miri: fix sysroot build

Author: RalfJung

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

@@ -136,7 +136,9 @@ pub fn setup(
     // But keep the overflow checks, they are cheap. This completely overwrites flags
     // the user might have set, which is consistent with normal `cargo build` that does
     // not apply `RUSTFLAGS` to the sysroot either.
-    let rustflags = &["-Cdebug-assertions=off", "-Coverflow-checks=on"];
+    // We also need to set `force-unstable-if-unmarked` to ensure the sysroot builds.
+    let rustflags =
+        &["-Cdebug-assertions=off", "-Coverflow-checks=on", "-Zforce-unstable-if-unmarked"];
 
     let mut after_build_output = String::new(); // what should be printed when the build is done.
     let notify = || {