[rustdoc-json] Free-floating `struct_field` items with enum tuple struct variants

[Enselic]

There is currently a problematic inconsistency with regards to the generated rustdoc JSON for tuple structs. If the tuple struct is an enum variant, a "parentless" or "free-floating" item is generated. This does not happen with regular tuple structs.

Consider this code in src/lib.rs.

pub struct S1(pub usize);

pub enum E1 {
    S2(usize)
}

pub struct S3 {
    pub s3: usize,
}

pub enum E2 {
    S4 {
        s4: usize,
    }
}

Now we generate the rustdoc JSON (I used rustc 1.60.0-nightly (ad46af247 2022-01-14)):

RUSTDOCFLAGS='-Z unstable-options --output-format json' cargo doc

The rustdoc JSON output will contain 4 struct_field items, corresponding to S1.0, S2.0, S3.s3 and S4.s4 above.

The JSON item for S1 includes a reference to S1.0 (some parts of the JSON are omitted for brevity):

        "0:3": {
            "id": "0:3",
            "crate_id": 0,
            "name": "S1",
            "kind": "struct",
            "inner": {
                "struct_type": "tuple",
                "fields": [
                    "0:5"            <-- this is `S1.0`
                ],
        },

And the JSON item for S3 includes a reference to S3.s3:

        "0:10": {
            "id": "0:10",
            "crate_id": 0,
            "name": "S3",
            "kind": "struct",
            "inner": {
                "struct_type": "plain",
                "fields": [
                    "0:11"       <-- this is `S3.s3`
                ],
            }
        },

And the JSON item for S4 includes a reference to S4.s4:

        "0:13": {
            "id": "0:13",
            "crate_id": 0,
            "name": "S4",
            "kind": "variant",
            "inner": {
                "variant_kind": "struct",
                "variant_inner": [
                    "0:14"       <-- this is `S4.s4`
                ]
            }
        },

However, and this is the bug, the JSON item for S2 does not contain a reference to S2.0:

        "0:7": {
            "id": "0:7",
            "crate_id": 0,
            "name": "S2",
            "kind": "variant",
            "inner": {
                "variant_kind": "tuple",
                "variant_inner": [
                    {
                        "kind": "primitive",
                        "inner": "usize"
                    }
                ]
            }
        },

But an item for S2.0 is still included in the output. Here are all struct_field items in the JSON output:

% jq  '.index | .[] | select(.crate_id == 0) | select(.kind == "struct_field")' target/doc/repro.json 
{
  "id": "0:11",
  "crate_id": 0,
  "name": "s3",
  "kind": "struct_field",
  "inner": {
    "kind": "primitive",
    "inner": "usize"
  }
}
{
  "id": "0:14",
  "crate_id": 0,
  "name": "s4",
  "kind": "struct_field",
  "inner": {
    "kind": "primitive",
    "inner": "usize"
  }
}
{
  "id": "0:9",
  "crate_id": 0,
  "name": "0",
  "kind": "struct_field",
  "inner": {
    "kind": "primitive",
    "inner": "usize"
  }
}
{
  "id": "0:5",
  "crate_id": 0,
  "name": "0",
  "kind": "struct_field",
  "inner": {
    "kind": "primitive",
    "inner": "usize"
  }
}

but "0:9" is "free-floating" or "parentless", because it is not referenced by any other item. These "free-floating" rustdoc JSON items that are not referenced by any other items are problematic, because tools that processes the rustdoc JSON can't do anything sensible with them. They can't be ignored, because they describe items that are part of the public API of a crate. But they can't be used either, because they can not be put in a context since they are not referenced by anything.

Fortunately, I think fixing this is pretty straightforward. First, let us note that for a regular struct, the fields of it are referenced regardless of if it is a regular struct or a tuple struct (code from https://github.com/rust-lang/rust/blob/master/src/rustdoc-json-types/lib.rs):

#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
pub struct Struct {
    pub struct_type: StructType,
    pub generics: Generics,
    pub fields_stripped: bool,
    pub fields: Vec<Id>,
    pub impls: Vec<Id>,
}

However, for enum variant structs, fields are only referenced for regular struct variants:

#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
#[serde(rename_all = "snake_case")]
#[serde(tag = "variant_kind", content = "variant_inner")]
pub enum Variant {
    Plain,
    Tuple(Vec<Type>),
    Struct(Vec<Id>),
}

Conceptually I think the fix should be to replace Tuple(Vec<Type>), with Tuple(Vec<Id>), in the snippet above. But I have not thought this through in depth, it just intuitively seems like the right way to tackle this.

[Enselic]

Proof-of-concept fix to experiment with can be found at Enselic#1.

I have done some casual verification of that change and it appears to solve the problem in a good way. But the fix requires more thought and testing.

[aDotInTheVoid]

This happens because we add all the inner items here

rust/src/librustdoc/clean/types.rs

Line 774 in 6c943ba

EnumItem(e) => e.variants.iter(),

but I think it's better to move to having fields as their own item