wherr Crate Documentation

wherr extends Rust's ? operator to append file and line number details to errors, aiding in debugging.

wherr is composed of two separate crates, due to Rust's limitation that prohibits mixing normal functions with procedural macros in a single crate:

1. wherr: This is the main library that offers the enhanced functionality for error handling in Rust.
2. wherr-macro: Contains the procedural macros specifically designed for the wherr crate.

Table of Contents

• Quick Start
• How it works
• Usage
  • Without #[wherr]
  • With #[wherr]
• API Reference
  • Wherr
  • wherrapper
  • wherr procedural macro
• Contributing
• License

Quick Start

Add the wherr crate to your Cargo.toml:

[dependencies]
wherr = "0.1"

Now, by simply annotating your functions with #[wherr], any error propagated using ? will also include the file and line number.

use wherr::wherr;

#[wherr]
fn some_function() -> Result<(), Box<dyn std::error::Error>> {
    /* ... */
    some_operation()?;
    /* ... */
}

How it works

The #[wherr] notation is a proc_macro_attribute, a powerful tool in Rust that allows for custom transformations of the code at compile time.

When an error is propagated with the ? operator in a function decorated with #[wherr], it captures the file and line number where the error occurred. This provides developers with a more informative error, aiding in precise debugging.

For instance, consider this function:

#[wherr]
fn add(s1: &str, s2: &str) -> Result<i64, Box<dyn std::error::Error>> {
    let radix = 10;
    let i1 = i64::from_str_radix(s1, radix)?;
    let i2 = i64::from_str_radix(s2, radix)?;
    Ok(i1 + i2)
}

Under the hood, the #[wherr] macro transforms this function to:

fn add(s1: &str, s2: &str) -> Result<i64, Box<dyn std::error::Error>> {
    let radix = 10;
    let i1 = wherr::wherrapper(i64::from_str_radix(s1, radix), file!(), line!())?;
    let i2 = wherr::wherrapper(i64::from_str_radix(s2, radix), file!(), line!())?;
    Ok(i1 + i2)
}

Now, you may be wondering: How does this wherrapper function make all this happen?

The wherrapper function receives the original Result, the file, and the line number. If the Result is an Ok, it's simply returned unchanged. For an Err, however, things are a bit more intricate. If the error hasn't already been wrapped in a Wherr type (meaning it doesn't yet have the file and line details), the function wraps it in a new Wherr type that contains the original error, as well as the file and line information. If the error is already a Wherr type, it's left unchanged, ensuring that we retain the original location of the error.

Here's a brief look at the wherrapper function:

pub fn wherrapper<T, E>(
    result: Result<T, E>,
    file: &'static str,
    line: u32,
) -> Result<T, Box<dyn std::error::Error>>
where
    E: Into<Box<dyn std::error::Error>>,
{
    match result {
        Ok(val) => Ok(val),
        Err(err) => {
            let boxed_err: Box<dyn std::error::Error> = err.into();
            if boxed_err.is::<Wherr>() {
                Err(boxed_err)
            } else {
                Err(Box::new(Wherr::new(boxed_err, file, line)))
            }
        }
    }
}

Through this mechanism, any error returned (propagated using ?) from a function annotated with #[wherr] will the precise location of where the error occurred in the code. This offers developers better insight during debugging sessions.

Usage

To understand the benefits of the wherr crate, let's first observe the problem it aims to solve.

If you already know Rust, feel free to skip ahead to the Without wherr section, since the following will breifly explain the Rust concepts of the Result<T, E> type, and the ? operator.

In these examples, we utilize the i64::from_str_radix(s1, radix) function from the standard library, which has the signature:

pub fn from_str_radix(src: &str, radix: u32) -> Result<i64, ParseIntError>

This function aims to convert a string slice, representing a number in a specified base, into an integer. It returns a Result type —- an enum in Rust. The Result enum comprises two variants: Ok and Err. Notably, in Rust, these variants can encapsulate data. For Result<i64, ParseIntError>, the Ok variant wraps an i64 value, whereas the Err variant encapsulates a ParseIntError value.

In the line:

let i = i64::from_str_radix(s, radix)?;

We see the usage of the ? operator. This operator is used in Rust for a concise error handling.

When placed after a function that returns a Result, it does two things:

1. If the function returns an Ok variant, the ? operator extracts the value inside Ok and assigns it to the variable (in this case i).

2. If the function returns an Err variant, the ? operator immediately returns this error, effectively short-circuiting any further operations in the function.

Let's make an experiment demonstrating both variants.

Without #[wherr]:

examples/basic_without_wherr.rs:

// Function to add two numbers represented as strings.
// Returns a Result with the sum within an `Ok` variant if successful,
// or an `Err` variant if there's an error.
fn add(s1: &str, s2: &str) -> Result<i64, Box<dyn std::error::Error>> {
    let radix = 10;
    let i1 = i64::from_str_radix(s1, radix)?;
    let i2 = i64::from_str_radix(s2, radix)?;
    Ok(i1 + i2)
}

fn main() {
    let x = add("123", "not a number");
    println!("x = {:?}", x);
}

cargo run --example basic_without_wherr

x = Err(ParseIntError { kind: InvalidDigit })

Note that the Err lacks file or line details.

