rustdoc-json: Not enough info after paths change.

[aDotInTheVoid]

@aDotInTheVoid I'm running into a serious issue making cargo-semver-checks support this format. I'd vote for reverting this change while we look for a better alternative — possibly by reworking what path is in paths.

All this information is available in the paths field, and there it's in a more usable form, as it also includes the defining crate.

This turns out to not be true in practice. The paths field includes the originally-defined path of the item, which is often private and not accessible. For example:

• Where Path::name previously contained Iterator (accessible via the prelude), the paths lookup produces core::iter::traits::iterator::Iterator which is private.
• Where Path::name previously contained std::str::SplitAsciiWhitespace, the paths lookup produces core::str::iter::SplitAsciiWhitespace which is private.
  etc.

This is making cargo-semver-checks unable to properly print types, function signatures, trait bounds, etc. and effectively disables a bunch of lints.

In theory, we could run c-s-c's importable paths analysis to derive a valid, publicly-importable path for the item instead of using the paths field. In practice, this is blocked in two ways:

• It requires cross-crate analysis, since most such items are in std/core/alloc etc. This is a goal, but as we've already chatted, it's many months out and blocked on rustc functionality etc.
• Even if we had cross-crate analysis, it requires the ability to either download or generate rustdoc JSON for std/core/alloc. I don't know of a way to generate it client-side, and AFAIK we can download their rustdoc JSON for nightly Rust only. This would break c-s-c's ability to support multiple stable Rust versions.

Originally posted by @obi1kenobi in #134880 (comment)

[aDotInTheVoid]

MCVE:

#![no_std]

pub type Foo = core::str::SplitAsciiWhitespace<'static>;

Which produces (abridged):

{
  "index": {
    "0": {
      "id": 0,
      "inner": {
        "type_alias": {
          "generics": {"params": [], "where_predicates": []},
          "type": {
            "resolved_path": {
              "args": {"angle_bracketed": {"args": [{"lifetime": "'static"}], "constraints": []}},
              "id": 1,
              "name": "SplitAsciiWhitespace"
            }
          }
        }
      },
      "name": "Foo"
    },
    "2": {
      "id": 2,
      "inner": {"module": {"is_crate": true, "is_stripped": false, "items": [0]}},
      "name": "mcve"
    }
  },
  "paths": {
    "0": {"crate_id": 0, "kind": "type_alias", "path": ["mcve", "Foo"]},
    "1": {"crate_id": 1, "kind": "struct", "path": ["core", "str", "iter", "SplitAsciiWhitespace"]}
  },
  "root": 2
}

And there's no way to produce core::str::SplitAsciiWhitespace from this, which is what a load of tools really want. (And core::str::iter::SplitAsciiWhitespace isn't publicly accessible)

[aDotInTheVoid]

@rustbot claim.

I'm going to submit a revert for #134880, then later we can try to reland (while still having the needed info somewhere)