diff --git a/ci.sh b/ci.sh
index a6daa806453..56a6f1228ab 100755
--- a/ci.sh
+++ b/ci.sh
@@ -25,8 +25,7 @@ function run_tests {
   ./miri test --locked
   if ! [ -n "${MIRI_TEST_TARGET+exists}" ]; then
     # Only for host architecture: tests with MIR optimizations
-    #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
+    MIRIFLAGS="-Z mir-opt-level=3" ./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 b0644a3d731..79ba8eebc19 100644
--- a/rust-version
+++ b/rust-version
@@ -1 +1 @@
-efbaa413061c2a6e52f06f00a60ee7830fcf3ea5
+c9ced8523bbb90561385aab305232f2167228a83