resolve: Do not put nonexistent crate meta into prelude

[petrochenkov]

Before the 2018 edition release there was some vague suggestion about adding a crate named meta to the standard distribution.
On this basis the name meta was "partially reserved" by putting meta into extern prelude (this means importing something named meta will result in an ambiguity error, for example).
This only caused confusion so far, and two years later there are no specific plans to add such crate.

If some standard crate (named meta or not) is added in the future, then cargo will hopefully already have ability to put it into extern prelude explicitly through Cargo.toml.
Otherwise, it could be added to extern prelude by the compiler at edition boundary.

Closes#73948

[rust-highfive]

r? @nikomatsakis

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[bors]

☔ The latest upstream changes (presumably #74862) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

Could someone from lang team to look at this?
I'm pretty sure meta isn't going to be a thing in 2018 edition time frame, so we can remove this future-proofing.

[joshtriplett]

This seems reasonable. Our policy changed since the 2018 edition, and we no longer reserve keyword-like things until there's a concrete proposal.

There's still support in the @rust-lang/lang team for using the name meta for any future introduction of things from the sysroot, to avoid introducing more top-level names. But it makes sense that doing so should require some indication in Cargo.toml`, rather than something fully implicit.

Given that:

@rfcbot merge

We'd welcome someone submitting concrete patches adding sysroot names under a built-in crate meta, along with cargo patches, though.

[rfcbot]

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

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @withoutboats

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), 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.

[emilyalbini]

As per @pnkfelix's request on Discord, the meta crate on crates.io was transferred to the Rust project (the author consented the transfer to anyone requesting it).

[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.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

[nikomatsakis]

r=me once rebased

[petrochenkov]

@bors r=nikomatsakis rollup

[bors]

📌 Commit 3522add has been approved by nikomatsakis