commit	b301802755c45592675d57cad062f7207b4d952a	[log] [tgz]
author	CreepySkeleton <creepy-skeleton@yandex.ru>	Thu Aug 15 15:39:45 2019
committer	CreepySkeleton <creepy-skeleton@yandex.ru>	Thu Aug 15 15:39:45 2019
tree	04f7a96df9835d133caa25b82aca70d4956ebc18
parent	842d134b0439f5c12a208f4a1271195e2bc888a2 [diff]

tree: 04f7a96df9835d133caa25b82aca70d4956ebc18

README.md

proc-macro-error

This crate aims to provide an error reporting mechanism that is usable inside proc-macros, can highlight a specific span, and can be migrated from panic!-based errors with minimal efforts.

Also, there's a facility to report multiple errors.

Usage

In your Cargo.toml:

proc-macro-error = "0.1"

In lib.rs:

extern crate proc_macro_error;
use proc_macro_error::{
    filter_macro_errors,
    span_error,
    call_site_error,
    ResultExt,
    OptionExt
};

// 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 one 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`!
}

Motivation and Getting started

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! 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 but it's experimental.

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.
It must be usable on stable.

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, see Usage

How it works

Effectively, it emulates try-catch mechanism on top of panics.

Essentially, the filter_macro_errors! macro is a

try {
    /* your code */
} catch (MacroError) {
    /* conversion to compile_error! */
}

span_error! and co are

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! 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.