# 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 [ability to append a dummy token stream][dummy] to your errors.
## Usage
In your `Cargo.toml`:
```toml
proc-macro-error = "0.2"
```
In `lib.rs`:
```rust,ignore
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 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`](https://doc.rust-lang.org/proc_macro/struct.Diagnostic.html)
but it's experimental. (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.
- 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](#usage)
# How it works
Effectively, it emulates try-catch mechanism on the top of panics.
Essentially, the [`filter_macro_errors!`] macro is (C++ like pseudo-code)
```C++
try {
/* your code */
} catch (MacroError) {
/* conversion to compile_error! */
} catch (MultiMacroErrors) {
/* conversion to multiple compile_error! invocations */
}
```
[`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 weren't triggered by [`span_error!`] and co 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.
[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)