], [`String`] and so on... /// #[macro_export] macro_rules! span_error { ($span:expr, $fmt:literal, $($args:expr),*) => {{ let msg = format!($fmt, $($args),*); // we use $span.into() so it would work with proc_macro::Span and // proc_macro2::Span all the same let err = $crate::MacroError::new($span.into(), msg); $crate::trigger_error(err) }}; ($span:expr, $msg:expr) => {{ // we use $span.into() so it would work with proc_macro::Span and // proc_macro2::Span all the same let err = $crate::MacroError::new($span.into(), $msg.to_string()); $crate::trigger_error(err) }}; ($err:expr) => { $crate::trigger_error($err) }; } /// Shortcut for `span_error!(Span::call_site(), msg...)`. This macro /// is still preferable over plain panic, see [Motivation](#motivation) #[macro_export] macro_rules! call_site_error { ($fmt:literal, $($args:expr),*) => {{ use $crate::span_error; let span = $crate::proc_macro2::Span::call_site(); span_error!(span, $fmt, $($args),*) }}; ($fmt:expr) => {{ use $crate::span_error; let span = $crate::proc_macro2::Span::call_site(); span_error!(span, $fmt) }}; } /// This macro is supposed to be used at the top level of your `proc-macro`, /// the function marked with a `#[proc_macro*]` attribute. It catches all the /// errors triggered by [`span_error!`], [`call_site_error!`], and [`trigger_error`]. /// Once caught, it converts it to a [`proc_macro::TokenStream`] /// containing a [`compile_error!`][compl_err] invocation. /// /// See the [module-level documentation](self) for usage example /// /// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html #[macro_export] macro_rules! filter_macro_errors { ($($code:tt)*) => { let f = move || -> $crate::proc_macro::TokenStream { $($code)* }; $crate::filter_macro_error_panics(f) }; } /// An error in a proc-macro. This struct preserves /// the given span so `rustc` can highlight the exact place in user code /// responsible for the error. /// /// You're not supposed to use this type directly, use [`span_error!`] and [`call_site_error!`]. #[derive(Debug)] pub struct MacroError { span: Span, msg: String, } impl MacroError { /// Create an error with the span and message provided. pub fn new(span: Span, msg: String) -> Self { MacroError { span, msg } } /// A shortcut for `MacroError::new(Span::call_site(), message)` pub fn call_site(msg: String) -> Self { MacroError::new(Span::call_site(), msg) } /// Convert this error into a [`TokenStream`] containing these tokens: `compiler_error!(<message>);`. /// All these tokens carry the span this error contains attached. /// /// [compl_err]: https://doc.rust-lang.org/std/macro.compile_error.html pub fn into_compile_error(self) -> TokenStream { let MacroError { span, msg } = self; quote_spanned! { span=> compile_error!(#msg); } } /// Abandon the old span and replace it with the given one. pub fn set_span(&mut self, span: Span) { self.span = span; } } /// 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 [`span_error!`]. fn unwrap_or_exit(self) -> Self::Ok; /// Behaves like [`Result::expect`]: if self is `Ok` yield the contained value, /// otherwise abort macro execution via [`span_error!`]. /// If it aborts then resulting message will be preceded with `message`. fn expect_or_exit(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 [`call_site_error!`]. /// 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_exit(self, msg: &str) -> Self::Some; } impl<T, E: Into<MacroError>> ResultExt for Result<T, E> { type Ok = T; fn unwrap_or_exit(self) -> T { match self { Ok(res) => res, Err(e) => trigger_error(e), } } fn expect_or_exit(self, message: &str) -> T { match self { Ok(res) => res, Err(e) => { let MacroError { msg, span } = e.into(); let msg = format!("{}: {}", message, msg); trigger_error(MacroError::new(span, msg)) } } } } impl<T> OptionExt for Option<T> { type Some = T; fn expect_or_exit(self, message: &str) -> T { match self { Some(res) => res, None => call_site_error!(message), } } } impl From<syn::Error> for MacroError { fn from(e: syn::Error) -> Self { MacroError::new(e.span(), e.to_string()) } } impl From<String> for MacroError { fn from(msg: String) -> Self { MacroError::call_site(msg) } } impl From<&str> for MacroError { fn from(msg: &str) -> Self { MacroError::call_site(msg.into()) } } impl ToString for MacroError { fn to_string(&self) -> String { self.msg.clone() } } /// Trigger error, aborting the proc-macro's execution. /// /// You're not supposed to use this function directly. /// Use [`span_error!`] or [`call_site_error!`] instead. pub fn trigger_error<T: Into<MacroError>>(err: T) -> ! { panic!(Payload(err.into())) } /// Execute the closure and catch all the panics triggered by /// [`trigger_error`], converting them to [`proc_macro::TokenStream`]. /// All the other panics will be passed through as is. /// /// You're not supposed to use this function directly, use [`filter_macro_errors!`] /// instead. pub fn filter_macro_error_panics<F>(f: F) -> proc_macro::TokenStream where F: FnOnce() -> proc_macro::TokenStream, { use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe}; match catch_unwind(AssertUnwindSafe(f)) { Ok(stream) => stream, Err(boxed) => match boxed.downcast::<Payload>() { Ok(err) => err.0.into_compile_error().into(), Err(other) => resume_unwind(other), }, } } struct Payload(MacroError); // SAFE: Payload is private, a user can't use it to make any harm. unsafe impl Send for Payload {} unsafe impl Sync for Payload {}