no mention of the builtin macro prelude

[lolbinarycat]

The builtin macro prelude is very similar to the language prelude in that it is still available even with no_implicit_prelude, but it differs in that:

1. it only contains function-style macros
2. it is not available to no_core crates

It contains macros like option_env and line. A full list is available in library/std/src/lib.rs.

I would like the reference to have a section on them so that rustdoc can provide a banner on builtin macros linking to the reference's section on builtin macros.

[ehuss]

Since no_core is not stable, is the existence of this prelude observable by the user? If not, then we probably wouldn't document it. Can you say why rustdoc can't link to the individual macro definitions as-is?

[lolbinarycat]

I want to link from the definition of builtin macros to a page explaining what builtin macros are.

If we want to ignore the existence of no_core, we could say they're part of the language prelude, but that's not what the reference does currently, currently we treat them as part of the library prelude, which is incorrect, since they are still available even under no_implicit_prelude.