# Unused return enum

## Description

- Vulnerability Category: `Validations and error handling`
- Vulnerability Severity: `Minor`
- Detectors: [`unused-return-enum`](https://github.com/CoinFabrik/scout-soroban/tree/main/detectors/unused-return-enum)
- Test Cases: [`unused-return-enum-1`](https://github.com/CoinFabrik/scout-soroban/tree/main/test-cases/unused-return-enum/unused-return-enum-1) [`unused-return-enum-2`](https://github.com/CoinFabrik/scout-soroban/tree/main/test-cases/unused-return-enum/unused-return-enum-2)

Soroban messages can return a `Result` enum with a custom error type. This is useful for the caller to know what went wrong when the message fails.

The definition in Rust of the `Result` enum is:

enum Result<T, E> {
    Ok(T),
    Err(E),
}

## Why is this bad?

If any of the variants (Ok/Err) is not used, the code could be simplified or it could imply a bug.

## Issue example

Consider the following `Soroban` contract:

#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
#[repr(u32)]
pub enum Error {
    /// An overflow was produced.
    Overflow = 1,
}



pub fn get_percentage_difference(balance1: u128, balance2: u128) -> Result<u128, Error> {
        let absolute_difference = balance1.abs_diff(balance2);
        let sum = balance1 + balance2;

        match 100u128.checked_mul(absolute_difference / sum) {
            Some(result) => result,
            None => panic!("Overflow"),
        };

        Err(Error::Overflow)
    }	

This is a `Soroban` message that returns the percentage difference between two values.

The function then returns an error enum variant `TradingPairErrors::Overflow`.
However, the function never returns a `Result` enum variant `Ok`, thus always
failing.

The vulnerable code example can be found [here](https://github.com/CoinFabrik/scout-soroban/tree/main/test-cases/unused-return-enum/unused-return-enum-1/remediated-example ).

## Remediated example

This function could be easily fixed by returning a `Result` enum variant `Ok`
when the percentage difference is calculated successfully. By providing a check in
the linter that ensures that all the variants of the `Result` enum are used, this
bug could have been avoided. This is shown in the example below:

#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
#[repr(u32)]
pub enum Error {
    /// An overflow was produced.
    Overflow = 1,
}


pub fn get_percentage_difference(balance1: u128, balance2: u128) -> Result<u128, Error> {
        let absolute_difference = balance1.abs_diff(balance2);
        let sum = balance1 + balance2;

        match 100u128.checked_mul(absolute_difference / sum) {
            Some(result) => Ok(result),
            None => Err(Error::Overflow),
        }
    }

The remediated code example can be found [here](https://github.com/CoinFabrik/scout-soroban/tree/main/test-cases/unused-return-enum//unused-return-enum-1/remediated-example).

## How is it detected?

It warns if a function that returns a Result type does not return the Result enum variant (Ok/Err).