Parse destructuring assignment

The only patterns we should parse are `..` in structs and `_`: the rest are either not supported or already valid expressions.

crates/ide_db/src/helpers.rs

@@ -186,6 +186,7 @@ pub fn for_each_tail_expr(expr: &ast::Expr, cb: &mut dyn FnMut(&ast::Expr)) {
         | ast::Expr::TupleExpr(_)
         | ast::Expr::WhileExpr(_)
         | ast::Expr::LetExpr(_)
+        | ast::Expr::UnderscoreExpr(_)
         | ast::Expr::YieldExpr(_) => cb(expr),
     }
 }