Contrapositive of `AsRef<str>` for convently taking owned `String`?

[jbe]

@spikespaz

[…] what is the trait bound for "take any type that can be coerced into an owned String, cloning if necessary, moving if not"?

It might be GenericCow<str> (edit: see example), but …

@spikespaz

[…] is harder to use. The caller must wrap their value in this type, and pass that to a function.

Edit: Also GenericCow<str> implies Borrow<str>, which is stricter than AsRef<str>. Thus GenericCow is the corresponding match for Borrow instead of AsRef.

Edit #2: On the other hand, Cow<'_, str> and GenericCow<str> are more generic as they support borrowing while you only wanted to require ownership in all cases. Thus Cow/GenericCow are different concepts: 1. borrowing (where Eq, Ord, and Hash behave consistent) instead of cheap conversion (where Eq, Ord, and Hash do not have to behave consistent), and 2. supporting both directions (either to a reference or to an owned value, while you only want an owned value).

@spikespaz

What if a function wants to operate on a type similar to Cow, but it requires ownership in all circumstances?

I would say then the equivalent to AsRef<str> is Into<String>, like you said. Note: Both are in std::convert.

@spikespaz

Into<String>: This may be too lose, but generally as I understand this trait implies a 1:1 map between the original from type and the String, meaning that no information is lost, it is a different representation. This does seem like the best option to me.

I think it's this.

@spikespaz

ToString: This probably isn't what we want. It could be, but only if T: ToString + !Display, because we probably don't want to be mutating the (lossy) representation that is usually returned from the blanket impl that comes with Display. Display is intended for just that: display to a human. It is not directly a conversion, it is a summary.

This is sometimes more lax, sometimes more strict (I think). More lax because It will accept everything with a string representation due to impl<T: ?Sized> ToString for T where T: Display + ?Sized, see implementators of ToString. More strict because it will not accept anything everything that is Into<String>, right?

@spikespaz

ToOwned<String>: I don't know what to think about this one. The name of the trait seems to be an exact description of how I would like the generic to behave. If the value is already owned, using .to_owned() does nothing and there is no wasted work. If the value is a reference, ToOwned implementation handles the behavior, which is usually to delegate to Clone.

This clones unnecessarily. See also this long thread.