Refactor the #[allow(dead_code)] propagation for impl items of traits

[mu001999]

Extracted from #157885.

This PR does the refactor and corrects the previous implementation.

The following will fail currently (play):

#![deny(dead_code)]
#![deny(unfulfilled_lint_expectations)]

#[allow(dead_code)]
pub trait Tr {
    fn foo(&self);
}

struct Foo;

impl Tr for Foo {
    fn foo(&self) {
        bar();
    }
}

#[expect(dead_code)]
fn bar() {}

fn main() {}

After this PR, we could handle the #[allow(dead_code)] propagation correctly, and should get perf improvement.

[mu001999]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ef781c8 (ef781c8005ba42aaa03739a3effec6ddb8e82cbe)
Base parent: 2f54787 (2f54787b636028a19294176d0fb27794452e6608)

[rust-timer]

Finished benchmarking commit (ef781c8): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-1.3%, -0.2%]	35
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	9
All ❌✅ (primary)	-0.5%	[-1.3%, -0.2%]	35

Max RSS (memory usage)

Results (secondary 5.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.9%	[5.9%, 5.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 3.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.7%	[2.7%, 7.2%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 470.877s -> 469.545s (-0.28%)
Artifact size: 400.19 MiB -> 400.97 MiB (0.19%)

[rustbot]

r? @chenyukang

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• Owners of files modified in this PR: compiler
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[mu001999]

@rustbot author

[mu001999]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e466d2a (e466d2ab296542be4a4e93a797980f81378d18a4)
Base parent: da51146 (da5114692c9ebe46b869488c5f34f92eb10b98c1)

[rust-timer]

Finished benchmarking commit (e466d2a): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.5%	[-1.3%, -0.1%]	37
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	10
All ❌✅ (primary)	-0.5%	[-1.3%, -0.1%]	37

Max RSS (memory usage)

Results (secondary 7.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	7.7%	[7.7%, 7.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -3.1%, secondary -3.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.1%	[-3.2%, -3.1%]	2
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	-3.1%	[-3.2%, -3.1%]	2

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 469.244s -> 471.227s (0.42%)
Artifact size: 400.19 MiB -> 400.33 MiB (0.03%)

[mu001999]

@rustbot ready