We actually dllexports all public symbols:

// lib.rs
#![crate_type = "dylib"]
#![crate_id = "r"]
#[no_mangle] pub fn func1() {} // exported
#[no_mangle] fn func2() {}
pub fn func3() {} // exported
fn func4() {}

C:\home\stone\tmp>dumpbin /exports r-1437d9a7-0.0.dll
...
File Type: DLL

  Section contains the following exports for r-1437d9a7-0.0.dll

    00000000 characteristics
    538549CB time date stamp Wed May 28 11:28:27 2014
        0.00 version
           1 ordinal base
           4 number of functions
           4 number of names

    ordinal hint RVA      name

          1    0 000015F0 _ZN5func320h830e15bc5d49336cjaa4v0.0E
          2    1 0000162C __morestack
          3    2 000015B0 func1
          4    3 00004000 rust_metadata_r_0.0_ac1193c82740e58b

but just by accident! If we don't explicitly export anything, ld exports everything public.

The "unintended export" will not occur if we add some explicit dllexports in native code:

// c.c
__declspec(dllexport)
void func5(void) {}

$ gcc -c -o c.o c.c
$ ar rcs ext.lib c.o

// lib.rs
#![crate_type = "dylib"]
#![crate_id = "r"]

#[link(name = "ext", kind = "static")]
extern "C" {
    pub fn func5();
}

#[no_mangle] pub fn func1() {} // exported
#[no_mangle] fn func2() {}
fn func3() { unsafe { func5(); } } // use dllexported symbol

$ rustc lib.rs

C:\home\stone\tmp>dumpbin /exports r-1437d9a7-0.0.dll
...
  Section contains the following exports for r-1437d9a7-0.0.dll

    00000000 characteristics
    538552D1 time date stamp Wed May 28 12:06:57 2014
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 0000161C func5

// a.rs
extern crate r;

fn main() {
    r::func1();
}

$ rustc lib.rs
$ rustc a.rs -L .
error: linking with `gcc` failed: exit code: 1
...
note: a.o:(.text+0x1a): undefined reference to `func1'

This is bad. We have to export them explicitly in this case.

Uh oh!

Add support for DllExport on Windows #7196

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