dead_code lint misses re-exported type aliases

[pnkfelix]

Summary: A library might want to have a single private type-parametric struct that is only exposed via a collection of named instantiations of that struct. The dead_code lint is issuing a false warning about dead code that is in fact reachable from the library via the re-exported type alias.

Consider the following library and client-code pair:

bug_lib.rs:

#![crate_type="lib"]

pub use src::aliases::B;
pub use src::hidden_core::make;

mod src {
    pub mod aliases {
        use super::hidden_core::A;
        pub type B = A<f32>;
    }

    pub mod hidden_core {
        use super::aliases::B;

        pub struct A<T>;

        pub fn make() -> B { A }

        impl<T> A<T> {
            pub fn foo(&mut self) { println!("called foo"); }
        }
    }
}

bug_client.rs

extern crate bug_lib;

use bug_lib::B;
use bug_lib::make;

fn main() {
    let mut an_A : B  = make();
    an_A.foo();
}

Transcript of rustc invocation illustrated false warning (and the fact that code is not dead):

% ./x86_64-apple-darwin/stage2/bin/rustc --version
rustc 0.11.0-pre (e402e75 2014-05-22 13:31:24 -0700)
host: x86_64-apple-darwin
% ./x86_64-apple-darwin/stage2/bin/rustc --out-dir /tmp /tmp/bug_lib.rs && ./x86_64-apple-darwin/stage2/bin/rustc -L /tmp/ /tmp/bug_client.rs
/tmp/bug_lib.rs:20:13: 20:62 warning: code is never used: `foo`, #[warn(dead_code)] on by default
/tmp/bug_lib.rs:20             pub fn foo(&mut self) { println!("called foo"); }
                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
% ./bug_client
called foo
%