vis note for no pub reexports glob import

[bvanjoi]

Fixes #115966

Only trigger the unused_import lint when it's not being used.

r? @petrochenkov

[petrochenkov]

Minimized reproduction:

mod m {
    pub(crate) type A = u8;
}

pub use m::*; // warning: glob import doesn't reexport anything because no candidate is public enough

fn main() {
    let _: A;
}

This warning uses a wrong model of glob import's behavior.
It assumes that everything smaller than the glob's nominal visibility (pub) is filtered away, while it's not actually filtered, just lowered in visibility.

[petrochenkov]

This warning uses a wrong model of glob import's behavior.
It assumes that everything smaller than the glob's nominal visibility (pub) is filtered away, while it's not actually filtered, just lowered in visibility.

Sigh, turns out this was the right model of glob import's behavior, and for globs we produced the same error as for single imports in the same case.
But then it was changed into a lint in #65539, I need to re-read that thread.

[petrochenkov]

On closer look, the lint currently works as expected, the change in this PR is not correct.

The only problem is that it is not really an unused imports lint.
The lint message can be extended to better reflect what this lint wants to say:

glob import doesn't reexport anything because no candidate is public enough

=>

glob import doesn't reexport anything with visibility `pub` because no imported item is public enough
NOTE: the most public imported item is `pub(crate)`
SUGGESTION: reduce the glob import's visibility or increase visibility of imported items

Let me check if there's any more appropriate category for lint than UNUSED_IMPORTS.

[petrochenkov]

Yeah, besides introducing a new lint name, which I'm hesitant to do, the only plausible candidates are UNUSED_IMPORTS and maybe HIDDEN_GLOB_REEXPORTS and UNREACHABLE_PUB.

Let's keep it UNUSED_IMPORTS for now and just improve the message wording and suggestions.
@rustbot author

[bvanjoi]

NOTE: the most public imported item is pub(crate)

We cannot obtain concrete information (such as pub(crate) or pub(super)) from ty::Visibility::Restricted, which implies that we need to store ast::Visibility into vis during the resolution process.

[petrochenkov]

Printing whatever is available from ty::Visibility would be enogh.

[bvanjoi]

I have postponed this lint report until the privacy check in order to print the maximum visibility information.

[bvanjoi]

ci is green.

@rustbot ready

[bvanjoi]

@rustbot ready

[petrochenkov]

@bors r+

[bors]

📌 Commit d0941f9 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit d0941f9 with merge 64d7e0d...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing 64d7e0d to master...

[rust-timer]

Finished benchmarking commit (64d7e0d): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-2.5%	[-2.9%, -2.0%]	2
All ❌✅ (primary)	-1.8%	[-1.8%, -1.8%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 672.331s -> 672.427s (0.01%)
Artifact size: 313.37 MiB -> 313.37 MiB (0.00%)