Turn Cow::is_borrowed,is_owned into associated functions.

[theemathas]

This is done because Cow implements Deref. Therefore, to avoid conflicts with an inner type having a method of the same name, we use an associated method, like Box::into_raw.

Tracking issue: #65143

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[theemathas]

r? libs-api

[joshtriplett]

So, +1 for documenting that users might want to call the functions this way if there's a conflict and they want to be explicit, but -1 for forcing the functions to be called this way. Yes, I understand that Cow implements Deref and these would shadow inner functions. However, we already have some methods on Cow: into_owned and to_mut. This doesn't in practice seem to pose a problem.

Did a crater run turn up some issue with these two new methods?

I also think these two new methods aren't especially useful if they can't be written in postfix form; at that point, you're not far off from just using matches! or let chains or waiting for is. So if there's some issue that stops us from making these methods, I think we should remove them entirely.

[bors]

☔ The latest upstream changes (presumably #138208) made this pull request unmergeable. Please resolve the merge conflicts.