--- a/rustc-1.29.0-src/src/librustc_llvm/lib.rs
--- b/rustc-1.29.0-src/src/librustc_llvm/lib.rs
@@ -23,6 +23,9 @@
 #![feature(link_args)]
 #![feature(static_nobundle)]
 
+// https://github.com/rust-lang/rust/issues/34486
+#[link(name = "ffi")] extern {}
+
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
 #[allow(unused_extern_crates)]
 extern crate rustc_cratesio_shim;