Rust: Force symbol re-export from libglean_ffi.

Workaround to force a re-export of the `no_mangle` symbols from `glean_ffi`

Due to how linking works and hides symbols the symbols from `glean_ffi` might not be
re-exported and thus not usable.
By forcing use of _at least one_ symbol in an exported function the functions will also be
rexported.
This is only required for debug builds (and `debug_assertions` is the closest thing we have to
check that).
In release builds we rely on LTO builds to take care of it.
As the tests will use libglean_ffi functionality running the tests
should ensure this actually happens.

See rust-lang/rust#50007

glean-core/bundle/src/lib.rs

@@ -3,3 +3,21 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 pub use glean_ffi;
+
+/// Workaround to force a re-export of the `no_mangle` symbols from `glean_ffi`
+///
+/// Due to how linking works and hides symbols the symbols from `glean_ffi` might not be
+/// re-exported and thus not usable.
+/// By forcing use of _at least one_ symbol in an exported function the functions will also be
+/// rexported.
+/// This is only required for debug builds (and `debug_assertions` is the closest thing we have to
+/// check that).
+/// In release builds we rely on LTO builds to take care of it.
+/// Our tests should ensure this actually happens.
+///
+/// See https://github.com/rust-lang/rust/issues/50007
+#[cfg(debug_assertions)]
+#[no_mangle]
+pub unsafe extern "C" fn _glean_force_reexport_donotcall() {
+    glean_ffi::glean_enable_logging();
+}