Require `-Z unstable-options` for unstable editions.

compiler/rustc_session/src/config.rs

@@ -1308,12 +1308,11 @@ fn parse_crate_edition(matches: &getopts::Matches) -> Edition {
         None => DEFAULT_EDITION,
     };
 
-    if !edition.is_stable() && !nightly_options::match_is_nightly_build(matches) {
+    if !edition.is_stable() && !nightly_options::is_unstable_enabled(matches) {
         early_error(
             ErrorOutputType::default(),
             &format!(
-                "edition {} is unstable and only \
-                     available for nightly builds of rustc.",
+                "edition {} is unstable and only available with -Z unstable-options.",
                 edition,
             ),
         )

src/test/ui/hello2021.rs

 // run-pass
 // edition:2021
+// compile-flags: -Zunstable-options
 
 fn main() {
     println!("hello, 2021");