Subcrate's privates leak, and then cause linker errors

[bfops]

I'm getting a parent crate to "accidentally" use private functions from a sub-crate private module. The compile succeeds, but then the link fails!

main.rs:

extern crate bar;

fn main() {
  let bar = bar::Bar::new();
  bar.foo.foo();
}

bar/mod.rs

mod priv_bar;

pub struct Bar {
  pub foo: priv_bar::Foo,
}

impl Bar {
  pub fn new() -> Bar {
    Bar { foo: priv_bar::Foo }
  }
}

bar/priv_bar.rs:

pub struct Foo;

impl Foo {
  pub fn foo(&self) {
    println!("hello world");
  }
}

If bar/priv_bar.rs is inlined into bar/mod.rs, this doesn't happen; the correct error message is printed.

[bfops]

  Compiling bar v0.0.0 (file:///home/ben/sync/devl/test)
bar/priv_bar.rs:4:3: 6:4 warning: method is never used: `foo`, #[warn(dead_code)] on by default
bar/priv_bar.rs:4   pub fn foo(&self) {
bar/priv_bar.rs:5     println!("hello world");
bar/priv_bar.rs:6   }
  Compiling foo v0.0.0 (file:///home/ben/sync/devl/test)
error: linking with `cc` failed: exit code: 1
note: "cc" "-Wl,--as-needed" "-m64" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/home/ben/sync/devl/test/target/debug/foo.o" "-o" "/home/ben/sync/devl/test/target/debug/foo" "-Wl,--whole-archive" "-l" "morestack" "-Wl,--no-whole-archive" "-Wl,--gc-sections" "-pie" "-nodefaultlibs" "/home/ben/sync/devl/test/target/debug/deps/libbar-969d3503f37f9633.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_unicode-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-11582ce5.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-11582ce5.rlib" "-L" "/home/ben/sync/devl/test/target/debug" "-L" "/home/ben/sync/devl/test/target/debug/deps" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "/home/ben/sync/devl/test/.rust/lib/x86_64-unknown-linux-gnu" "-L" "/home/ben/sync/devl/test/lib/x86_64-unknown-linux-gnu" "-Wl,-Bstatic" "-Wl,-Bdynamic" "-l" "dl" "-l" "pthread" "-l" "rt" "-l" "gcc_s" "-l" "pthread" "-l" "c" "-l" "m" "-l" "compiler-rt"
note: /home/ben/sync/devl/test/target/debug/foo.o: In function `foo::main':
/home/ben/sync/devl/test/main.rs:5: undefined reference to `priv_bar::Foo::foo::ha1d68178bb28c0bckaa'
collect2: error: ld returned 1 exit status

error: aborting due to previous error
Could not compile `foo`.

[alexcrichton]

I believe this is a dupe of rust-lang/rust#16734, so closing in favor of that.