Document the name of the crate that is the playground.

[Apersoma]

Currently is it not really possible to write actual doc tests because you can't import the thing your documenting.
i.e.

/// foo does foo things.
///
/// # Example
/// ```
/// use <what crate name?>::foo;
///
/// assert_eq!(foo(3u8), vec![92f64]);
/// ```
pub fn foo(x: u8) -> Vec<f64> {
    ...
}

[shepmaster]

I'm hesitant to do so because any documentation becomes an implicit binding contract and then people expect a specific behavior forever and ever.

Practically speaking, you are shown the answer every time you run the code:

   Compiling playground v0.0.1

You can also grab the value out of Cargo's environment variables:

eprintln!("{}", env!("CARGO_PKG_NAME"));

Or look at the Cargo.toml here in the repository.

[Apersoma]

I'm hesitant to do so because any documentation becomes an implicit binding contract and then people expect a specific behavior forever and ever.

Practically speaking, you are shown the answer every time you run the code:

   Compiling playground v0.0.1

Adding to the docs that it is displayed in the result of the build would work and is future proof.

[shepmaster]

and is future proof.

It's not, really. That output is from Cargo, which the playground has no control over.