TooOwned

[kornelski]

What it does

Warn against use of Cow<String> or Cow<Vec<u8>>, instead of Cow<str> or Cow<[u8]>.

[u8] is an unusual type, and it's not obvious that Cow<[u8]> can hold a Vec<u8>, so users can mistakenly use Cow<Vec<u8>> instead. However, Cow<Vec<u8>> wasn't meant to be used. It creates an extra level of indirection, and Cow::Borrowed is limited to holding a less-general &Vec instead of a slice. Both Cow<[u8]> and Cow<Vec<u8>> have the same size, so the other form doesn't even benefit from holding a thin pointer.

Same for CStr, OsStr, Path. From implementation perspective, I think it could warn when Cow<X> is used, and there exists Cow<Y, Owned = X>, and X != Y.

Advantage

It helps users discover the intended types for use in Cow, where the type argument is for a non-owned type, not the owned one.

Drawbacks

None?

Example

Cow<'_, Vec<u8>>

Could be written as:

Cow<'_, [u8]>

[llogiq]

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