Understanding Borrow and ToOwned

[jbe]

To summarize the answers to the original post:

Question #1:

@jbe

I don't understand why Borrow has the borrowed type as a type parameter (e.g. Borrow<B> )? Is there any case where an owned type will allow borrowing as several different types (under the rules of Borrow that require Eq and others to be consistent)?

Answer #1:

@H2CO3

It's perfectly fine to borrow an owned type as two different borrowed types. I don't see why this would necessarily violate consistency with Eq / Ord / Hash . Here's an implementation where they work together just fine.

Question #2:

@jbe

So why does Borrow not use an associated type as target type?

Answer #2:

@quinedot

Every T implements Borrow<T>.Thus every other implementation results in an implementation for more than one type parameter (including the other std impls).

To give an example: String implements both Borrow<String> and Borrow<str>, so I can borrow it either as String or as str. And Borrow will assure that Ord behaves the same on the respective borrowed type (unlike AsRef).

This reminds me of the discussion in What is the difference between as_ref() & '&'. I would say that it's actually the main point of Borrow::borrow to be generic and to allow borrowing different types. Otherwise, we could just use &.