Cow type (Clone-on-write)

[danakj]

Clone-on-write lets you store a reference but on write it can convert to an owned version of the same type.

std.rs/cow

To work with it as an lvalue more, and avoid template writing, we may want to add a method or two:

• set_owned()

Context and use case: https://chromium.slack.com/archives/CGGPN5GDT/p1698242024888729?thread_ts=1698166154.431099&cid=CGGPN5GDT

The particular case I would've suggested it for is a GURL, so that I can do something like this:

Cow<GURL> request_url = ...; // some const GURL& on the RHS
if (my_condition) {
  request_url = Cow::Owned(some_transformation(request_url));
}
// Now use request_url as usual

Instead of doing:

const GURL* request_url = &some_url;
GURL transformed_url;
if (my_condition) {
  transformed_url = some_transformation(*request_url);
  request_url = &transformed_url;
}
// Now use request_url as usual

The goal is just to minimize work in the common case where my_condition is false; and to nicely encapsulate the lifetime concerns introduced by using a raw pointer in the current approach.