Imports/reexports of built-in macros are not supported

[petrochenkov]

use column; // error: cannot import a built-in macro

fn main() {}

The reason behind this error is that if you import something with a DefId, that DefId should be supported by all stages of compilation (metadata emitting specifically), but built-in macros don't have such a DefId.

The crate ID in their DefId has special value CrateNum::BuiltinMacros that is supported only inside rustc_resolve and causes ICEs every time it leaves it.

[petrochenkov]

Possible fix 1.

Use LOCAL_CRATE as a CrateNum in built-in macros' DefIds.

Built-in macros have fixed def-indices, so we should be able to recognize a built-in macro when loading metadata from other crate and thus know how to expand it.

Using LOCAL_CRATE will mean that each crate reexporting built-in macros will reexport it's own different version (with a separate DefId).
This is not correct and observable through import conflicts (imports of the same macro will conflict while they shouldn't).
This can perhaps be fixed by changing DefIds (resetting crate num to LOCAL_CRATE) on the fly during metadata loading (?). Not sure.

[petrochenkov]

Possible fix 2.

Introduce a special CrateNum for things defined by the language rather than any crate and support it through the whole compilation (aka CrateNum::BuiltinMacros done right).

Built-in types could potentially use it as well if it becomes necessary for them to have DefIds.

[petrochenkov]

The primary motivation for fixing this is reexporting built-in macros from the standard library.

This way those macros will have a reliable way to call them (e.g. ::std::column!()) from other macros (the __rust_unstable_column!() problem) even without having stable def-site hygiene in the language.

[petrochenkov]

Fixed in #62086