Write a function that takes a string of slashes (\ and /) and returns all of those slashes drawn downwards in a line connecting them.

main.rs

use std::{io::stdout, io::Write};

fn main() {
    indent_slashes(&mut stdout(), "\\\\\\\\//\\\\\\////").unwrap();
}

/// takes a string of slashes (\ and /) and returns all of those slashes drawn
/// downwards in a line connecting them.
///
/// Example:
///
/// ```
/// $ verticalSlashes('\\\//\/\\')
/// \
///  \
///   \
///   /
///  /
///  \
///  /
///  \
///   \
/// $ verticalSlashes('\\\\')
/// \
///  \
///   \
///    \
/// ```
fn indent_slashes<W: Write>(w: &mut W, pat: &str) -> std::io::Result<()> {
    let mut indent = 0;
    for c in pat.chars() {
        match c {
            '\\' => {
                write!(w, "{}\\\n", " ".repeat(indent))?;
                indent += 1;
            }
            '/' => {
                if indent == 0 {
                    return Err(std::io::Error::from(std::io::ErrorKind::InvalidData));
                }
                indent -= 1;
                write!(w, "{}/\n", " ".repeat(indent))?;
            }
            _ => return Err(std::io::Error::from(std::io::ErrorKind::InvalidData)),
        }
    }

    Ok(())
}

#[cfg(test)]
mod tests {
    use std::io::{stderr, BufWriter};

    use super::*;

    #[test]
    fn happy_path() {
        let expected = "\\
 \\
  \\
  /
 /
 \\
  \\
  /
";

        let mut buf = BufWriter::new(vec![]);
        indent_slashes(&mut buf, "\\\\\\//\\\\/").unwrap();
        assert_eq!(expected.as_bytes(), buf.buffer());
    }

    #[test]
    fn overflow() {
        assert!(indent_slashes(&mut stderr(), "\\/////").is_err())
    }

    #[test]
    fn buffer_full() {
        let mut buf: &mut [u8] = &mut [0; 8];
        assert!(indent_slashes(&mut buf, "\\/\\\\\\\\///").is_err())
    }
}