influence type inference.
* - `braced_empty_structs` - Allows use of empty structs and enum variants with braces.
+* - `stmt_expr_attributes` - Allows attributes on expressions and
+ non-item statements.
+
If a feature is promoted to a language feature, then all existing programs will
start to receive compilation warnings about `#![feature]` directives which enabled
the new feature (because the directive is no longer necessary). However, if a