Sub-bindings, e.g. ref x @ Some(ref y)
are now allowed under #![feature(bindings_after_at)]
and checked to make sure that memory safety is upheld.
In certain cases it is possible for sub-bindings to violate memory safety. Updates to the borrow checker in a future version of Rust may remove this restriction, but for now patterns must be rewritten without sub-bindings.
Before:
match Some("hi".to_string()) { ref op_string_ref @ Some(s) => {}, None => {}, }
After:
match Some("hi".to_string()) { Some(ref s) => { let op_string_ref = &Some(s); // ... }, None => {}, }
The op_string_ref
binding has type &Option<&String>
in both cases.
See also Issue 14587.