rustdoc examples in std have an explicit `extern crate std` when loaded in playpen

[huonw]

(Tiny papercut)

e.g. the first example in std::io is

use std::io;

for line in io::stdin().lines() {
    print!("{}", line.unwrap());
}

but the data sent to playpen has a pointless extern crate std;:

extern crate std;
fn main() {
    use std::io;

    for line in io::stdin().lines() {
        print!("{}", line.unwrap());
    }
}

[huonw]

Strangely, some examples don't have crates, e.g. the first example in std::option is sent to playpen looking like

fn divide(numerator: f64, denominator: f64) -> Option<f64> {
    if denominator == 0.0 {
        None
    } else {
        Some(numerator / denominator)
    }
}

// The return value of the function is an option
let result = divide(2.0, 3.0);

// Pattern match to retrieve the value
match result {
    // The division was valid
    Some(x) => println!("Result: {}", x),
    // The division was invalid
    None    => println!("Cannot divide by 0")
}

i.e. no extern crate std. And, even when originating in core::option there's no extern crate core;.

However, this doesn't seem to be a universal feature of reexports, e.g. Arcs example is "fine":

• from std it includes extern crate std.
• from sync it includes extern crate sync

(And, just for reference, this doesn't appear to be a problem of module doc strings vs type doc strings, e.g. the example on Option.as_ref suffers the same fate as the other option examples.)