pass -Zunstable-options to tests

src/tools/miri/cargo-miri/src/phases.rs

@@ -312,6 +312,7 @@ pub fn phase_rustc(args: impl Iterator<Item = String>, phase: RustcPhase) {
             // Ask rustc for the filename (since that is target-dependent).
             let mut rustc = miri_for_host(); // sysroot doesn't matter for this so we just use the host
             rustc.arg("--print").arg("file-names");
+            rustc.arg("-Zunstable-options"); // needed for JSON targets
             for flag in ["--crate-name", "--crate-type", "--target"] {
                 for val in get_arg_flag_values(flag) {
                     rustc.arg(flag).arg(val);

src/tools/miri/ci/ci.sh

@@ -173,7 +173,7 @@ case $HOST_TARGET in
     # Host
     MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 CARGO_MIRI_ENV=1 run_tests
     # Custom target JSON file
-    TEST_TARGET=tests/x86_64-unknown-kernel.json MIRI_NO_STD=1 run_tests_minimal no_std
+    TEST_TARGET=tests/x86_64-unknown-kernel.json MIRI_NO_STD=1 MIRIFLAGS="-Zunstable-options" run_tests_minimal no_std
     # Not officially supported tier 2
     MANY_SEEDS=16 TEST_TARGET=x86_64-pc-solaris run_tests
     ;;