Fix playpen links to not all be the same

[estsauver]

In #20732, that all links in some modules point to the same code
examples was reported. The ID's generated for documents in
librustdoc are not all unique, which means the code rendered as
text is not being properly selected.

This change makes the link to the code section that is next to
the current link.

[rust-highfive]

r? @pcwalton

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[estsauver]

r? steveklabnik

If you have suggestions for how to fix the underlying html generation, I'd love to make that fix so that the docs divs all have unique ids.

~Earl

[steveklabnik]

I'd like @huonw or @gankro or someone who knows rustdoc better to review this, I have no idea if this is right or not.

[Gankra]

This definitely fixes the problem, but it's basically patching over the flaw that the ids are incorrectly generated in rustdoc itself.

[Gankra]

The problem appears to be in: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/markdown.rs#L196-L213

There's clearly some code to increment a counter, but that's obviously not working.

[Gankra]

Yeah it's predictably using a thread-local static: https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/markdown.rs#L158

I haven't been following all the TLS reforms, but maybe something changed that broke this?

[alexcrichton]

I think it's fine to fix the JS instead, if we can use siblings then there's no need for the counter than @gankro pointed out and it could just be removed (or the counter could be fixed).

[Gankra]

Killing TLSes in rustdoc is a big 👍 from me

[estsauver]

@alexcrichton I would really like for each of these div's to either have unique id's or hopefully use classes eventually.

Can I suggest we take this approach initially and open a subsequent bug for updating the counter to not use TLS's? I'd love to dive into that project but could use some help/a mentor to get a hand on what's going on internally.

[alexcrichton]

Sure! In that case, could the selector be altered to .rusttest instead of using an id which is possibly duplicated elsewhere on the page?

Otherwise, the relevant code should be pretty easy to remove, I believe you'll just need to remove these lines:

• rust/src/librustdoc/html/markdown.rs

  Line 158 in 055cc2e

  thread_local!(static TEST_IDX: Cell<uint> = Cell::new(0));

• rust/src/librustdoc/html/markdown.rs

  Lines 199 to 204 in 055cc2e

  	let idx = TEST_IDX.with(|slot| {
  	let i = slot.get();
  	slot.set(i + 1);
  	i
  	});

• rust/src/librustdoc/html/markdown.rs

  Line 213 in 055cc2e

  format!("rust-example-rendered-{}", idx)

And then just fix the fallout from that. Either way is fine by me!

[estsauver]

I wasn't sure how to do some of the argument wrapping/whitespace on line 206 in html/markdown.rs.

I saw this TBD style guide, but is there another good source for "good style" rust currently?

https://github.com/rust-lang/rust-guidelines/blob/master/style/README.md

[alexcrichton]

Looks good to me, thanks!