In some situations, rustc may provide a diagnostic suggestion in an external crate, which in general it shouldn't do. These external crates may be in cargo's registry cache, which the user should not be modifying. For example, cargo fix may stomp on changes outside of the package (which is a separate issue rust-lang/cargo#9857). Usually rustc is good about avoiding this, so I'm not sure what is wrong in these situations.

In general, I expect rustc to not provide suggestions to modify dependencies.

The crater run found several instances of this, each for different underlying reasons (all dealing with macros).

Example 1 — unused tt

Example dependency:

// bar.rs
#[macro_export]
macro_rules! m {
    ($i:tt) => {
        $crate::m!(foo $i);
    };
    (foo $i:tt) => {
        let $i = 1;
    }
}

Example local crate:

// foo.rs
pub fn foo() {
    bar::m!(abc);
}

Resulting suggestion:

warning: unused variable: `abc`
 --> /Users/eric/Temp/crater-2021/foo/bar/src/lib.rs:7:13
  |
7 |         let $i = 1;
  |             ^^ help: if this is intentional, prefix it with an underscore: `_abc`
  |
  = note: `#[warn(unused_variables)]` on by default

A key point of this example is that the macro uses tt instead of ident. ident will not issue an unused warning.

Found in the 2021 crater run for:

• https://tiny.cc/oaaiuz/gh/rchaser53.at-coder-localplayground/log.txt

Example 2 — Weird $body suggestion

The following makes a suggestion to remove unnecessary braces, but the suggestion doesn't actually remove any braces.

// Using cpython 0.2.1
use cpython::*;

fn hello(py: Python) -> PyResult<PyString> {
    Ok(PyString::new(py, "Rust says: Hello world"))
}

py_module_initializer!(example, initexample, PyInit_example, |py, m| {
    m.add(py, "hello", py_fn!(py, hello()))?;
    Ok(())
});

Suggestion:

warning: unnecessary braces around block return value
   --> /Users/eric/.cargo/registry/src/github.com-1ecc6299db9ec823/cpython-0.2.1/src/argparse.rs:386:52
    |
386 |     ( $py:expr, $iter:expr, $body:block, [] ) => { $body };
    |                                                    ^^^^^ help: remove these braces
    |
    = note: `#[warn(unused_braces)]` on by default

warning: `foo` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

Found in the 2021 crater run for:

• https://tiny.cc/oaaiuz/gh/AustinHaugerud.mb_ext_script/log.txt
• https://tiny.cc/oaaiuz/gh/expobrain.python-rust-library-example/log.txt
• https://tiny.cc/oaaiuz/gh/mazurwiktor.aoaddons-python/log.txt

Example 3 — Hidden JSON suggestion

In this example, the human-readable text doesn't mention the dependency, but the JSON includes a MachineApplicable suggestion.

// diesel 1.4.7
#[macro_use]
extern crate diesel;
use diesel::table;

table! {
    use diesel::sql_types::*;
    use std::fs;

    users {
        id -> Integer,
        name -> VarChar,
        favorite_color -> Nullable<VarChar>,
    }
}

This emits two duplicate diagnostics in human form:

warning: unused import: `std::fs`
 --> src/lib.rs:7:9
  |
7 |     use std::fs;
  |         ^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: unused import: `std::fs`
 --> src/lib.rs:7:9
  |
7 |     use std::fs;
  |         ^^^^^^^

warning: `foo` (lib) generated 2 warnings

Buried in the JSON output is a machine-applicable change which modifies diesel:

{
    "byte_end": 9126,
    "byte_start": 9108,
    "column_end": 55,
    "column_start": 37,
    "expansion":
    {},
    "file_name": "/Users/eric/.cargo/registry/src/github.com-1ecc6299db9ec823/diesel-1.4.7/src/macros/mod.rs",
    "is_primary": true,
    "label": null,
    "line_end": 299,
    "line_start": 299,
    "suggested_replacement": "",
    "suggestion_applicability": "MachineApplicable",
    "text":
    [
        {
            "highlight_end": 55,
            "highlight_start": 37,
            "text": "            imports = [$($imports)* use $($import)::+;],"
        }
    ]
}

Found in the 2021 crater run for:

• https://tiny.cc/oaaiuz/gh/pigeon-app.pigeon-backend/log.txt

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (5eacec9ec 2021-08-28)
binary: rustc
commit-hash: 5eacec9ec7e112a0de1011519a57c45586d58414
commit-date: 2021-08-28
host: x86_64-apple-darwin
release: 1.56.0-nightly
LLVM version: 13.0.0