lib.rs

pub mod matrix {
    use num_traits::Num;
    pub struct Matrix<T> {
        pub rows: usize,
        pub cols: usize,
        data: Vec<T>,
    }

    impl<T> Matrix<T>
    where
        T: Num + Copy,
    {
        pub fn new(rows: usize, cols: usize) -> Self {
            assert!(rows > 0 && cols > 0);
            Matrix {
                rows: rows,
                cols: cols,
                data: vec![T::zero(); rows * cols],
            }
        }

        pub fn from(v: Vec<Vec<T>>) -> Self {
            let mut a = Matrix::<T>::new(v.len(), v[0].len());
            for (i, e) in v.iter().flatten().enumerate() {
                a.data[i] = *e;
            }
            a
        }

        pub fn transpose(self) -> Self {
            self
        }
    }

    #[macro_export]
    macro_rules! mat {
        ($([$($x:expr),* $(,)*]),+ $(,)*) => {{
            crate::matrix::Matrix::from(vec![$([$($x,)*].to_vec(),)*])
        }}
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        #[test]
        #[should_panic]
        fn test_panic_on_0_size() {
            let a = Matrix::<u8>::new(0, 0);
            let b = Matrix::<u8>::new(1, 0);
            let a = Matrix::<u8>::new(0, 1);
        }

        fn test_create_matrix_macro() {
            let a = mat![[1, 2], [3, 4]];
            assert_eq!(a.data, [1, 2, 3, 4]);
        }
    }
}

main.rs

#[macro_use]
use matrixsolver;
use matrixsolver::matrix::*;

fn main() {
    println!("oh brother");
    let a = mat![[1, 2], [3, 4]];
    println!("{} {}", a.rows, a.cols);
}

Gives: error: cannot find macro mat in this scope