move --force-rebuild behind unstable flag

src/cargo/util/command_prelude.rs

@@ -318,8 +318,13 @@ pub trait ArgMatchesExt {
         let mut build_config = BuildConfig::new(config, self.jobs()?, &self.target(), mode)?;
         build_config.message_format = message_format;
         build_config.release = self._is_present("release");
-        build_config.build_plan = self._is_present("build-plan");
         build_config.force_rebuild = self._is_present("force-rebuild");
+        if build_config.force_rebuild && !config.cli_unstable().unstable_options {
+            Err(failure::format_err!(
+                "`--force-rebuild` flag is unstable, pass `-Z unstable-options` to enable it"
+            ))?;
+        };
+        build_config.build_plan = self._is_present("build-plan");
         if build_config.build_plan && !config.cli_unstable().unstable_options {
             Err(failure::format_err!(
                 "`--build-plan` flag is unstable, pass `-Z unstable-options` to enable it"

tests/testsuite/check.rs

@@ -210,7 +210,13 @@ fn force_rebuild_displays_error() {
         .with_stderr_contains("[..]warning: unused import[..]")
         .run();
 
-    foo.cargo("check --force-rebuild")
+    // for now this requires the unstable feature flag, so expect an error here
+    let output = foo.cargo("check --force-rebuild")
+        .exec_with_output();
+    assert!(output.is_err());
+
+    foo.cargo("check -Z unstable-options --force-rebuild")
+        .masquerade_as_nightly_cargo() // remove this when `-Z unstable-options` is no longer required
         .with_stderr_contains("[..]warning: unused import[..]")
         .run();
 }