Add 5 more error explanations. Update E0015's explanation. Add an error code.

[nham]

The E0397 explanation, as I've written it, isn't really an explanation, but I'm not sure what to put here. I will happily take suggestions.

Partially addresses #25851

[rust-highfive]

r? @Aatch

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[Aatch]

I think it makes more sense to link to the text of the RFC rather than the PR.

[Aatch]

The fact that enum variants and tuple structs are function calls is just confusing here. Now const fn is a thing, it's probably better to just say "only const fns can be called in static or constant expressions, since they obviously would have seen this error by using a non-const fn call.

[nham]

Thanks, good point.

[nham]

@Aatch I agree, but I don't know of a way to wrap URLs in Markdown, and the lines have to be <= 80 chars. I can do this if I don't set the URL out separately, but generally this is less readable when printed out on the command line. For example, it would look like this:

The only function calls allowed in static or constant expressions are
constructors for enum variants and structs, or `const fn`s. Rust currently does
not support more general compile-time function execution.

See [RFC 911](https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn
.md) for more details on the design of `const fn`s.

Edit: Now that I think about it, I'm not sure that this works either. Github renders the URL here as https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn%0A.md

[nham]

Ahh, I didn't see that the url for the text of RFC 911 is actually short enough to link to here. I've changed that. RFC 246's url is still too long

[Aatch]

While I'd still prefer a link to the RFC text, I can see the issue. It's not too big of problem though.

[Aatch]

@bors r+ fdf3ce7 rollup