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: 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 {

Diff for: 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

Diff for: 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 {

Diff for: 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
 }

Diff for: 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.

Diff for: 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
 

Diff for: 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 {

Diff for: 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
 

Diff for: 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
 

Diff for: 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
 

Diff for: tests/ui/contracts/contract-const-fn.all_pass.stderr

@@ -0,0 +1,11 @@
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/contract-const-fn.rs:17:12
+   |
+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
+

Diff for: tests/ui/contracts/contract-const-fn.rs

@@ -0,0 +1,56 @@
+//! Check if we can annotate a constant function with contracts.
+//!
+//! The contract is only checked at runtime, and it will not fail if evaluated statically.
+//! This is an existing limitation due to the existing architecture and the lack of constant
+//! closures.
+//!
+//@ revisions: all_pass runtime_fail_pre runtime_fail_post
+//
+//@ [all_pass] run-pass
+//
+//@ [runtime_fail_pre] run-fail
+//@ [runtime_fail_post] run-fail
+//
+//@ [all_pass] compile-flags: -Zcontract-checks=yes
+//@ [runtime_fail_pre] compile-flags: -Zcontract-checks=yes
+//@ [runtime_fail_post] compile-flags: -Zcontract-checks=yes
+#![feature(contracts)]
+//~^ WARN the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes [incomplete_features]
+
+extern crate core;
+use core::contracts::*;
+
+#[requires(x < 100)]
+const fn less_than_100(x: u8) -> u8 {
+    x
+}
+
+// This is wrong on purpose.
+#[ensures(|ret| *ret)]
+const fn always_true(b: bool) -> bool {
+    b
+}
+
+const ZERO: u8 = less_than_100(0);
+// This is no-op because the contract cannot be checked at compilation time.
+const TWO_HUNDRED: u8 = less_than_100(200);
+
+/// Example from <https://github.com/rust-lang/rust/issues/136925>.
+#[ensures(move |ret: &u32| *ret > x)]
+const fn broken_sum(x: u32, y: u32) -> u32 {
+    x + y
+}
+
+fn main() {
+    assert_eq!(ZERO, 0);
+    assert_eq!(TWO_HUNDRED, 200);
+    assert_eq!(broken_sum(0, 1), 1);
+    assert_eq!(always_true(true), true);
+
+    #[cfg(runtime_fail_post)]
+    let _ok = always_true(false);
+
+    // Runtime check should fail.
+    #[cfg(runtime_fail_pre)]
+    let _200 = less_than_100(200);
+}

Diff for: tests/ui/contracts/contract-const-fn.runtime_fail_post.stderr

@@ -0,0 +1,11 @@
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/contract-const-fn.rs:17:12
+   |
+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
+

Diff for: tests/ui/contracts/contract-const-fn.runtime_fail_pre.stderr

@@ -0,0 +1,11 @@
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/contract-const-fn.rs:17:12
+   |
+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
+

Diff for: tests/ui/contracts/internal_machinery/contract-ast-extensions-nest.rs

@@ -21,6 +21,7 @@
 fn nest(x: Baz) -> i32
     contract_requires(|| x.baz > 0)
     contract_ensures(|ret| *ret > 100)
+    //~^ WARN unreachable expression [unreachable_code]
 {
     loop {
         return x.baz + 50;

Diff for: tests/ui/contracts/internal_machinery/contract-intrinsics.rs

@@ -26,11 +26,11 @@ fn main() {
     #[cfg(any(default, unchk_pass, chk_fail_requires))]
     core::intrinsics::contract_check_requires(|| false);
 
-    let doubles_to_two = { let old = 2; move |ret| ret + ret == old };
+    let doubles_to_two = { let old = 2; move |ret: &u32 | ret + ret == old };
     // Always pass
-    core::intrinsics::contract_check_ensures(&1, doubles_to_two);
+    core::intrinsics::contract_check_ensures(1, doubles_to_two);
 
     // Fail if enabled
     #[cfg(any(default, unchk_pass, chk_fail_ensures))]
-    core::intrinsics::contract_check_ensures(&2, doubles_to_two);
+    core::intrinsics::contract_check_ensures(2, doubles_to_two);
 }

Diff for: tests/ui/contracts/internal_machinery/contract-lang-items.rs

@@ -15,14 +15,14 @@
 #![feature(contracts)] // to access core::contracts
 //~^ WARN the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes [incomplete_features]
 #![feature(contracts_internals)] // to access check_requires lang item
-
+#![feature(core_intrinsics)]
 fn foo(x: Baz) -> i32 {
     let injected_checker = {
         core::contracts::build_check_ensures(|ret| *ret > 100)
     };
 
     let ret = x.baz + 50;
-    injected_checker(ret)
+    core::intrinsics::contract_check_ensures(ret, injected_checker)
 }
 
 struct Baz { baz: i32 }