"cargo fix" produces an error message and fails to apply the suggested fixes to the file

[dthul]

I used cargo fix to transition my project to Rust 2018 and noticed that one of my source files made it error out so I extracted the following minimal example:

Cargo.toml

[package]
name = "cargofixtest"
version = "0.1.0"
authors = ["Daniel Thul <daniel.thul@gmail.com>"]

[dependencies]
pyo3 = "0.3.1"

lib.rs

#![feature(use_extern_macros, specialization)]

extern crate pyo3;
use pyo3::prelude::*;

#[pyclass]
pub struct Test {
    token: PyToken,
}

#[pymethods]
impl Test {
    #[new]
    fn __new__(obj: &PyRawObject) -> PyResult<()> {
        obj.init(|token| Test { token: token })
    }

    fn fun(&self, py: Python) -> PyResult<()> {
        Ok(())
    }
}

cargo fix tries to replace fun(&self, py: Python) with fun(&self, _py: Python) but fails with the following output:

    Checking cargofixtest v0.1.0 (file:///C:/Users/dthul/repos/cargofix)
warning: failed to automatically apply fixes suggested by rustc to crate `cargofix`

after fixes were automatically applied the compiler reported errors within these files:

  * src\lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/cargo/issues
quoting the full output of this command we'd be very appreciative!

warning: unused variable: `py`
  |
  = note: #[warn(unused_variables)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.59s

It does not fail when removing the __new__ method and its #[new] annotation.

[dthul]

Looks like similar problems have already been reported (#5775, #5785, #5798, #5799).
Sorry for the duplicate.

[alexcrichton]

Thanks for the report and no worries if this is a duplicate! This issue is sort of a blending of #5738 and #5799, with two bugs here:

• First, cargo fix shouldn't fix anything related to foreign macros, and that should be fixed by rustc: Disallow machine applicability in foreign macros rust#52756.
• Second, cargo fix --prepare-for shouldn't attempt to fix anything other than edition compatibility lints. That's covered by Cargo fix --prepare-for should only apply edition fixes #5738.

In that sense I'm gonna close this issue in favor of those two issues. Thanks regardless though for the report! I'd rather close duplicate issues than not know about issues :)