rustdoc: parse item and reexport markdown separate

[notriddle]

This change is a pre-requisite for LaTeX support, because the markdown parser doesn't support enabling and disabling extensions in the middle of a document, and there's no way to add that. This means string concatenating a document that has LaTeX Math disabled with a document that has it enabled can't be done.

As part of this change, a bug related to intra-doc links is fixed. This shows up when the reexport and the item both have intra-doc links with the same visible path, but where they resolve to different items. The bug is demonstrated in
tests/rustdoc-html/reexport/link-with-same-name-but-different-destination.rs.

The other test case changes demonstrate that this is, technically, a breaking change. When I ran a Crater test for docs that rely on this behavior, though, it seemed most authors weren't relying on it.

[rustbot]

These commits modify tests/rustdoc-json.
rustdoc-json is a public (but unstable) interface.

Please ensure that if you've changed the output:

• It's intentional.
• The FORMAT_VERSION in src/librustdoc-json-types is bumped if necessary.

cc @obi1kenobi

rustdoc-json-types is a public (although nightly-only) API. If possible, consider changing src/librustdoc/json/conversions.rs; otherwise, make sure you bump the FORMAT_VERSION constant.

cc @CraftSpider, @Enselic, @obi1kenobi

[rustbot]

r? @GuillaumeGomez

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• Random selection from GuillaumeGomez, lolbinarycat

[obi1kenobi]

I'm curious, when might this Vec have more than one element?

It might be useful to include an example in the doc comment, for ease of use.

View changes since the review

[notriddle]

Good call. I've added an example and some description to the docs.

The full markdown docstring of this item. Empty if there is no documentation at all,
vec![Doc { text: "" }] if there is some documentation but it is empty (EG #[doc = ""]),
and multiple items if a reexport and the original both have docstrings.

/// Reexport docs
pub use Thing1 as Thing2;

/// Original docs
pub struct Thing1;

{
    "name": "Thing2",
    "docs": [
        { "text": "Reexport docs", "links": [] },
        { "text": "Original docs", "links": [] },
    ],
    ...
}

[obi1kenobi]

Ah interesting! Nice, thank you.

Is any particular order of elements guaranteed? That's the only remaining thing I'd consider adding.