./x miri: fix sysroot build

1 files changed, 4 insertions, 0 deletions

diff --git a/src/bootstrap/src/core/build_steps/test.rs b/src/bootstrap/src/core/build_steps/test.rs
index 4fa91c1a571..161157acffe 100644
--- a/src/bootstrap/src/core/build_steps/test.rs
+++ b/src/bootstrap/src/core/build_steps/test.rs
@@ -2756,6 +2756,10 @@ impl Step for Crate {
             // `lib.rs` file, and a `lib.miri.rs` file exists in the same folder, we build that
             // instead. But crucially we only do that for the library, not the test builds.
             cargo.env("MIRI_REPLACE_LIBRS_IF_NOT_TEST", "1");
+            // std needs to be built with `-Zforce-unstable-if-unmarked`. For some reason the builder
+            // does not set this directly, but relies on the rustc wrapper to set it, and we are not using
+            // the wrapper -- hence we have to set it ourselves.
+            cargo.rustflag("-Zforce-unstable-if-unmarked");
             cargo
         } else {
             // Also prepare a sysroot for the target.