Rust uses markdown pretty much everywhere that we would use POD in Perl.

//!, ///=pod

Rust uses // for regular line comments, but /// for markdown comments. We can even use markdown's triple backticks ``` to include Rust source code. When we do this, it automatically becomes a doctest! That is, cargo test --doc runs the code we include in markdown comments. This helps us keep our docs up to date (cargo test includes --doc by default). I think perhaps the worst error in Perl is a SYNOPSIS section in our POD that doesn't work because the code changed, but the POD didn't. It's annoyingly easy to do.

For example, here is the apfs function from the epochs crate.

fn main() {
/// APFS time is the number of nanoseconds since the Unix epoch
/// (*cf.*, [APFS filesystem format](
/// ```
/// use epochs::apfs;
/// let ndt = apfs(1_234_567_890_000_000_000).unwrap();
/// assert_eq!(ndt.to_string(), "2009-02-13 23:31:30");
/// ```
pub fn apfs(num: i64) -> Option<NaiveDateTime> {
    epoch2time(num, 1_000_000_000, 0)

It contains a description of the function as well as a doctest. The compiler itself ignores it, like any other comment, but rustdoc will grab that description and run that test. Currently, rustdoc only generates HTML output that we must then view in a browser. There is no command line tool for looking up things in the docs like perldoc yet.

I've documented all of my functions in a similar way, so rustdoc generates this whole page for me. I didn't write that page, it was all generated from the source code when I uploaded it to!

Now, /// documents whatever immediately follows it, like the function above. There is also //! which goes after. That summary line for the whole crate came from one of those at the top.