`core::panic` documentation is useless

[jplatte]

For some reason, the documentation for core::panic does not contain any of the useful pieces of information of std::panic's documentation and instead references a non-existent module std::macros.

[czipperz]

So it turns out the panic macro is defined in both libcore and libstd. Both have documentation. They are defined differently. Presumably libstd is the "better" implementation while libcore is the fallback when nostd is enabled.

I'm considering opening a PR that copies the documentation. Thoughts?

[czipperz]

I believe the documentation points the the other one because it doesn't want duplication of comments. Which is bad. But it might be better for std to point to core rather than the other way around. Or something like that.

[jplatte]

To avoid duplicating the text while still having the same doc comment for both macros, one could use the external_doc feature: https://doc.rust-lang.org/nightly/unstable-book/language-features/external-doc.html

[czipperz]

AFAIK there isn't a usual place to put these sorts of markdown files. Where would you recommend? We could move libcore/macros.rs to libcore/macros/mod.rs and then store all the markdown files in there?

[jplatte]

Fixed by #66395