**Cargo.toml**
```toml
[package]
name = "multiplier"
version = "0.1.0"
edition = "2021"

[dependencies]
```

**src/lib.rs**
```rust
pub fn solution(a: i32, b: i32) -> i32 {
    a * b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_positive_numbers() {
        assert_eq!(solution(4, 5), 20);
    }

    #[test]
    fn test_negative_numbers() {
        assert_eq!(solution(-3, 6), -18);
    }

    #[test]
    fn test_zero() {
        assert_eq!(solution(0, 10), 0);
    }
}
```

**Build**
```bash
cargo build
```

**Test**
```bash
cargo test
```