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.

16 files changed, 39 insertions, 66 deletions

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index 0b7a7630431..c3f57e11183 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/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)
     }
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 28f596ac092..c89112fc5a3 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/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);
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index 290e47b42b5..e54fa89328c 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/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 {
diff --git a/library/core/src/contracts.rs b/library/core/src/contracts.rs
index 82922642018..53f459debab 100644
--- a/library/core/src/contracts.rs
+++ b/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,
diff --git a/library/core/src/intrinsics/mod.rs b/library/core/src/intrinsics/mod.rs
index 8812cb66526..90f686a6728 100644
--- a/library/core/src/intrinsics/mod.rs
+++ b/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
diff --git a/tests/ui/contracts/contract-attributes-nest.chk_pass.stderr b/tests/ui/contracts/contract-attributes-nest.chk_pass.stderr
index e7c42ad98a5..9ca95b8bb01 100644
--- a/tests/ui/contracts/contract-attributes-nest.chk_pass.stderr
+++ b/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
 
diff --git a/tests/ui/contracts/contract-attributes-nest.rs b/tests/ui/contracts/contract-attributes-nest.rs
index 7c35e54c18b..e1e61b88f28 100644
--- a/tests/ui/contracts/contract-attributes-nest.rs
+++ b/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 {
diff --git a/tests/ui/contracts/contract-attributes-nest.unchk_fail_post.stderr b/tests/ui/contracts/contract-attributes-nest.unchk_fail_post.stderr
index e7c42ad98a5..9ca95b8bb01 100644
--- a/tests/ui/contracts/contract-attributes-nest.unchk_fail_post.stderr
+++ b/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
 
diff --git a/tests/ui/contracts/contract-attributes-nest.unchk_fail_pre.stderr b/tests/ui/contracts/contract-attributes-nest.unchk_fail_pre.stderr
index e7c42ad98a5..9ca95b8bb01 100644
--- a/tests/ui/contracts/contract-attributes-nest.unchk_fail_pre.stderr
+++ b/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
 
diff --git a/tests/ui/contracts/contract-attributes-nest.unchk_pass.stderr b/tests/ui/contracts/contract-attributes-nest.unchk_pass.stderr
index e7c42ad98a5..9ca95b8bb01 100644
--- a/tests/ui/contracts/contract-attributes-nest.unchk_pass.stderr
+++ b/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
 
diff --git a/tests/ui/contracts/contract-captures-via-closure-noncopy.stderr b/tests/ui/contracts/contract-captures-via-closure-noncopy.stderr
index 4a47671fee1..b6f2e014e0a 100644
--- a/tests/ui/contracts/contract-captures-via-closure-noncopy.stderr
+++ b/tests/ui/contracts/contract-captures-via-closure-noncopy.stderr
@@ -16,6 +16,7 @@ LL | #[core::contracts::ensures({let old = x; move |ret:&Baz| ret.baz == old.baz
    | |                                        within this `{closure@$DIR/contract-captures-via-closure-noncopy.rs:12:42: 12:57}`
    | |                                        this tail expression is of type `{closure@contract-captures-via-closure-noncopy.rs:12:42}`
    | unsatisfied trait bound
+   | required by a bound introduced by this call
    |
    = help: within `{closure@$DIR/contract-captures-via-closure-noncopy.rs:12:42: 12:57}`, the trait `std::marker::Copy` is not implemented for `Baz`
 note: required because it's used within this closure
diff --git a/tests/ui/contracts/internal_machinery/contract-ast-extensions-nest.rs b/tests/ui/contracts/internal_machinery/contract-ast-extensions-nest.rs
index 7f9c3fe28ce..6d8cd3949ee 100644
--- a/tests/ui/contracts/internal_machinery/contract-ast-extensions-nest.rs
+++ b/tests/ui/contracts/internal_machinery/contract-ast-extensions-nest.rs
@@ -21,7 +21,6 @@
 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 --git a/tests/ui/contracts/internal_machinery/contract-intrinsics.rs b/tests/ui/contracts/internal_machinery/contract-intrinsics.rs
index f94dfbde75f..c62b8cca75a 100644
--- a/tests/ui/contracts/internal_machinery/contract-intrinsics.rs
+++ b/tests/ui/contracts/internal_machinery/contract-intrinsics.rs
@@ -28,9 +28,9 @@ fn main() {
 
     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(doubles_to_two, 1);
 
     // 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(doubles_to_two, 2);
 }
diff --git a/tests/ui/contracts/internal_machinery/contract-lang-items.rs b/tests/ui/contracts/internal_machinery/contract-lang-items.rs
index 26042cf688f..73c59194531 100644
--- a/tests/ui/contracts/internal_machinery/contract-lang-items.rs
+++ b/tests/ui/contracts/internal_machinery/contract-lang-items.rs
@@ -22,7 +22,7 @@ fn foo(x: Baz) -> i32 {
     };
 
     let ret = x.baz + 50;
-    core::intrinsics::contract_check_ensures(ret, injected_checker)
+    core::intrinsics::contract_check_ensures(injected_checker, ret)
 }
 
 struct Baz { baz: i32 }
diff --git a/tests/ui/contracts/internal_machinery/internal-feature-gating.rs b/tests/ui/contracts/internal_machinery/internal-feature-gating.rs
index 1b76eef6780..6e5a7a3f950 100644
--- a/tests/ui/contracts/internal_machinery/internal-feature-gating.rs
+++ b/tests/ui/contracts/internal_machinery/internal-feature-gating.rs
@@ -6,7 +6,7 @@ fn main() {
     //~^ ERROR use of unstable library feature `contracts_internals`
     core::intrinsics::contract_check_requires(|| true);
     //~^ ERROR use of unstable library feature `contracts_internals`
-    core::intrinsics::contract_check_ensures(&1, |_|true);
+    core::intrinsics::contract_check_ensures( |_|true, &1);
     //~^ ERROR use of unstable library feature `contracts_internals`
 
     core::contracts::build_check_ensures(|_: &()| true);
diff --git a/tests/ui/contracts/internal_machinery/internal-feature-gating.stderr b/tests/ui/contracts/internal_machinery/internal-feature-gating.stderr
index 7302694a787..1e39bd62e24 100644
--- a/tests/ui/contracts/internal_machinery/internal-feature-gating.stderr
+++ b/tests/ui/contracts/internal_machinery/internal-feature-gating.stderr
@@ -41,7 +41,7 @@ LL |     core::intrinsics::contract_check_requires(|| true);
 error[E0658]: use of unstable library feature `contracts_internals`
   --> $DIR/internal-feature-gating.rs:9:5
    |
-LL |     core::intrinsics::contract_check_ensures(&1, |_|true);
+LL |     core::intrinsics::contract_check_ensures( |_|true, &1);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #128044 <https://github.com/rust-lang/rust/issues/128044> for more information