Defining a public method on a private type alias causes linking failures

[chris-morgan]

Suppose we have a crate named c.

src/lib.rs:

pub struct A<T>(T);

type B = A<()>;
impl B {
    pub fn new() -> Self {
        A(())
    }
}

src/lib.rs:5:5: 7:6 warning: method is never used: `new`, #[warn(dead_code)] on by default
src/lib.rs:5     pub fn new() -> Self {
src/lib.rs:6         A(())
src/lib.rs:7     }

Given that type aliases are supposed to be purely aliases, new should not be considered dead code. The fact that B is not public should be immaterial.

Then we try to use it in src/main.rs:

extern crate c;

fn main() {
    let _ = c::A::new();
}

error: linking with `cc` failed: exit code: 1
note: "cc" "-Wl,--as-needed" "-m64" "-L" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/tmp/c/target/debug/c.0.o" "-o" "/tmp/c/target/debug/c" "-Wl,--whole-archive" "-l" "morestack" "-Wl,--no-whole-archive" "-Wl,--gc-sections" "-pie" "-nodefaultlibs" "/tmp/c/target/debug/libc.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_unicode-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-74fa456f.rlib" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-74fa456f.rlib" "-L" "/tmp/c/target/debug" "-L" "/tmp/c/target/debug/deps" "-L" "/home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "/tmp/c/.rust/lib/x86_64-unknown-linux-gnu" "-L" "/tmp/c/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" "-Wl,-rpath,$ORIGIN/../../../../home/chris/.multirust/toolchains/nightly/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-rpath,/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-l" "compiler-rt"
note: /tmp/c/target/debug/c.0.o: In function `c::main':
/tmp/c/src/main.rs:4: undefined reference to `B::new::h8f30536929b2bf42paa'
collect2: error: ld returned 1 exit status

So the Rust compiler thinks it exists, but hasn’t gone creating the symbol for it and so it fails in linking.

[alexcrichton]

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