expand: Change how #![cfg(FALSE)] behaves on crate root

[petrochenkov]

Previously it removed all other attributes from the crate root.
Now it removes only attributes below itself (during both regular expansion and pre-configuration).

So it becomes possible to configure some global crate properties even for fully unconfigured crates.

Fixes #104633
Part of #110082

[rustbot]

r? @WaffleLapkin

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[petrochenkov]

This needs a crater run and the breakage list will be similar to #108221 (comment) and caused by crater peculiarities - it builds all crates for x86 host, including those that are supposed to be used with embedded targets and not supposed to be built for the host.

@bors try

[bors]

⌛ Trying commit 8142da2de449dc5f1d51a703b2b00de197eada89 with merge d57d2d63e6f308d854fedbafd91d7721b7c651af...

[bors]

☀️ Try build successful - checks-actions
Build commit: d57d2d63e6f308d854fedbafd91d7721b7c651af (d57d2d63e6f308d854fedbafd91d7721b7c651af)

[petrochenkov]

@craterbot check

[craterbot]

👌 Experiment pr-110141 created and queued.
🤖 Automatically detected try build d57d2d63e6f308d854fedbafd91d7721b7c651af
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-110141 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-110141 is completed!
📊 25 regressed and 3 fixed (261080 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[petrochenkov]

get_if_addrs-sys-0.1.1 - denying removed lint above cfg(FALSE) (it's a warning, but this crate denies warnings)

chaudharypraveen98.actix-stackoverflow-api.039a1e82a210b3d55933f5dc672309f17040d7ad - cannot reproduce

All other regressions come from crates for embedded targets, maybe even from a common dependency which has something like #![cfg(target = "my-embedded-target")] at its crate root.
During crater run all of those crates are built for x86 host (which is not their intended use case).
Previously that fully unconfigured crate wasn't considered no_std and linked libstd, as a result the x86 host build succeeded.
After this PR that fully uncondifured crate becomes no_std and stop linking libstd breaking x86 host build.

[petrochenkov]

Still need to write the change description for lang team.

[petrochenkov]

Change description for the lang team.

This PR changes how #![cfg(FALSE)] works when applied to a crate root.

#![attr_before]
#![cfg(FALSE)] // <--
#![attr_after]

fn item1() {}

fn item2() {}

fn main() {}

Unlike cfg(FALSE) on other nodes, crate root cfg cannot remove the whole node it's applied to, the crate still exists after it is applied.
So it only cleanups the contents of the crate.

Before this PR cfg(FALSE) removed all items and all crate-level attributes in the crate.
So the crate could lose its name and type if specified with #![crate_(name,type)], could lose #![no_std] and start linking to std (#110082), and could lose #![feature] attributes and produce errors during parsing unstable syntax (#104633).

After this PR cfg(FALSE) performs less cleanup and only removes all items in the crate, and all crate-level attributes after itself.
This way we can configure global crate properties like name, type, std linking, list of features, etc even in case of fully unconfigured crates, we just need to put the relevant crate-level attributes above the #![cfg(FALSE)].

Result of expanding the example above:

// Before this PR

// After this PR

#![attr_before]

The change applies to both pre-configuration (#108221) which is relevant to a few attributes used very early, and to regular crate expansion that is relevant to all other attributes.

[nikomatsakis]

@rfcbot fcp merge

Discussed in @rust-lang/lang meeting and we felt that this change was consistent with other behavior at crate root. Moving to merge.

[rfcbot]

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

• @Aaron1011
• @cjgillot
• @davidtwco
• @eddyb
• @estebank
• @joshtriplett
• @lcnr
• @matthewjasper
• @michaelwoerister
• @nagisa
• @nikomatsakis
• @oli-obk
• @petrochenkov
• @pnkfelix
• @scottmcm
• @tmandry
• @wesleywiser

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[GuillaumeGomez]

It failed in #112470.

@bors r-

[petrochenkov]

Rebased.
@bors r=WaffleLapkin

[bors]

📌 Commit 46becfd has been approved by WaffleLapkin

It is now in the queue for this repository.