Fix lowering of empty macro expressions in trailing position

Author: Jonas Schievink

crates/hir-def/src/body/lower.rs

@@ -551,9 +551,17 @@ impl ExprCollector<'_> {
                 }
             }
             ast::Expr::MacroStmts(e) => {
-                let statements = e.statements().filter_map(|s| self.collect_stmt(s)).collect();
+                let statements: Box<[_]> =
+                    e.statements().filter_map(|s| self.collect_stmt(s)).collect();
                 let tail = e.expr().map(|e| self.collect_expr(e));
 
+                if e.syntax().children().next().is_none() {
+                    // HACK: make sure that macros that expand to nothing aren't treated as a `()`
+                    // expression when used in block tail position.
+                    cov_mark::hit!(empty_macro_in_trailing_position_is_removed);
+                    return None;
+                }
+
                 self.alloc_expr(Expr::MacroStmts { tail, statements }, syntax_ptr)
             }
             ast::Expr::UnderscoreExpr(_) => self.alloc_expr(Expr::Underscore, syntax_ptr),

crates/hir-ty/src/tests/regression.rs

@@ -1648,3 +1648,20 @@ fn main() {
         "#]],
     );
 }
+
+#[test]
+fn trailing_empty_macro() {
+    cov_mark::check!(empty_macro_in_trailing_position_is_removed);
+    check_no_mismatches(
+        r#"
+macro_rules! m2 {
+    ($($t:tt)*) => {$($t)*};
+}
+
+fn macrostmts() -> u8 {
+    m2! { 0 }
+    m2! {}
+}
+    "#,
+    );
+}