resolve: skip underscore character during candidate lookup

[bvanjoi]

Fixes #116164

In use statement, an underscore is merely a placeholder symbol and does not bind to any name. Therefore, it can be safely ignored.

[rustbot]

r? @cjgillot

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[cjgillot]

I am under the impression that this fixes a symptom instead of the issue. _ should not create a binding at all, should it?

[bvanjoi]

_ should not create a binding at all

Currently, the binding for as _ aims to ensure the correct resolution for trait:

mod m {
    pub trait Trait {
        fn t() {}
    }
    impl<T> Trait for T {}
}

use m::Trait as _;

fn f() {
    <()>::t();
}

I'm uncertain if it's justified to change the existing behavior solely for #116164, or perhaps it should be treated as a separate issue.

[bvanjoi]

Incidentally, to avoid binding _ in all cases exclude traits, I think we should amend the condition in imports from:

Ok(binding) if binding.is_importable() => {
    // define
}

to:

Ok(binding) if binding.is_importable() => {
    if ident.name == kw::Undersoce && ns != Namespace::Trait { 
        // `Namespace::Trait` implies that we need to handle traits separately,
        // which may result in substantial changes.
        return ;
    }
    // define
}

[cjgillot]

Thanks for the explanation @bvanjoi. Let's go with this implementation then.
@bors r+

[bors]

📌 Commit cfb819f has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit cfb819f with merge 16b7b39...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 16b7b39 to master...

[rust-timer]

Finished benchmarking commit (16b7b39): 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)	1.9%	[1.1%, 3.2%]	17
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.0%	[-8.2%, -7.9%]	2
All ❌✅ (primary)	1.9%	[1.1%, 3.2%]	17

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: 627.002s -> 629.278s (0.36%)
Artifact size: 273.33 MiB -> 273.33 MiB (0.00%)