Explain why macros cannot be imported from the same crate

[johnthagen]

I was reading the Macro Changes page:

Note that you'll still need #[macro_use] to use macros you've defined in your own crate; this feature only works for importing macros from external crates.

A couple questions that immediately came to mind:

• Why? Could a link be added for people to read more about this?
• Is it planned that in the future you can import macros from your own crate?

I think it would be helpful to add a little background about this, as I'm sure others will have similar questions.

[johnthagen]

These seem to be related:

• Tracking issue: declarative macros 2.0 rust#39412
• https://github.com/rust-lang/rfcs/blob/master/text/1561-macro-naming.md

[johnthagen]

@petrochenkov Shared a short explanation for this on the internals forum.

[ehuss]

In #232 this section has moved to https://github.com/rust-lang/edition-guide/blob/302a115e8f71876dfc884aebb0ca5ccb02b8a962/src/rust-2018/path-changes.md#macros.