Added an external hack function

uniffi/src/lib.rs

@@ -639,6 +639,16 @@ where
     }
 }
 
+// This function is not intended to be called, but can be used as for the to workaround
+// https://github.com/rust-lang/rust/issues/50007
+//
+// Basically, if another library adds uniffi as an external crate, then builds into a dylib, that
+// library normally won't have the symbols from uniffi.  However, if that library calls an extern
+// function from function, then it *will* have the symbols.  So this is a dummy function that the
+// other library can use to call to work around 50007.
+#[no_mangle]
+pub extern "C" fn extern_symbol_hack() {}
+
 #[cfg(test)]
 mod test {
     use super::*;