Feature request: `#[doc(canonical)]`

I don't have time for a full write-up of this right now, so I'm just opening an issue so I have somewhere to point people when they open rustdoc issues.

Motivation:

• search items are duplicated (https://github.com/rust-lang/rust/issues/15723)
• rustdoc sometimes links to the original definition, even for facade crates that use inlining: https://github.com/rust-lang/rust/issues/22083
  • ditto, but for intra-doc links: https://github.com/rust-lang/rust/issues/78467 (deemed to be a duplicate of the issue above)
• rustdoc sometimes links to a re-export, even when it is named differently than the link the user wrote: https://github.com/rust-lang/rust/issues/83976

doc(canonical) could tell rustdoc the 'one true path' the item should appear at.

Possible difficulties:

• doc(canonical) is necessarily unique across all crates. rustdoc should give an error if it's duplicated.
• rustdoc currently stores search-index.json per-crate. This will need a redesign if rustdoc only know to hide the item after the documentation has been generated and it goes onto the next crate

Unresolved questions:

• Does inlining still make sense with doc(canonical)? Should all other usages just link to the canonical crate? How does this work if the canonical usage is in a reverse-dependency?

cc @camelid

Start by reading the linked rustdoc issues on duplicated search items, original-definition links, and re-export links. Then review the proposed doc(canonical) behavior, including duplicate detection and the per-crate search-index.json design. Done requires resolving the listed inlining and reverse-dependency questions and producing a complete design.