There was a question whether this is equally performant. There are
multiple good reasons why it should be:
1. `get_or_insert_with` is marked `#[inline]`
2. Any good optimizer will inline a function that is used exactly once
3. 1 and 2 conclude that the closure will get inlined
4. Computing self.tx can then be moved to the only branch where it is
required.
5. Even if get_or_insert_with didn't get optimized, which is extremely
unlikely, the `tx` field is at the beginning of the struct and it
probably has pointer alignment (`Deref` suggests it's a pointer).
Alignment larger than pointer is not used, so we can expect the
fields to be ordered as-defined. (This is not guaranteed by Rust but
there's not good reason to change the order in this case.) We can
assume that offset to tx is zero in most cases which means no
computation is actually needed so the expression before closure is
no-op short of passing it into the closure as an argument.
At the time of writing `#[inline]` can be seen at
https://doc.rust-lang.org/src/core/option.rs.html#933
Martin Habovstiak
Riccardo Casatta