Fix hotplug backend and add test

src/librustc_driver/lib.rs

@@ -204,6 +204,14 @@ fn load_backend_from_dylib(sess: &Session, backend_name: &str) -> Box<TransCrate
         _lib: DynamicLibrary,
         trans: Box<TransCrate>,
     }
+
+    impl Drop for ExternTransCrate {
+        fn drop(&mut self) {
+            // Make sure trans gets dropped before _lib as bad things happen otherwise
+            self.trans = Box::new(::rustc_trans_utils::trans_crate::DummyTransCrate)
+        }
+    }
+
     impl TransCrate for ExternTransCrate {
         fn print(&self, req: PrintRequest, sess: &Session) {
             self.trans.print(req, sess);

src/test/run-make/hotplug_codegen_backend/Makefile

@@ -0,0 +1,10 @@
+include ../tools.mk
+
+all:
+	/bin/echo || exit 0 # This test requires /bin/echo to exist
+	$(RUSTC) the_backend.rs --crate-name the_backend --crate-type dylib \
+		-o $(TMPDIR)/the_backend.dylib
+	sleep 10
+	$(RUSTC) some_crate.rs --crate-name some_crate --crate-type bin -o $(TMPDIR)/some_crate \
+		-Z codegen-backend=$(TMPDIR)/the_backend.dylib -Z unstable-options
+	grep -x "This has been \"compiled\" succesfully." $(TMPDIR)/some_crate

src/test/run-make/hotplug_codegen_backend/some_crate.rs

@@ -0,0 +1,3 @@
+fn main() {
+    ::std::process::exit(1);
+}

src/test/run-make/hotplug_codegen_backend/the_backend.rs

@@ -0,0 +1,72 @@
+#![feature(rustc_private)]
+
+extern crate syntax;
+extern crate rustc;
+extern crate rustc_trans_utils;
+
+use std::any::Any;
+use std::sync::mpsc;
+use syntax::symbol::Symbol;
+use rustc::session::{Session, CompileIncomplete};
+use rustc::session::config::OutputFilenames;
+use rustc::ty::TyCtxt;
+use rustc::ty::maps::Providers;
+use rustc::middle::cstore::MetadataLoader;
+use rustc::dep_graph::DepGraph;
+use rustc_trans_utils::trans_crate::{TransCrate, MetadataOnlyTransCrate};
+
+struct TheBackend(Box<TransCrate>);
+
+impl TransCrate for TheBackend {
+    fn metadata_loader(&self) -> Box<MetadataLoader> {
+        self.0.metadata_loader()
+    }
+
+    fn provide(&self, providers: &mut Providers) {
+        self.0.provide(providers);
+    }
+
+    fn provide_extern(&self, providers: &mut Providers) {
+        self.0.provide_extern(providers);
+    }
+
+    fn trans_crate<'a, 'tcx>(
+        &self,
+        tcx: TyCtxt<'a, 'tcx, 'tcx>,
+        _rx: mpsc::Receiver<Box<Any + Send>>
+    ) -> Box<Any> {
+        use rustc::hir::def_id::LOCAL_CRATE;
+        
+        Box::new(tcx.crate_name(LOCAL_CRATE) as Symbol)
+    }
+
+    fn join_trans_and_link(
+        &self,
+        trans: Box<Any>,
+        sess: &Session,
+        _dep_graph: &DepGraph,
+        outputs: &OutputFilenames,
+    ) -> Result<(), CompileIncomplete> {
+        use std::io::Write;
+        use rustc::session::config::CrateType;
+        use rustc_trans_utils::link::out_filename;
+        let crate_name = trans.downcast::<Symbol>()
+            .expect("in join_trans_and_link: trans is not a Symbol");
+        for &crate_type in sess.opts.crate_types.iter() {
+            if crate_type != CrateType::CrateTypeExecutable {
+                sess.fatal(&format!("Crate type is {:?}", crate_type));
+            }
+            let output_name =
+                out_filename(sess, crate_type, &outputs, &*crate_name.as_str());
+            let mut out_file = ::std::fs::File::create(output_name).unwrap();
+            write!(out_file, "This has been \"compiled\" succesfully.").unwrap();
+        }
+        Ok(())
+    }
+}
+
+/// This is the entrypoint for a hot plugged rustc_trans
+#[no_mangle]
+pub extern "C" fn __rustc_codegen_backend(sess: &Session) -> Box<TransCrate> {
+    Box::new(TheBackend(MetadataOnlyTransCrate::new(sess)))
+}