comperr 0.2.0

A minimal, lightweight crate for emitting span-accurate compile-time errors from procedural macros.
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<div align="center">

# comperr

[![Rust Version](https://img.shields.io/badge/rustc-2024%20edition-blue.svg)](https://doc.rust-lang.org/edition-guide/rust-2024/)
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[![License MIT](https://img.shields.io/crates/l/comperr)](https://codeberg.org/razkar/comperr/src/branch/main/LICENSE-MIT)
[![License Apache-2.0](https://img.shields.io/crates/l/comperr)](https://codeberg.org/razkar/comperr/src/branch/main/LICENSE-APACHE)
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A minimal, zero dependency crate for emitting span-accurate compile-time errors from procedural macros.

```rust
return error(span, "expected a string literal");
```

</div>

## What is this?

When writing proc-macros, emitting a `compile_error!` the naive way loses the span, meaning the error points at the wrong place in the user's source code. `comperr` constructs the error token by token, attaching the correct span to each one so the diagnostic lands exactly where you want it.

Zero dependencies. Built for proc-macro authors.

## Installation

```sh
cargo add comperr
```

## Quick Start

One-shot with the free function:

```rust
use comperr::error;
use proc_macro2::{Span, TokenStream};

pub fn my_macro(input: TokenStream) -> TokenStream {
    return error(Span::call_site(), "something went wrong");
}
```

Or using the `Error` struct directly:

```rust
use comperr::Error;
use proc_macro2::{Span, TokenStream};

pub fn my_macro(input: TokenStream) -> TokenStream {
    let e = Error::new(Span::call_site(), "something went wrong");
    return e.to_compile_error();
}
```

Batch multiple errors together:

```rust
use comperr::Error;
use proc_macro2::{Span, TokenStream};

pub fn my_macro(input: TokenStream) -> TokenStream {
    let mut err = Error::new(Span::call_site(), "first error");
    err.combine(Error::new(Span::call_site(), "second error"));
    return err.to_compile_error();
}
```

## Features

* **Span-accurate `compile_error!` emission**
* **Free function for one-shot use**
* **`Error` struct for more control**
* **Batch multiple errors with `Error::combine()`**
* **Accepts any `Into<String>` message**
* **One-dep, super lightweight**

## How It Works

Every token in a `TokenStream` carries a `Span` that tells the compiler where in the source code that token came from. When `compile_error!` is invoked, the compiler reads the span off the tokens it receives and uses that to place the diagnostic. By constructing each token manually and calling `.set_span()` on it, the resulting error points at the original source location rather than at a meaningless internal position.

## Requirements

- Rust 2024 edition
- A proc-macro crate (this library is for macro authors, not end users)

## License

Licensed under either of:

- Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or <http://www.apache.org/licenses/LICENSE-2.0>)
- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)

at your option.

Cheers, RazkarStudio
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