contracts: fix lowering final declaration without trailing semicolon

Lowering for contract delcarations incorrectly handled the final
declaration statement when it didn't end in a semicolon. This
change fixes the issue.

Author: dawidl022

compiler/rustc_ast_lowering/src/contract.rs

@@ -23,7 +23,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         // The order in which things are lowered is important! I.e to
         // refer to variables in contract_decls from postcond/precond,
         // we must lower it first!
-        let contract_decls = self.lower_stmts(&contract.declarations).0;
+        let contract_decls = self.lower_decls(contract);
 
         match (&contract.requires, &contract.ensures) {
             (Some(req), Some(ens)) => {
@@ -124,6 +124,18 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
     }
 
+    fn lower_decls(&mut self, contract: &rustc_ast::FnContract) -> &'hir [rustc_hir::Stmt<'hir>] {
+        let (decls, decls_tail) = self.lower_stmts(&contract.declarations);
+
+        if let Some(e) = decls_tail {
+            // include the tail expression in the declaration statements
+            let tail = self.stmt_expr(e.span, *e);
+            self.arena.alloc_from_iter(decls.into_iter().map(|d| *d).chain([tail].into_iter()))
+        } else {
+            decls
+        }
+    }
+
     /// Lower the precondition check intrinsic.
     fn lower_precond(&mut self, req: &Box<rustc_ast::Expr>) -> rustc_hir::Stmt<'hir> {
         let lowered_req = self.lower_expr_mut(&req);

tests/ui/contracts/requires-block-stmt.rs

@@ -0,0 +1,22 @@
+//@ run-pass
+//@ 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;
+
+// Compound statements (those using [ExpressionWithBlock]
+// (https://doc.rust-lang.org/beta/reference/expressions.html#railroad-ExpressionWithBlock))
+// like blocks, if-expressions, and loops require no trailing semicolon. This
+// regression test captures the case where the last statement in the contract
+// declarations has no trailing semicolon.
+#[requires(
+    {}
+    true
+)]
+fn foo() {}
+
+fn main() {
+    foo()
+}

tests/ui/contracts/requires-block-stmt.stderr

@@ -0,0 +1,11 @@
+warning: the feature `contracts` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/requires-block-stmt.rs:4:12
+   |
+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
+