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.

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);