Add error explanation for E0502

[timothy-mcroy]

I am questioning the order of presentation on the suggested code fixes, but I'm not sure what would be best. Thoughts?
#32777

r? @GuillaumeGomez

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @GuillaumeGomez (or someone else) soon.

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[GuillaumeGomez]

Extra whitespace.

[GuillaumeGomez]

I think it's not completely well explained. For example, you say that if you want the value before modifying it, you can just copy it. But it's absolutely wrong in many cases (in every case where the type doesn't implement Clone trait). We should try to keep this explanation as simple as possible. Showing that you can use a reference on the type inside its own scope can be useful, but is it really interesting to show it here?

So in conclusion, could you make the explanation simpler please? Something like "please check you don't have any other reference to your variable before trying to get a mutable access to it", followed by an example.

[timothy-mcroy]

Sure thing. Thanks for the critique. I was worried about the Clone requirement, tbh. Do you feel that suggested fixes aren't appropriate here, or simply that mine aren't helpful?

[GuillaumeGomez]

I think they're not helpful. However, providing an url to the rust book was an excellent idea.

[timothy-mcroy]

I have simplified the explanation.

[timothy-mcroy]

Do you want me to squash the previous commit?

Also, there will be a merge conflict with my changes on #33294. Is there anything that I need to do about that?

[GuillaumeGomez]

Oh right, I didn't pay attention to commits. Please squash them. Thanks for notifying me about it.

For the conflict that'll occur, the only choice is to wait that one of the two PRs is merged before fixing the conflict and then merge the other one.

[timothy-mcroy]

@steveklabnik Phasers set to merge.

[bors]

☔ The latest upstream changes (presumably #33376) made this pull request unmergeable. Please resolve the merge conflicts.

[GuillaumeGomez]

@timothy-mcroy: You need to update your PR.

[timothy-mcroy]

Ah, yeah. Thanks for the reminder. Finals week. :/

[GuillaumeGomez]

Extra empty line.

[GuillaumeGomez]

Two nits to fixed and it's good to get merged! (good luck for your finals!)

[timothy-mcroy]

Nits are fixed. Thanks!

[GuillaumeGomez]

Thanks!

r=me @steveklabnik

[bors]

☔ The latest upstream changes (presumably #33658) made this pull request unmergeable. Please resolve the merge conflicts.

[timothy-mcroy]

Oh bors. Why must you be so angry with me? I'll get this updated.

[timothy-mcroy]

Ready to roll. @steveklabnik

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit bbb7844 has been approved by steveklabnik

[sanxiyn]

@bors r-

This failed Travis. When you add error explanation, you need to remove the error from the list at the end of the file. See 7a9f4c2 for an example.

[timothy-mcroy]

Gahhh, how did I forget that that time. Embarrassing. :/

[sanxiyn]

@bors r+ rollup

[bors]

📌 Commit acfe199 has been approved by sanxiyn