Improve contracts intrisics and remove wrapper function

1. Document the new intrinsics.
2. Make the intrinsics actually check the contract if enabled, and
   remove `contract::check_requires` function.
3. Use panic with no unwind in case contract is using to check for
   safety, we probably don't want to unwind. Following the same
   reasoning as UB checks.

core/src/contracts.rs

@@ -1,38 +1,21 @@
 //! Unstable module containing the unstable contracts lang items and attribute macros.
+#![cfg(not(bootstrap))]
 
-#[cfg(not(bootstrap))]
-pub use crate::macros::builtin::contracts_ensures as ensures;
-#[cfg(not(bootstrap))]
-pub use crate::macros::builtin::contracts_requires as requires;
-
-/// 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_internals", issue = "133866" /* 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");
-    }
-}
+pub use crate::macros::builtin::{contracts_ensures as ensures, contracts_requires as requires};
 
 /// 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_internals", issue = "133866" /* compiler-team#759 */)]
 #[lang = "contract_build_check_ensures"]
 #[track_caller]
-pub fn build_check_ensures<Ret, C>(c: C) -> impl (FnOnce(Ret) -> Ret) + Copy
+pub fn build_check_ensures<Ret, C>(cond: C) -> impl (Fn(Ret) -> Ret) + Copy
 where
-    C: for<'a> FnOnce(&'a Ret) -> bool + Copy + 'static,
+    C: for<'a> Fn(&'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");
-        }
+        crate::intrinsics::contract_check_ensures(&ret, cond);
         ret
     }
 }

core/src/intrinsics/mod.rs

@@ -4082,18 +4082,32 @@ pub const fn contract_checks() -> bool {
     false
 }
 
+/// Check if the pre-condition `cond` has been met.
+///
+/// By default, if `contract_checks` is enabled, this will panic with no unwind if the condition
+/// returns false.
 #[cfg(not(bootstrap))]
 #[unstable(feature = "rustc_contracts_internals", issue = "133866" /* compiler-team#759 */)]
+#[lang = "contract_check_requires"]
 #[rustc_intrinsic]
-pub fn contract_check_requires<C: FnOnce() -> bool>(c: C) -> bool {
-    c()
+pub fn contract_check_requires<C: Fn() -> bool>(cond: C) {
+    if contract_checks() && !cond() {
+        // Emit no unwind panic in case this was a safety requirement.
+        crate::panicking::panic_nounwind("failed requires check");
+    }
 }
 
+/// Check if the post-condition `cond` has been met.
+///
+/// By default, if `contract_checks` is enabled, this will panic with no unwind if the condition
+/// returns false.
 #[cfg(not(bootstrap))]
 #[unstable(feature = "rustc_contracts_internals", issue = "133866" /* compiler-team#759 */)]
 #[rustc_intrinsic]
-pub fn contract_check_ensures<'a, Ret, C: FnOnce(&'a Ret) -> bool>(ret: &'a Ret, c: C) -> bool {
-    c(ret)
+pub fn contract_check_ensures<'a, Ret, C: Fn(&'a Ret) -> bool>(ret: &'a Ret, cond: C) {
+    if contract_checks() && !cond(ret) {
+        crate::panicking::panic_nounwind("failed ensures check");
+    }
 }
 
 /// The intrinsic will return the size stored in that vtable.