Diamon Kata in Rust

lib.rs

pub fn diamond(c: char) -> String {
    ('A'..=c)
        .chain(('A'..c).rev())
        .map(|row| line(c, row))
        .collect::<Vec<_>>()
        .join("\n")
}

fn line(c: char, row: char) -> String {
    ('A'..=c)
        .rev()
        .chain('B'..=c)
        .map(move |col| if row == col { row } else { ' ' })
        .collect::<String>()
}

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

    #[test]
    fn diamond_a() {
        assert_eq!("A", diamond('A'));
    }
    #[test]
    fn diamond_b() {
        assert_eq!(
            " A \
           \nB B\
           \n A ",
            diamond('B')
        );
    }

    #[test]
    fn diamond_c() {
        assert_eq!(
            "  A  \
           \n B B \
           \nC   C\
           \n B B \
           \n  A  ",
            diamond('C')
        );
    }

    #[test]
    fn diamond_z() {
        assert_eq!(
            diamond('Z'),
            "                         A                         \
           \n                        B B                        \
           \n                       C   C                       \
           \n                      D     D                      \
           \n                     E       E                     \
           \n                    F         F                    \
           \n                   G           G                   \
           \n                  H             H                  \
           \n                 I               I                 \
           \n                J                 J                \
           \n               K                   K               \
           \n              L                     L              \
           \n             M                       M             \
           \n            N                         N            \
           \n           O                           O           \
           \n          P                             P          \
           \n         Q                               Q         \
           \n        R                                 R        \
           \n       S                                   S       \
           \n      T                                     T      \
           \n     U                                       U     \
           \n    V                                         V    \
           \n   W                                           W   \
           \n  X                                             X  \
           \n Y                                               Y \
           \nZ                                                 Z\
           \n Y                                               Y \
           \n  X                                             X  \
           \n   W                                           W   \
           \n    V                                         V    \
           \n     U                                       U     \
           \n      T                                     T      \
           \n       S                                   S       \
           \n        R                                 R        \
           \n         Q                               Q         \
           \n          P                             P          \
           \n           O                           O           \
           \n            N                         N            \
           \n             M                       M             \
           \n              L                     L              \
           \n               K                   K               \
           \n                J                 J                \
           \n                 I               I                 \
           \n                  H             H                  \
           \n                   G           G                   \
           \n                    F         F                    \
           \n                     E       E                     \
           \n                      D     D                      \
           \n                       C   C                       \
           \n                        B B                        \
           \n                         A                         "
        );
    }
}