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

compiler/rustc_ast_lowering/src/expr.rs

@@ -397,12 +397,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         expr: &'hir hir::Expr<'hir>,
         span: Span,
-        check_ident: Ident,
-        check_hir_id: HirId,
+        cond_ident: Ident,
+        cond_hir_id: HirId,
     ) -> &'hir hir::Expr<'hir> {
-        let checker_fn = self.expr_ident(span, check_ident, check_hir_id);
+        let cond_fn = self.expr_ident(span, cond_ident, cond_hir_id);
         let span = self.mark_span_with_reason(DesugaringKind::Contract, span, None);
-        self.expr_call(span, checker_fn, std::slice::from_ref(expr))
+        let call_expr = self.expr_call_lang_item_fn_mut(
+            span,
+            hir::LangItem::ContractCheckEnsures,
+            arena_vec![self; *expr, *cond_fn],
+        );
+        self.arena.alloc(call_expr)
     }
 
     pub(crate) fn lower_const_block(&mut self, c: &AnonConst) -> hir::ConstBlock {

compiler/rustc_hir/src/lang_items.rs

@@ -429,6 +429,7 @@ language_item_table! {
     // Experimental lang items for implementing contract pre- and post-condition checking.
     ContractBuildCheckEnsures, sym::contract_build_check_ensures, contract_build_check_ensures_fn, Target::Fn, GenericRequirement::None;
     ContractCheckRequires,     sym::contract_check_requires,      contract_check_requires_fn,      Target::Fn, GenericRequirement::None;
+    ContractCheckEnsures,     sym::contract_check_ensures,      contract_check_ensures_fn,      Target::Fn, GenericRequirement::None;
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -228,15 +228,11 @@ pub fn check_intrinsic_type(
         };
         (n_tps, 0, 0, inputs, output, hir::Safety::Unsafe)
     } else if intrinsic_name == sym::contract_check_ensures {
-        // contract_check_ensures::<'a, Ret, C>(&'a Ret, C)
-        // where C: impl Fn(&'a Ret) -> bool,
+        // contract_check_ensures::<Ret, C>(Ret, C) -> Ret
+        // where C: for<'a> Fn(&'a Ret) -> bool,
         //
-        // so: two type params, one lifetime param, 0 const params, two inputs, no return
-
-        let p = generics.param_at(0, tcx);
-        let r = ty::Region::new_early_param(tcx, p.to_early_bound_region_data());
-        let ref_ret = Ty::new_imm_ref(tcx, r, param(1));
-        (2, 1, 0, vec![ref_ret, param(2)], tcx.types.unit, hir::Safety::Safe)
+        // so: two type params, 0 lifetime param, 0 const params, two inputs, no return
+        (2, 0, 0, vec![param(0), param(1)], param(0), hir::Safety::Safe)
     } else {
         let safety = intrinsic_operation_unsafety(tcx, intrinsic_id);
         let (n_tps, n_cts, inputs, output) = match intrinsic_name {

library/core/src/contracts.rs

@@ -6,16 +6,15 @@ 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
 }

library/core/src/intrinsics/mod.rs

@@ -4086,28 +4086,55 @@ pub const fn contract_checks() -> bool {
 ///
 /// By default, if `contract_checks` is enabled, this will panic with no unwind if the condition
 /// returns false.
+///
+/// Note that this function is a no-op during constant evaluation.
 #[cfg(not(bootstrap))]
-#[unstable(feature = "contracts_internals", issue = "128044" /* compiler-team#759 */)]
+#[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" /* compiler-team#759 */)]
-#[rustc_intrinsic]
-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");
-    }
+#[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: for<'a> Fn(&'a Ret) -> bool + Copy>(
+    ret: Ret,
+    cond: C,
+) -> Ret {
+    const_eval_select!(
+        @capture[Ret, C: for<'a> Fn(&'a 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
+        }
+    )
 }
 
 /// The intrinsic will return the size stored in that vtable.

tests/ui/contracts/contract-attributes-nest.chk_pass.stderr

@@ -7,5 +7,16 @@ LL | #![feature(contracts)]
    = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+warning: unreachable expression
+  --> $DIR/contract-attributes-nest.rs:23:1
+   |
+LL | #[core::contracts::ensures(|ret| *ret > 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
+...
+LL |         return x.baz + 50;
+   |         ----------------- any code following this expression is unreachable
+   |
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 2 warnings emitted
 

tests/ui/contracts/contract-attributes-nest.rs

@@ -21,6 +21,7 @@
 
 #[core::contracts::requires(x.baz > 0)]
 #[core::contracts::ensures(|ret| *ret > 100)]
+//~^ WARN unreachable expression [unreachable_code]
 fn nest(x: Baz) -> i32
 {
     loop {

tests/ui/contracts/contract-attributes-nest.unchk_fail_post.stderr

@@ -7,5 +7,16 @@ LL | #![feature(contracts)]
    = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+warning: unreachable expression
+  --> $DIR/contract-attributes-nest.rs:23:1
+   |
+LL | #[core::contracts::ensures(|ret| *ret > 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
+...
+LL |         return x.baz + 50;
+   |         ----------------- any code following this expression is unreachable
+   |
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 2 warnings emitted
 

tests/ui/contracts/contract-attributes-nest.unchk_fail_pre.stderr

@@ -7,5 +7,16 @@ LL | #![feature(contracts)]
    = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+warning: unreachable expression
+  --> $DIR/contract-attributes-nest.rs:23:1
+   |
+LL | #[core::contracts::ensures(|ret| *ret > 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
+...
+LL |         return x.baz + 50;
+   |         ----------------- any code following this expression is unreachable
+   |
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 2 warnings emitted
 

tests/ui/contracts/contract-attributes-nest.unchk_pass.stderr

@@ -7,5 +7,16 @@ LL | #![feature(contracts)]
    = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-warning: 1 warning emitted
+warning: unreachable expression
+  --> $DIR/contract-attributes-nest.rs:23:1
+   |
+LL | #[core::contracts::ensures(|ret| *ret > 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable expression
+...
+LL |         return x.baz + 50;
+   |         ----------------- any code following this expression is unreachable
+   |
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 2 warnings emitted