tweaks and fixes for doc(include)

[QuietMisdreavus]

This PR makes a handful of changes around #[doc(include="file.md")] (#44732):

• Turns errors when loading files into full errors. This matches the original RFC text.
• Makes the missing_docs lint check for #[doc(include="file.md")] as well as regular #[doc="text"] attributes.
• Loads files included by #[doc(include="file.md")] into dep-info, mirroring the behavior of include_str!() and friends.
• Adds or modifies tests to check for all of these.

[rust-highfive]

r? @michaelwoerister

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[jonhoo]

Hmm, I wonder if the missing_docs check also needs to test if the included file is empty. Otherwise, arguably, docs are indeed missing.

[SimonSapin]

@jonhoo What happens with #[doc = ""] or ///?

[QuietMisdreavus]

Right now, empty doc comments and doc attributes will satisfy the missing_docs lint. I can go ahead and add a check for doc attributes that are empty, but that would be an additional enhancement to the missing_docs lint, not just updating it to work for doc(include).

[SimonSapin]

I think that an empty included doc file should be consistent with empty doc-comments. If we want to change what that does, maybe discuss it in a separate issue or PR.

[estebank]

@bors r+

[bors]

📌 Commit cbbb73b has been approved by estebank

[frewsxcv]

@bors rollup