miri: fix sysroot build

Author: RalfJung

src/bootstrap/src/core/build_steps/test.rs

@@ -2755,6 +2755,10 @@ 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.

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 = || {