resolve: Do not error on access to proc macros imported with #[macro_use]

[petrochenkov]

This error is artificial, but previously, when #[macro_use] extern crate x; was stable, but non-derive proc macros were not, it worked like kind of a feature gate. Now both features are stable, so the error is no longer necessary.

This PR simplifies how #[macro_use] extern crate x; works - it takes all items from macro namespace of x's root and puts them into macro prelude from which they all can now be accessed.

[rust-highfive]

r? @aturon

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[petrochenkov]

r? @alexcrichton

[alexcrichton]

While this seems plausible to me, I'd defer to @rust-lang/lang in this regard as it's moreso a language change they'd likely wish to discuss.

Folks on @rust-lang/lang, how do you feel about #[macro_use], a hopefully legacy attribute soon, working for new shiny new procedural macro attributes we've stabilized?

[joshtriplett]

Given that we're moving towards using use for macros, and use works for procedural macros, do we want to make #[macro_use] work for this?

[petrochenkov]

Given that #[macro_use] extern crate ... is being retired, I'd say that it largely doesn't matter from language usability point of view whether this case is allowed or not.
My primary motivation for doing this is simplification of the rules and removal of technical debt from the compiler.

[joshtriplett]

@petrochenkov Fair enough. Sounds reasonable to me.

[Centril]

I also think it sounds reasonable :)
...we should discuss this further on the meeting,
...but to expedite things, let's:

@rfcbot fcp merge

[rfcbot]

Team member @Centril has proposed to merge this. The next step is review by the rest of the tagged teams:

• @Centril
• @aturon
• @cramertj
• @eddyb
• @joshtriplett
• @nikomatsakis
• @nrc
• @pnkfelix
• @scottmcm
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[bors]

☔ The latest upstream changes (presumably #53471) made this pull request unmergeable. Please resolve the merge conflicts.

[scottmcm]

If this is just allowing the compiler to delete code, not changing idioms,

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

[petrochenkov]

Yay, FCP complete.
@bors r=alexcrichton

[bors]

📌 Commit e411bb3 has been approved by alexcrichton

[bors]

⌛ Testing commit e411bb3 with merge cb6d2df...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing cb6d2df to master...