Fix unreachable expression warning

Invert the order that we pass the arguments to the
`contract_check_ensures` function to avoid the warning when the tail
of the function is unreachable.

Note that the call itself is also unreachable, but we have already
handled that case by ignoring unreachable call for contract calls.

Author: celinval

compiler/rustc_ast_lowering/src/expr.rs

@@ -401,11 +401,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         cond_hir_id: HirId,
     ) -> &'hir hir::Expr<'hir> {
         let cond_fn = self.expr_ident(span, cond_ident, cond_hir_id);
-        let span = self.mark_span_with_reason(DesugaringKind::Contract, span, None);
         let call_expr = self.expr_call_lang_item_fn_mut(
             span,
             hir::LangItem::ContractCheckEnsures,
-            arena_vec![self; *expr, *cond_fn],
+            arena_vec![self; *cond_fn, *expr],
         );
         self.arena.alloc(call_expr)
     }

compiler/rustc_ast_lowering/src/item.rs

@@ -1209,8 +1209,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let precond = if let Some(req) = &contract.requires {
                     // Lower the precondition check intrinsic.
                     let lowered_req = this.lower_expr_mut(&req);
+                    let req_span = this.mark_span_with_reason(
+                        DesugaringKind::Contract,
+                        lowered_req.span,
+                        None,
+                    );
                     let precond = this.expr_call_lang_item_fn_mut(
-                        req.span,
+                        req_span,
                         hir::LangItem::ContractCheckRequires,
                         &*arena_vec![this; lowered_req],
                     );
@@ -1220,6 +1225,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 };
                 let (postcond, body) = if let Some(ens) = &contract.ensures {
                     let ens_span = this.lower_span(ens.span);
+                    let ens_span =
+                        this.mark_span_with_reason(DesugaringKind::Contract, ens_span, None);
                     // Set up the postcondition `let` statement.
                     let check_ident: Ident =
                         Ident::from_str_and_span("__ensures_checker", ens_span);

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -236,7 +236,7 @@ pub fn check_intrinsic_type(
         // where C: for<'a> Fn(&'a Ret) -> bool,
         //
         // 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)
+        (2, 0, 0, vec![param(0), param(1)], param(1), 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

@@ -2,15 +2,20 @@
 
 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).
+/// This is an identity function used as part of the desugaring of the `#[ensures]` attribute.
 ///
-/// This call helps with type inference for the predicate.
+/// This is an existing hack to allow users to omit the type of the return value in their ensures
+/// attribute.
+///
+/// Ideally, rustc should be able to generate the type annotation.
+/// The existing lowering logic makes it rather hard to add the explicit type annotation,
+/// while the function call is fairly straight forward.
 #[unstable(feature = "contracts_internals", issue = "128044" /* compiler-team#759 */)]
+// Similar to `contract_check_requires`, we need to use the user-facing
+// `contracts` feature rather than the perma-unstable `contracts_internals`.
+// Const-checking doesn't honor allow internal unstable logic used by contract expansion.
 #[rustc_const_unstable(feature = "contracts", issue = "128044")]
 #[lang = "contract_build_check_ensures"]
-#[track_caller]
 pub const fn build_check_ensures<Ret, C>(cond: C) -> C
 where
     C: Fn(&Ret) -> bool + Copy + 'static,

library/core/src/intrinsics/mod.rs

@@ -3453,6 +3453,10 @@ pub const fn contract_checks() -> bool {
 ///
 /// Note that this function is a no-op during constant evaluation.
 #[unstable(feature = "contracts_internals", issue = "128044")]
+// Calls to this function get inserted by an AST expansion pass, which uses the equivalent of
+// `#[allow_internal_unstable]` to allow using `contracts_internals` functions. Const-checking
+// doesn't honor `#[allow_internal_unstable]`, so for the const feature gate we use the user-facing
+// `contracts` feature rather than the perma-unstable `contracts_internals`
 #[rustc_const_unstable(feature = "contracts", issue = "128044")]
 #[lang = "contract_check_requires"]
 #[rustc_intrinsic]
@@ -3478,12 +3482,15 @@ pub const fn contract_check_requires<C: Fn() -> bool + Copy>(cond: C) {
 /// Note that this function is a no-op during constant evaluation.
 #[cfg(not(bootstrap))]
 #[unstable(feature = "contracts_internals", issue = "128044")]
+// Similar to `contract_check_requires`, we need to use the user-facing
+// `contracts` feature rather than the perma-unstable `contracts_internals`.
+// Const-checking doesn't honor allow internal unstable logic used by contract expansion.
 #[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 {
+pub const fn contract_check_ensures<C: Fn(&Ret) -> bool + Copy, Ret>(cond: C, ret: Ret) -> Ret {
     const_eval_select!(
-        @capture[Ret, C: Fn(&Ret) -> bool + Copy] { ret: Ret, cond: C } -> Ret :
+        @capture[C: Fn(&Ret) -> bool + Copy, Ret] { cond: C, ret: Ret } -> Ret :
         if const {
             // Do nothing
             ret

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

@@ -7,16 +7,5 @@ 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: 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
+warning: 1 warning emitted
 

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

@@ -21,7 +21,6 @@
 
 #[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

@@ -7,16 +7,5 @@ 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: 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
+warning: 1 warning emitted
 

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

@@ -7,16 +7,5 @@ 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: 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
+warning: 1 warning emitted
 

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

@@ -7,16 +7,5 @@ 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: 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
+warning: 1 warning emitted