Add test that min const fns can't call unstable min const fns even with the feature gate active

src/test/ui/consts/min_const_fn/min_const_fn_libstd_stability.rs

@@ -13,7 +13,7 @@
             we're apparently really bad at it",
             issue = "0")]
 
-#![feature(rustc_const_unstable, const_fn, foo)]
+#![feature(rustc_const_unstable, const_fn, foo, foo2)]
 #![feature(staged_api)]
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -35,4 +35,12 @@ const fn bar2() -> u32 { foo2() } //~ ERROR can only call other `min_const_fn`
 // conformity is required, even with `const_fn` feature gate
 const fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` operations
 
+// check whether this function cannot be called even with the feature gate active
+#[unstable(feature = "foo2", issue="0")]
+const fn foo2_gated() -> u32 { 42 }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+// can't call non-min_const_fn
+const fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `min_const_fn`
+
 fn main() {}

src/test/ui/consts/min_const_fn/min_const_fn_libstd_stability.stderr

@@ -16,5 +16,11 @@ error: only int, `bool` and `char` operations are stable in const fn
 LL | const fn bar3() -> u32 { (5f32 + 6f32) as u32 } //~ ERROR only int, `bool` and `char` operations
    |                          ^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: can only call other `min_const_fn` within a `min_const_fn`
+  --> $DIR/min_const_fn_libstd_stability.rs:44:32
+   |
+LL | const fn bar2_gated() -> u32 { foo2_gated() } //~ ERROR can only call other `min_const_fn`
+   |                                ^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors