diff --git a/ci.sh b/ci.sh
index 56a6f1228a..a6daa80645 100755
--- a/ci.sh
+++ b/ci.sh
@@ -25,7 +25,8 @@ function run_tests {
   ./miri test --locked
   if ! [ -n "${MIRI_TEST_TARGET+exists}" ]; then
     # Only for host architecture: tests with MIR optimizations
-    MIRIFLAGS="-Z mir-opt-level=3" ./miri test --locked
+    #FIXME: Only testing opt level 1 due to <https://github.com/rust-lang/rust/issues/77564>.
+    MIRIFLAGS="-Z mir-opt-level=1" ./miri test --locked
   fi
   # "miri test" has built the sysroot for us, now this should pass without
   # any interactive questions.
diff --git a/rust-version b/rust-version
index 622fa14d1c..b0644a3d73 100644
--- a/rust-version
+++ b/rust-version
@@ -1 +1 @@
-a835b483fe0418b48ca44afb65cd0dd6bad4eb9b
+efbaa413061c2a6e52f06f00a60ee7830fcf3ea5