add test ensuring we cannot call const-stable unstable functions

Author: RalfJung

Diff for: tests/ui/consts/auxiliary/unstable_but_const_stable.rs

@@ -0,0 +1,13 @@
+#![feature(staged_api, rustc_attrs, intrinsics)]
+#![stable(since="1.0.0", feature = "stable")]
+
+extern "rust-intrinsic" {
+    #[unstable(feature = "unstable", issue = "42")]
+    #[rustc_const_stable(feature = "stable", since = "1.0.0")]
+    #[rustc_nounwind]
+    pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
+}
+
+#[unstable(feature = "unstable", issue = "42")]
+#[rustc_const_stable(feature = "stable", since = "1.0.0")]
+pub const fn some_unstable_fn() {}

Diff for: tests/ui/consts/unstable-const-stable.rs

@@ -0,0 +1,14 @@
+//@ aux-build:unstable_but_const_stable.rs
+
+extern crate unstable_but_const_stable;
+use unstable_but_const_stable::*;
+
+fn main() {
+    some_unstable_fn(); //~ERROR use of unstable library feature
+    unsafe { write_bytes(4 as *mut u8, 0, 0) }; //~ERROR use of unstable library feature
+}
+
+const fn const_main() {
+    some_unstable_fn(); //~ERROR use of unstable library feature
+    unsafe { write_bytes(4 as *mut u8, 0, 0) }; //~ERROR use of unstable library feature
+}

Diff for: tests/ui/consts/unstable-const-stable.stderr

@@ -0,0 +1,43 @@
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:7:5
+   |
+LL |     some_unstable_fn();
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:8:14
+   |
+LL |     unsafe { write_bytes(4 as *mut u8, 0, 0) };
+   |              ^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:12:5
+   |
+LL |     some_unstable_fn();
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:13:14
+   |
+LL |     unsafe { write_bytes(4 as *mut u8, 0, 0) };
+   |              ^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.