Enable contracts for const functions

Use `const_eval_select!()` macro to enable contract checking only at
runtime. The existing contract logic relies on closures,
which are not supported in constant functions.

This commit also removes one level of indirection for ensures clauses,
however, it currently has a spurious warning message when the bottom
of the function is unreachable.

Author: celinval

Diff for: core/src/contracts.rs

@@ -5,16 +5,15 @@ pub use crate::macros::builtin::{contracts_ensures as ensures, contracts_require
 /// 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).
+///
+/// This call helps with type inference for the predicate.
 #[unstable(feature = "contracts_internals", issue = "128044" /* compiler-team#759 */)]
+#[rustc_const_unstable(feature = "contracts", issue = "128044")]
 #[lang = "contract_build_check_ensures"]
 #[track_caller]
-pub fn build_check_ensures<Ret, C>(cond: C) -> impl (Fn(Ret) -> Ret) + Copy
+pub const fn build_check_ensures<Ret, C>(cond: C) -> C
 where
-    C: for<'a> Fn(&'a Ret) -> bool + Copy + 'static,
+    C: Fn(&Ret) -> bool + Copy + 'static,
 {
-    #[track_caller]
-    move |ret| {
-        crate::intrinsics::contract_check_ensures(&ret, cond);
-        ret
-    }
+    cond
 }

Diff for: core/src/intrinsics/mod.rs

@@ -3450,20 +3450,55 @@ pub const fn contract_checks() -> bool {
 ///
 /// By default, if `contract_checks` is enabled, this will panic with no unwind if the condition
 /// returns false.
-#[unstable(feature = "contracts_internals", issue = "128044" /* compiler-team#759 */)]
+///
+/// Note that this function is a no-op during constant evaluation.
+#[unstable(feature = "contracts_internals", issue = "128044")]
+#[rustc_const_unstable(feature = "contracts", issue = "128044")]
 #[lang = "contract_check_requires"]
 #[rustc_intrinsic]
-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");
-    }
+pub const fn contract_check_requires<C: Fn() -> bool + Copy>(cond: C) {
+    const_eval_select!(
+        @capture[C: Fn() -> bool + Copy] { cond: C } :
+        if const {
+                // Do nothing
+        } else {
+            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.
+///
+/// Note that this function is a no-op during constant evaluation.
+#[cfg(not(bootstrap))]
+#[unstable(feature = "contracts_internals", issue = "128044")]
+#[rustc_const_unstable(feature = "contracts", issue = "128044")]
+#[lang = "contract_check_ensures"]
+#[rustc_intrinsic]
+pub const fn contract_check_ensures<Ret, C: Fn(&Ret) -> bool + Copy>(ret: Ret, cond: C) -> Ret {
+    const_eval_select!(
+        @capture[Ret, C: Fn(&Ret) -> bool + Copy] { ret: Ret, cond: C } -> Ret :
+        if const {
+            // Do nothing
+            ret
+        } else {
+            if contract_checks() && !cond(&ret) {
+                // Emit no unwind panic in case this was a safety requirement.
+                crate::panicking::panic_nounwind("failed ensures check");
+            }
+            ret
+        }
+    )
+}
+
+/// This is the old version of contract_check_ensures kept here for bootstrap only.
+#[cfg(bootstrap)]
 #[unstable(feature = "contracts_internals", issue = "128044" /* compiler-team#759 */)]
 #[rustc_intrinsic]
 pub fn contract_check_ensures<'a, Ret, C: Fn(&'a Ret) -> bool>(ret: &'a Ret, cond: C) {

Diff for: core/src/lib.rs

@@ -101,7 +101,6 @@
 #![feature(bstr)]
 #![feature(bstr_internals)]
 #![feature(cfg_match)]
-#![feature(closure_track_caller)]
 #![feature(const_carrying_mul_add)]
 #![feature(const_eval_select)]
 #![feature(core_intrinsics)]