`cargo fix` support for `exported-private-dependencies`

[epage]

Problem

rust-lang/rfcs#3516 calls for making exported-private-dependencies into an error in new editions. This means it needs to be machine-applicable so cargo fix can migrate it.

The problem is the most useful thing for a machine-applicable fix is to update Cargo.toml but rustc has no knowledge of that.

Proposed Solution

Intercept the message from rustc, before cargo fix (or the "machine applicable lints counter) sees this and add the machine-applicable content.

Notes

No response

[epage]

What we need is

• lint code
• dependency name (toml key)

Looking at the compiler message

{
  "reason": "compiler-message",
  "package_id": "cargo-pub-priv 0.1.0 (path+file:///home/epage/src/personal/dump/cargo-pub-priv)",
  "manifest_path": "/home/epage/src/personal/dump/cargo-pub-priv/Cargo.toml",
  "target": {
    "kind": [
      "lib"
    ],
    "crate_types": [
      "lib"
    ],
    "name": "cargo-pub-priv",
    "src_path": "/home/epage/src/personal/dump/cargo-pub-priv/src/lib.rs",
    "edition": "2021",
    "doc": true,
    "doctest": true,
    "test": true
  },
  "message": {
    "rendered": "warning: trait `FooTrait` from private dependency 'foo' in public interface\n  --> src/lib.rs:11:1\n   |\n11 | pub trait UseFoo: foo::FooTrait {}\n   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\n",
    "children": [],
    "code": {
      "code": "exported_private_dependencies",
      "explanation": null
    },
    "level": "warning",
    "message": "trait `FooTrait` from private dependency 'foo' in public interface",
    "spans": [
      {
        "byte_end": 189,
        "byte_start": 158,
        "column_end": 32,
        "column_start": 1,
        "expansion": null,
        "file_name": "src/lib.rs",
        "is_primary": true,
        "label": null,
        "line_end": 11,
        "line_start": 11,
        "suggested_replacement": null,
        "suggestion_applicability": null,
        "text": [
          {
            "highlight_end": 32,
            "highlight_start": 1,
            "text": "pub trait UseFoo: foo::FooTrait {}"
          }
        ]
      }
    ]
  }
}

We have the code but not the dependency name. It'd be ugly and brittle but we could parse the dependency name out of "from private dependency '{name}' in public interface". We ship with rustc so we can make sure this stays up to date as rustc changes, though it'll cause CI failures.

Expectation

• Parse the json blob, ensuring its a message and has the right code, extracting the dependency name, and constructing a machine applicable lint.
  • Depending on how replacements are encoded, we might need to add an item to children
• Test to verify we can auto-fix code
• Test the above with dependency renames
• Test that the "run cargo fix" message shows up for these with cargo check.

[epage]

Depending on the details, we might need #13096

[linyihai]

Is it currently not supported to obtain the specific line number of a certain configuration item?
Give a example, I like to know the line of license and regex in the Cargo.toml.

It looks like this issue need this ability to fixed.

And some discussiton can be found in zulip is it possible to include position data for cargo metadata

[epage]

@Muscraft was going to add code to cargo to get the needed spans in a follow up to #13172. If you need it before, reach out to them for notes they have on the topic

[linyihai]

@Muscraft was going to add code to cargo to get the needed spans in a follow up to #13172. If you need it before, reach out to them for notes they have on the topic

#13172 was merged recently. I'd to take a shot :-) .

[epage]

Note that I was referring to a PR they'll be working on after that PR. That PR doesn't have the functionality you would need.