[libcore/cmp] Expand Ord/PartialOrd Derivable doc for enum types

[behnam]

Expand Derivable docblock section for Ord and PartialOrd to cover
enum types, in addition to the existing language explaining it for
struct types.

[rust-highfive]

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[behnam]

Context: compare with Derivable section of PartialEq, which explains behavior for struct and enum separately: https://doc.rust-lang.org/std/cmp/trait.PartialEq.html#derivable. The language added in this diff is based on that.

[behnam]

Something slightly related here: at the moment, looking at the library docs and the book, it's not clear how PartialEq can be implemented for enum values, specially when they are not Eq.

For example, let's say we have

enum BookFormat {
Paperback,
Hardback,
Ebook,
Unknown,
}

where we expect Unknown != Unknown.

Then, in this case, the eq() implementation needs to use match self to detect Unknown value, and trying to use == results in unconditional recursion.

Do you think this is something that can be address in libcore/cmp.rs?

[arielb1]

r? @BurntSushi

[BurntSushi]

@bors r+

[bors]

📌 Commit 330dab8 has been approved by BurntSushi

[BurntSushi]

@behnam Seems like you probably just want to implement PartialEq explicitly? (If I'm misunderstanding, maybe open a separate issue?)

[behnam]

Thanks, @BurntSushi. Yeah, let me make a good example and file a separate issue.

[arielb1]

@bors rollup