Linker error with non-reachable type

[alexcrichton]

// foo.rs
pub use foo::Bar;

pub fn test() -> Bar<i32> {
    loop {}
}

mod foo {
    pub type Bar<T> = Result<T, Baz>;

    pub struct Baz;

    impl Baz {
        pub fn foo(&self) {}
    }
}

// bar.rs
extern crate foo;

fn main() {
    match foo::test() {
        Ok(..) => {}
        Err(e) => e.foo()
    }
}

$ rustc --crate-type lib foo.rs      
foo.rs:13:9: 13:29 warning: code is never used: `foo`, #[warn(dead_code)] on by default
foo.rs:13         pub fn foo(&self) {}
                  ^~~~~~~~~~~~~~~~~~~~
$ rustc bar.rs -L.    
error: linking with `cc` failed: exit code: 1
note: cc '-m64' '-L' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib' '-o' 'bar' 'bar.o' '-Wl,--whole-archive' '-lmorestack' '-Wl,--no-whole-archive' '-nodefaultlibs' '-Wl,--gc-sections' '-pie' '-Wl,--as-needed' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-4e7c5e5c.rlib' '/home/alex/libfoo.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libsync-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustrt-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunicode-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-4e7c5e5c.rlib' '/home/alex/code/rust/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-4e7c5e5c.rlib' '-L' '.' '-L' '/home/alex/.rust' '-L' '/home/alex' '-Wl,--whole-archive' '-Wl,-Bstatic' '-Wl,--no-whole-archive' '-Wl,-Bdynamic' '-ldl' '-lpthread' '-lgcc_s' '-lpthread' '-lc' '-lm' '-lcompiler-rt'
note: bar.o: In function `main::hbeb79415393a4115faa':
bar.rs:(.text._ZN4main20hbeb79415393a4115faaE+0x45): undefined reference to `foo::Baz::foo::h3d69757c908833a5Yaa'
collect2: error: ld returned 1 exit status

error: aborting due to previous error

There are a number of problems with this, and they may likely all be inter-related.

• linker error
• dead code which is not actually dead code
• it should be an error that Baz is a publicly visible private type.

Nominating due to the publicly visible private type not generating an error.

[alexcrichton]

Another example: #20201 (comment)

[steveklabnik]

This example now builds just fine with Cargo, but the example from the comment still fails with rustc.

[davidrhyswhite]

I came across a related issue today, put an example of the problematic project here: https://github.com/davidrhyswhite/rust-structs-example

Essentially when running cargo run --example example I get a linking error. The Parent struct is public has a public member called child which is a second struct with a public method. Calling parent.child.method() within the example causes a linking error.

Also a little more information from the rust IRC channel:

davidrhyswhite - Don't suppose anyone could help me understand why I get a 'linking with cc failed exit code 1' when I have a struct nested inside another struct and attempt to call a method on the child struct?
davidrhyswhite - Have a git repo with an example here: https://github.com/davidrhyswhite/rust-structs-example
krdln          - davidrhyswhite: Changing mod child to pub mod child works. You should file a bug, but I'm not shure what should be expected behaviour
mbrubeck       - davidrhyswhite: `cargo build` succeeds for me with only a dead_code warning
davidrhyswhite - Yeah cargo build was fine it was running 'cargo run --example example' that fails for me
mbrubeck       - ah, got it
krdln          - davidrhyswhite: Changing `use child` into `pub use child` works too
davidrhyswhite - Awesome thanks krdln been stuck on this for a while
mbrubeck       - davidrhyswhite: an example is compiled as a separate crate, so it can only call exported (`pub`) code from the main crate.
mbrubeck       - It'd be nice to have better diagnostics instead of just a linker error in cases like this...
krdln          - davidrhyswhite: Rust should either forbid returning private things from public funtions, or it should forbid calling method in example. Either way, file a bug
davidrhyswhite - mbrubeck: Okay, but I though since the child member on the Parent struct was pub and the method was pub too it would allow me to call it
krdln          - davidrhyswhite: Notice that you get warned about Child::method when building a lib
mbrubeck       - since `child::Child::method` is not a publicly-accessible path, it's not exported by the library, so other crates can't link to it.
aawe           - so the rust linker could use a patch for better error messages then
davidrhyswhite - mbrubeck: Thanks, that makes a bit more sense now.
mbrubeck       - aawe: Yeah, it'd be nice if rustc could thrown an error while compiling example.rs, instead of waiting for linking to fail.
mbrubeck       - The error is actually ends up coming from the system linker, gcc in my case.