contracts: added lang items that act as hooks for rustc-injected code to invoke.

see test for an example of the kind of injected code that is anticipated here.

library/core/src/contracts.rs

@@ -0,0 +1,33 @@
+//! Unstable module containing the unstable contracts lang items and attribute macros.
+
+/// Emitted by rustc as a desugaring of `#[requires(PRED)] fn foo(x: X) { ... }`
+/// into: `fn foo(x: X) { check_requires(|| PRED) ... }`
+#[cfg(not(bootstrap))]
+#[unstable(feature = "rustc_contracts", issue = "none" /* compiler-team#759 */)]
+#[lang = "contract_check_requires"]
+#[track_caller]
+pub fn check_requires<C: FnOnce() -> bool>(c: C) {
+    if core::intrinsics::contract_checks() {
+        assert!(core::intrinsics::contract_check_requires(c), "failed requires check");
+    }
+}
+
+/// Emitted by rustc as a desugaring of `#[ensures(PRED)] fn foo() -> R { ... [return R;] ... }`
+/// into: `fn foo() { let _check = build_check_ensures(|ret| PRED) ... [return _check(R);] ... }`
+/// (including the implicit return of the tail expression, if any).
+#[cfg(not(bootstrap))]
+#[unstable(feature = "rustc_contracts", issue = "none" /* compiler-team#759 */)]
+#[lang = "contract_build_check_ensures"]
+#[track_caller]
+pub fn build_check_ensures<Ret, C>(c: C) -> impl (FnOnce(Ret) -> Ret) + Copy
+where
+    C: for<'a> FnOnce(&'a Ret) -> bool + Copy + 'static,
+{
+    #[track_caller]
+    move |ret| {
+        if core::intrinsics::contract_checks() {
+            assert!(core::intrinsics::contract_check_ensures(&ret, c), "failed ensures check");
+        }
+        ret
+    }
+}