diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index 86d940cd733da56608338d5cdbb219fff4adab23..b3cf30672c82f330509ecce63145536426b208f8 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -452,7 +452,14 @@ fn run(self, builder: &Builder<'_>) {
 
             cargo.add_rustc_lib_path(builder, compiler);
 
-            if !try_run(builder, &mut cargo.into()) {
+            let mut cargo = Command::from(cargo);
+            if !try_run(builder, &mut cargo) {
+                return;
+            }
+
+            // # Run `cargo test` with `-Zmir-opt-level=4`.
+            cargo.env("MIRIFLAGS", "-O -Zmir-opt-level=4");
+            if !try_run(builder, &mut cargo) {
                 return;
             }
 
diff --git a/src/tools/miri b/src/tools/miri
index 12dac5c0f7acd106401aa14fec758f0ff552f678..2cdd1744b896e8129322229f253f95fd7ad491f1 160000
--- a/src/tools/miri
+++ b/src/tools/miri
@@ -1 +1 @@
-Subproject commit 12dac5c0f7acd106401aa14fec758f0ff552f678
+Subproject commit 2cdd1744b896e8129322229f253f95fd7ad491f1