rustbuild: Fix LC_ID_DYLIB directives on OSX

src/bootstrap/bin/rustc.rs

@@ -158,6 +158,15 @@ fn main() {
         // to change a flag in a binary?
         if env::var("RUSTC_RPATH") == Ok("true".to_string()) {
             let rpath = if target.contains("apple") {
+
+                // Note that we need to take one extra step on OSX to also pass
+                // `-Wl,-instal_name,@rpath/...` to get things to work right. To
+                // do that we pass a weird flag to the compiler to get it to do
+                // so. Note that this is definitely a hack, and we should likely
+                // flesh out rpath support more fully in the future.
+                if stage != "0" {
+                    cmd.arg("-Z").arg("osx-rpath-install-name");
+                }
                 Some("-Wl,-rpath,@loader_path/../lib")
             } else if !target.contains("windows") {
                 Some("-Wl,-rpath,$ORIGIN/../lib")

src/librustc/session/config.rs

@@ -928,6 +928,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "print some statistics about MIR"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],
           "encode MIR of all functions into the crate metadata"),
+    osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
+          "pass `-install_name @rpath/...` to the OSX linker"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_trans/back/linker.rs

@@ -207,7 +207,12 @@ impl<'a> Linker for GnuLinker<'a> {
         if self.sess.target.target.options.is_like_osx {
             self.cmd.args(&["-dynamiclib", "-Wl,-dylib"]);
 
-            if self.sess.opts.cg.rpath {
+            // Note that the `osx_rpath_install_name` option here is a hack
+            // purely to support rustbuild right now, we should get a more
+            // principled solution at some point to force the compiler to pass
+            // the right `-Wl,-install_name` with an `@rpath` in it.
+            if self.sess.opts.cg.rpath ||
+               self.sess.opts.debugging_opts.osx_rpath_install_name {
                 let mut v = OsString::from("-Wl,-install_name,@rpath/");
                 v.push(out_filename.file_name().unwrap());
                 self.cmd.arg(&v);