Add -Z allow_features=... flag

src/test/ui/feature-gate/allow-features-empty.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z allow_features=
+// Note: This test uses rustc internal flags because they will never stabilize.
+
+#![feature(rustc_diagnostic_macros)] //~ ERROR
+
+#![feature(rustc_const_unstable)] //~ ERROR
+
+#![feature(lang_items)] //~ ERROR
+
+fn main() {}

src/test/ui/feature-gate/allow-features-empty.stderr

@@ -0,0 +1,21 @@
+error[E0725]: the feature `rustc_diagnostic_macros` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:4:12
+   |
+LL | #![feature(rustc_diagnostic_macros)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0725]: the feature `rustc_const_unstable` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:6:12
+   |
+LL | #![feature(rustc_const_unstable)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+
+error[E0725]: the feature `lang_items` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:8:12
+   |
+LL | #![feature(lang_items)]
+   |            ^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0725`.

src/test/ui/feature-gate/allow-features.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z allow_features=rustc_diagnostic_macros,lang_items
+// Note: This test uses rustc internal flags because they will never stabilize.
+
+#![feature(rustc_diagnostic_macros)]
+
+#![feature(rustc_const_unstable)] //~ ERROR
+
+#![feature(lang_items)]
+
+fn main() {}

src/test/ui/feature-gate/allow-features.stderr

@@ -0,0 +1,9 @@
+error[E0725]: the feature `rustc_const_unstable` is not in the list of allowed features
+  --> $DIR/allow-features.rs:6:12
+   |
+LL | #![feature(rustc_const_unstable)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0725`.