add regression test for issue 67053

Author: lqd

src/test/ui/consts/unstable-const-fn-in-libcore.rs

@@ -0,0 +1,29 @@
+// This is a non-regression test for const-qualification of unstable items in libcore
+// as explained in issue #67053.
+// const-qualification could miss some `const fn`s if they were unstable and the feature
+// gate was not enabled in libcore.
+
+#![stable(feature = "core", since = "1.6.0")]
+#![feature(const_if_match)]
+#![feature(rustc_const_unstable)]
+#![feature(staged_api)]
+
+enum Opt<T> {
+    Some(T),
+    None,
+}
+
+impl<T> Opt<T> {
+    #[rustc_const_unstable(feature = "foo")]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    const fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
+    //~^ ERROR destructors cannot be evaluated at compile-time
+    //~| ERROR destructors cannot be evaluated at compile-time
+        match self {
+            Opt::Some(t) => t,
+            Opt::None => f(), //~ ERROR E0015
+        }
+    }
+}
+
+fn main() {}

src/test/ui/consts/unstable-const-fn-in-libcore.stderr

@@ -0,0 +1,22 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/unstable-const-fn-in-libcore.rs:24:26
+   |
+LL |             Opt::None => f(),
+   |                          ^^^
+
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/unstable-const-fn-in-libcore.rs:19:53
+   |
+LL |     const fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
+   |                                                     ^ constant functions cannot evaluate destructors
+
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/unstable-const-fn-in-libcore.rs:19:47
+   |
+LL |     const fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
+   |                                               ^^^^ constant functions cannot evaluate destructors
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0015, E0493.
+For more information about an error, try `rustc --explain E0015`.