Contracts core intrinsics.

These are hooks to:

  1. control whether contract checks are run
  2. allow 3rd party tools to intercept and reintepret the results of running contracts.

tests/ui/contracts/contract-intrinsics.rs

@@ -0,0 +1,23 @@
+//@ run-pass
+//@ revisions: yes no none
+//@ [yes] compile-flags: -Zcontract-checks=yes
+//@ [no] compile-flags: -Zcontract-checks=no
+#![feature(cfg_contract_checks, rustc_contracts, core_intrinsics)]
+
+fn main() {
+    #[cfg(none)] // default: disabled
+    assert_eq!(core::intrinsics::contract_checks(), false);
+
+    #[cfg(yes)] // explicitly enabled
+    assert_eq!(core::intrinsics::contract_checks(), true);
+
+    #[cfg(no)] // explicitly disabled
+    assert_eq!(core::intrinsics::contract_checks(), false);
+
+    assert_eq!(core::intrinsics::contract_check_requires(|| true), true);
+    assert_eq!(core::intrinsics::contract_check_requires(|| false), false);
+
+    let doubles_to_two = { let old = 2; move |ret| ret + ret == old };
+    assert_eq!(core::intrinsics::contract_check_ensures(&1, doubles_to_two), true);
+    assert_eq!(core::intrinsics::contract_check_ensures(&2, doubles_to_two), false);
+}

tests/ui/feature-gates/feature-gate-cfg-contract-checks.rs

@@ -0,0 +1,5 @@
+#![crate_type = "lib"]
+
+pub fn contract_checks_are_enabled() -> bool {
+    cfg!(contract_checks) //~ ERROR `cfg(contract_checks)` is experimental
+}

tests/ui/feature-gates/feature-gate-cfg-contract-checks.stderr

@@ -0,0 +1,13 @@
+error[E0658]: `cfg(contract_checks)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg-contract-checks.rs:4:10
+   |
+LL |     cfg!(contract_checks)
+   |          ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #133866 <https://github.com/rust-lang/rust/issues/133866> for more information
+   = help: add `#![feature(cfg_contract_checks)]` 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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.