Using .unwrap() extracts the Ok value or panics on error. While the panic shows the file and line number, it only indicates the .unwrap() location, not the error's origin. Even with RUST_BACKTRACE=1 or RUST_BACKTRACE=full, the error's origin remains elusive. As it's a returned value, possibly passed through many functions, without embedded file or line info, retrieval is impossible.

examples/unwrap_without_wherr.rs:

// The add() function is as previously defined and is omitted here for clarity.

fn main() {
    let x = add("123", "not a number").unwrap();
    println!("x = {:?}", x);
}

cargo run --example unwrap_without_wherr

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
ParseIntError { kind: InvalidDigit }', wherr/examples/unwrap_without_wherr.rs:12:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The line wherr/examples/unwrap_without_wherr.rs:12 corresponds to:

    let x = add("123", "not a number").unwrap();

Now, let's have a look at the same examples, but this time with #[wherr] enabled.

With #[wherr]:

By adding #[wherr] macro to the function, errors will automatically be wrapped in a Wherr struct with a file and line field telling us where the error happened. The original error is preserved and accessible via the inner field.

examples/basic_with_wherr.rs:

use wherr::wherr;

// Function to add two numbers represented as strings.
// Returns a Result with the sum within an `Ok` variant if successful,
// or an `Err` variant if there's an error.
#[wherr]
fn add(s1: &str, s2: &str) -> Result<i64, Box<dyn std::error::Error>> {
    let radix = 10;
    let i1 = i64::from_str_radix(s1, radix)?;
    let i2 = i64::from_str_radix(s2, radix)?;
    Ok(i1 + i2)
}

fn main() {
    let x = add("123", "not a number");
    println!("x = {:?}", x);
}

cargo run --example basic_with_wherr

x = Err(Error at wherr/examples/basic_with_wherr.rs:10. Original error: ParseIntError { kind: InvalidDigit })

The line wherr/examples/basic_with_wherr.rs:10 corresponds to:

    let i2 = i64::from_str_radix(s2, radix)?;

examples/unwrap_with_wherr.rs:

use wherr::wherr;

// Function to add two numbers represented as strings.
// Returns a Result with the sum within an `Ok` variant if successful,
// or an `Err` variant if there's an error.
#[wherr]
fn add(s1: &str, s2: &str) -> Result<i64, Box<dyn std::error::Error>> {
    let radix = 10;
    let i1 = i64::from_str_radix(s1, radix)?;
    let i2 = i64::from_str_radix(s2, radix)?;
    Ok(i1 + i2)
}

fn main() {
    let x = add("123", "not a number").unwrap();
    println!("x = {:?}", x);
}

cargo run --example unwrap_with_wherr

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
Error at wherr/examples/unwrap_with_wherr.rs:10.
Original error: ParseIntError { kind: InvalidDigit }', wherr/examples/unwrap_with_wherr.rs:15:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The line wherr/examples/unwrap_with_wherr.rs:10 corresponds to:

    let i2 = i64::from_str_radix(s2, radix)?;

And the line wherr/examples/unwrap_with_wherr.rs:15 corresponds to:

    let x = add("123", "not a number").unwrap();

Here is the diff (additional newlines added for clarity):

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
-ParseIntError { kind: InvalidDigit }', wherr/examples/unwrap_without_wherr.rs:12:40
+Error at wherr/examples/unwrap_with_wherr.rs:10.
+Original error: ParseIntError { kind: InvalidDigit }', wherr/examples/unwrap_with_wherr.rs:15:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The file and line info can also be extracted from the Wherr struct, that wraps the original Err:

    match add("123", "not a number") {
        Ok(sum) => {
            println!("sum = {}", sum);
        }
        Err(e) => {
            if let Some(wherr) = e.downcast_ref::<wherr::Wherr>() {
                println!(
                    "Error at file: '{}', line: {}. Original error: {}",
                    wherr.file, wherr.line, wherr.inner
                );
            } else {
                println!("Unexpected error: {}", e);
            }
        }
    }

It also works through multiple nested layers of ?. The Err is only wrapped inside a Wherr once, and then propagated unchanged upwards.

API Reference

Wherr

Represents an error that includes file and line number information.

pub struct Wherr {
    inner: Box<dyn std::error::Error>,
    file: &'static str,
    line: u32,
}

Methods:

• new(err: Box<dyn std::error::Error>, file: &'static str, line: u32) -> Self: Creates a new Wherr error that wraps another error, providing additional context.

wherrapper

This internal utility function is used by the procedural macro to wrap errors with file and line information.

pub fn wherrapper<T, E>(
    result: Result<T, E>,
    file: &'static str,
    line: u32,
) -> Result<T, Box<dyn std::error::Error>>

wherr procedural macro

A procedural macro that auto-wraps errors (using the ? operator) inside a function with file and line number details.

#[wherr]
fn some_function() -> Result<(), Box<dyn std::error::Error>> { /* ... */ }

Contributing

If you're interested in contributing to wherr, please follow standard Rust community guidelines and submit a PR on our repository.

License

Please refer to the LICENSE file in the root directory of the crate for license details.