cargo fix fails to apply all -Wunreachable-pub hints

[matthiaskrgr]

git clone https://github.com/rust-lang/cargo/
cd cargo
git checkout 0d1f1bbeabd5b43a7f3ecfa16540af8e76d5efb4
RUSTFLAGS="-Wunreachable-pub" cargo fix

   Compiling cargo v0.34.0 (/home/matthias/vcs/github/cargo)
warning: failed to automatically apply fixes suggested by rustc to crate `cargo`

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

  * src/cargo/core/compiler/context/mod.rs
  * src/cargo/core/compiler/mod.rs
  * src/cargo/core/resolver/context.rs
  * src/cargo/util/mod.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: unreachable `pub` item
  --> src/cargo/macros.rs:43:1
   |
43 | pub struct DisplayAsDebug<T>(pub T);
   | ---^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   | |
   | help: consider restricting its visibility: `pub(crate)`
   |
   = note: requested on the command line with `-W unreachable-pub`
   = help: or consider exporting it for use by other crates

(followed my a lot more warnings).

cargo 1.33.0-nightly (2cf1f5dda 2018-12-11)
release: 1.33.0
commit-hash: 2cf1f5dda2f7ed84e94c4d32f643e0f1f15352f0
commit-date: 2018-12-11

[ehuss]

It looks like the unreachable_pub lint is a little flaky. It looks like there were some attempts to improve it (rust-lang/rust#51866), but there are still some bad suggestions. I think it is mostly having problems with re-exports.

Here's one problem

#![warn(unreachable_pub)]

pub use self::m::f;

mod m {
    pub use self::imp::f;  // Recommends pub(crate), but that causes an error above.
    mod imp {
        pub fn f() {}
    }
}

And another one where it interacts poorly with unused_imports, which is harder to reduce:

#![warn(unreachable_pub)]
#![deny(unused_imports)]

pub mod core {
    pub mod compiler {
        mod context {
            mod compilation_files {
                pub struct Metadata;
                pub struct CompilationFiles;
                impl CompilationFiles {
                    pub fn metadata() -> Metadata { Metadata }
                }
            }
            use self::compilation_files::CompilationFiles;
            pub use self::compilation_files::Metadata;  // Suggests pub(crate)
                                // but that causes unused_imports to complain.

            pub struct Context;
            impl Context {
                pub fn files() -> CompilationFiles { CompilationFiles }
            }
        }
        pub use self::context::Context;
    }
}

[alexcrichton]

I've opened rust-lang/rust#57410 and rust-lang/rust#57411 for this, so closing in favor of those.