gist-rust-playpen.user.js

// ==UserScript==
// @name        gist-rust-playpen
// @description Adds a Rust Playpen button to Gists that contain a Rust file.
// @namespace   whipsch
// @include     https://gist.github.com/*
// @version     1
// @grant       none
// ==/UserScript==

if (document.querySelector("div.file > div.type-rust")) {
  var gist_id = null;
  var target = document.querySelector("meta[name='octolytics-dimension-gist_name']");
  if (target) {
    gist_id = target.getAttribute("content");
  }

  target = document.querySelector("div.repository-sidebar > div.only-with-full-nav");
  if (gist_id && target) {
    var button = document.createElement("a");
    button.setAttribute("href", "https://play.rust-lang.org/?gist=" + gist_id);
    button.setAttribute("target", "_blank");
    button.setAttribute("class", "btn btn-block");

    var icon = document.createElement("span");
    icon.setAttribute("class", "octicon octicon-gear");

    button.append(icon);
    button.append(document.createTextNode(" Rust Playpen"));

    target.append(button);
  }
}