Rustdoc JSON might not report all implemented traits on a public type

[obi1kenobi]

Given a public type, I expected all traits implemented by that type to be part of rustdoc JSON output.

However, rustdoc JSON output sometimes does not include information about all the traits implemented by a type unless --document-private-items is specified.

For example, in this clap semver regression:

• The following invocation on v3.1.18 generates JSON stating that UnwindSafe is implemented for ArgMatches: cargo rustdoc --lib --all-features -- --document-private-items -Zunstable-options --output-format json
• However, the following invocation has no mention of UnwindSafe for ArgMatches at all: cargo rustdoc --lib --all-features -- -Zunstable-options --output-format json

Related to:
obi1kenobi/cargo-semver-checks#32

Meta

rustc --version --verbose:

rustc 1.64.0-nightly (93ffde6f0 2022-07-23)
binary: rustc
commit-hash: 93ffde6f04d3d24327a4e17a2a2bf4f63c246235
commit-date: 2022-07-23
host: x86_64-unknown-linux-gnu
release: 1.64.0-nightly
LLVM version: 14.0.6

[camelid]

cc @CraftSpider @aDotInTheVoid

[aDotInTheVoid]

@rustbot modify labels: +E-needs-mcve

[aDotInTheVoid]

Interestingly, rustdoc-json is providing no impls at all:

"0:4070:1595": {
      "attrs": [],
      "crate_id": 0,
      "deprecation": null,
      "docs": "Container for parse results.\n\nUsed to get information about the arguments that were supplied to the program at runtime by\nthe user. New instances of this struct are obtained by using the [`Command::get_matches`] family of\nmethods.\n\n# Examples\n\n```no_run\n# use clap::{Command, Arg};\nlet matches = Command::new(\"MyApp\")\n    .arg(Arg::new(\"out\")\n        .long(\"output\")\n        .required(true)\n        .takes_value(true))\n    .arg(Arg::new(\"debug\")\n        .short('d')\n        .multiple_occurrences(true))\n    .arg(Arg::new(\"cfg\")\n        .short('c')\n        .takes_value(true))\n    .get_matches(); // builds the instance of ArgMatches\n\n// to get information about the \"cfg\" argument we created, such as the value supplied we use\n// various ArgMatches methods, such as ArgMatches::value_of\nif let Some(c) = matches.value_of(\"cfg\") {\n    println!(\"Value for -c: {}\", c);\n}\n\n// The ArgMatches::value_of method returns an Option because the user may not have supplied\n// that argument at runtime. But if we specified that the argument was \"required\" as we did\n// with the \"out\" argument, we can safely unwrap because `clap` verifies that was actually\n// used at runtime.\nprintln!(\"Value for --output: {}\", matches.value_of(\"out\").unwrap());\n\n// You can check the presence of an argument\nif matches.is_present(\"out\") {\n    // Another way to check if an argument was present, or if it occurred multiple times is to\n    // use occurrences_of() which returns 0 if an argument isn't found at runtime, or the\n    // number of times that it occurred, if it was. To allow an argument to appear more than\n    // once, you must use the .multiple_occurrences(true) method, otherwise it will only return 1 or 0.\n    if matches.occurrences_of(\"debug\") > 2 {\n        println!(\"Debug mode is REALLY on, don't be crazy\");\n    } else {\n        println!(\"Debug mode kind of on\");\n    }\n}\n```\n[`Command::get_matches`]: crate::Command::get_matches()",
      "id": "0:4070:1595",
      "inner": {
        "fields": [],
        "fields_stripped": true,
        "generics": {"params": [], "where_predicates": []},
        "impls": [],
        "struct_type": "plain"
      },
      "kind": "struct",
      "links": {"crate::Command::get_matches()": "0:3723:1609"},
      "name": "ArgMatches",
      "span": {
        "begin": [70, 0],
        "end": [79, 1],
        "filename": "src/parse/matches/arg_matches.rs"
      },
      "visibility": "public"
    },

[Enselic]

Missing impls is probably a recent regression, see #100204

[aDotInTheVoid]

MCVE

mod arg_matches {
    #[derive(Debug, Clone, Default, PartialEq, Eq)]
    pub struct ArgMatches();
}
pub use arg_matches::ArgMatches;

produces (full)

