'Run' button on the nightly error index doesn't open playground #36910

In the compiler error index, like in other parts of the documentation, it is possible to click a 'Run' button to open a playground containing the code from a <code> block. Unlike in other parts of the documentation, this does not work, and opens the same page in a new tab instead.

The issue in the page's HTML is that the href attribute of the a element that shows the button is blank:

<a class="test-arrow" target="_blank" href="">Run</a>.

In other parts of the documentation (e.g. the rust reference), this is not the case:

<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0A%20%20%20%20let%20a%20%3D%20%22foobar%22%3B%0Alet%20b%20%3D%20%22foo%5C%0A%20%20%20%20%20%20%20%20%20bar%22%3B%0A%0Aassert_eq!(a%2Cb)%3B%0A%7D">Run</a>.

This is likely an issue in the HTML generation, but I have not pinned down where exactly the generation goes wrong.