diff --git a/miri b/miri
index 1f46f04c13e..6de2391137a 100755
--- a/miri
+++ b/miri
@@ -45,7 +45,7 @@ fi
 # We set the rpath so that Miri finds the private rustc libraries it needs.
 # We enable debug-assertions to get tracing.
 # We enable line-only debuginfo for backtraces.
-export RUSTFLAGS="-C link-args=-Wl,-rpath,$LIBDIR -C debug-assertions -C debuginfo=1"
+export RUSTFLAGS="-C link-args=-Wl,-rpath,$LIBDIR -C debug-assertions -C debuginfo=1 $RUSTC_EXTRA_FLAGS"
 
 ## Helper functions
 
diff --git a/src/eval.rs b/src/eval.rs
index ae57bcf98b9..39da414de14 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -40,7 +40,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
         MemoryExtra::new(StdRng::seed_from_u64(config.seed.unwrap_or(0)), config.validate),
     );
     // Complete initialization.
-    EnvVars::init(&mut ecx, config.communicate);
+    EnvVars::init(&mut ecx);
 
     // Setup first stack-frame
     let main_instance = ty::Instance::mono(ecx.tcx.tcx, main_id);
diff --git a/src/shims/env.rs b/src/shims/env.rs
index c941bf4f50e..eb5722a5a3d 100644
--- a/src/shims/env.rs
+++ b/src/shims/env.rs
@@ -13,9 +13,8 @@ pub struct EnvVars {
 impl EnvVars {
     pub(crate) fn init<'mir, 'tcx>(
         ecx: &mut InterpCx<'mir, 'tcx, Evaluator<'tcx>>,
-        communicate: bool,
     ) {
-        if communicate {
+        if ecx.machine.communicate {
             for (name, value) in std::env::vars() {
                 let value = alloc_env_value(value.as_bytes(), ecx.memory_mut());
                 ecx.machine.env_vars.map.insert(name.into_bytes(), value);
diff --git a/travis.sh b/travis.sh
index c06dbaee363..6f14be44ef0 100755
--- a/travis.sh
+++ b/travis.sh
@@ -8,6 +8,7 @@ else
   FOREIGN_TARGET=i686-unknown-linux-gnu
 fi
 export CARGO_EXTRA_FLAGS="--all-features"
+export RUSTC_EXTRA_FLAGS="-D warnings"
 
 # Prepare
 echo "Build and install miri"