{
  "index": {
    "0:0:1564": {
      "id": "0:0:1564",
      "inner": {
        "is_crate": true,
        "is_stripped": false,
        "items": ["0:4"]
      },
      "kind": "module",
      "links": {},
      "name": "clap",
      "visibility": "public"
    },
    "0:4": {
      "id": "0:4",
      "inner": {"glob": false, "id": "0:7:1563", "name": "ArgMatches", "source": "arg_matches::ArgMatches"},
      "kind": "import",
      "name": null,
      "visibility": "public"
    },
    "0:7:1563": {
      "id": "0:7:1563",
      "inner": {"fields": [], "fields_stripped": false, "impls": [], "struct_type": "tuple"},
      "kind": "struct",
      "name": "ArgMatches",
      "visibility": "public"
    }
  },
  "root": "0:0:1564"
}

and with --document-private-items (full)

{
  "index": {
    "0:0:1564": {
      "id": "0:0:1564",
      "inner": {
        "is_crate": true,
        "is_stripped": false,
        "items": ["0:3:1562", "0:4"]
      },
      "kind": "module",
      "name": "clap",
      "visibility": "public"
    },
    "0:3:1562": {
      "id": "0:3:1562",
      "inner": {
        "is_crate": false,
        "is_stripped": false,
        "items": ["0:7:1563"]
      },
      "kind": "module",
      "name": "arg_matches",
      "visibility": "crate"
    },
    "0:4": {
      "id": "0:4",
      "inner": {"glob": false, "id": "0:7:1563", "name": "ArgMatches", "source": "arg_matches::ArgMatches"},
      "kind": "import",
      "name": null,
      "visibility": "public"
    },
    "0:7:1563": {
      "id": "0:7:1563",
      "inner": {
        "fields": [],
        "fields_stripped": false,
        "impls": [
          // Auto
          "a:2:9259:2290-0:7:1563", // RefUnwindSafe
          "a:2:3268:211-0:7:1563",  // Send
          "a:2:9258:2291-0:7:1563", // UnwindSafe
          "a:2:3280:220-0:7:1563",  // Sync
          "a:2:3309:1807-0:7:1563", // Unpin
          // Blanket
          "b:2:2871-0:7:1563", // BorrowMut
          "b:2:4001-0:7:1563", // Any
          "b:2:3197-0:7:1563", // Into
          "b:2:3212-0:7:1563", // TryFrom
          "b:2:3207-0:7:1563", // TryInto
          "b:2:2868-0:7:1563", // Borrow
          "b:2:3201-0:7:1563", // From
          "b:5:787-0:7:1563",  // ToOwned
          // Derives
          "0:9",  // Debug
          "0:11", // Clone
          "0:13", // Default
          "0:15", // StructuralPartialEq
          "0:16", // PartialEq
          "0:18", // StructuralEq
          "0:19"  // Eq
        ],
        "struct_type": "tuple"
      },
      "kind": "struct",
      "name": "ArgMatches",
      "visibility": "public"
    }
  },
  "root": "0:0:1564"
}

@rustbot modify labels: -E-needs-mcve

[aDotInTheVoid]

This is almost certanly due to #99287

searched nightlies: from nightly-2022-07-01 to nightly-2022-08-08
regressed nightly: nightly-2022-07-17
searched commit range: 23e21bd...d5e7f47
regressed commit: d5e7f47

bisected with cargo-bisect-rustc v0.6.4

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc 2022-07-01 --script ./script.sh --preserve

script.sh

#!/bin/bash
set -eou
rustdoc -w json -Z unstable-options mcve.rs
python3 ./check.py

mcve.rs

mod arg_matches {
    #[derive(Debug, Clone, Default, PartialEq, Eq)]
    pub struct ArgMatches();
}
pub use arg_matches::ArgMatches;

check.py

import json
with open("doc/mcve.json") as f:
    rdj = json.load(f)
idx = rdj["index"]
for item in idx.values():
    if item["kind"]=="struct" and item["name"] == "ArgMatches":
        impls = item["inner"]["impls"]
        break
if impls:
    exit(0)
else:
    exit(1)

[aDotInTheVoid]

More minimal (no_core)

#![feature(no_core)]
#![no_core]

mod bar {
    pub struct Baz;
    impl Baz {
        pub fn doit() {}
    }
}

pub use bar::Baz;

Is correct in html

and wrong in json

"0:2:1561": {
      "id": "0:2:1561",
      "inner": {
        "fields": [],
        "fields_stripped": false,
        "generics": {"params": [], "where_predicates": []},
        "impls": [],
        "struct_type": "unit"
      },
      "kind": "struct",
      "name": "Baz",
      "visibility": "public"
    }