Simplify dead code lint

[cjgillot]

This PR scratches a few itches I had when looking at that code.

The perf improvement comes from keeping the scanned set through several marking phases. This pretty much divides by 2 the number of HIR traversals.

[cjgillot]

@bors try @rust-timer queue

[rust-bors]

⌛ Trying commit fdb0651 with merge 2d9299b…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 2d9299b (2d9299bf7e56a5bbe4eb425be9beb136afa85ac2, parent: f34ba774c78ea32b7c40598b8ad23e75cdac42a6)

[cjgillot]

@bors try @rust-timer queue

[rust-bors]

⌛ Trying commit effc509 with merge be8ce86…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: be8ce86 (be8ce86c3741a46b0e888e7a4f5d7b4dbe5da9a2, parent: f34ba774c78ea32b7c40598b8ad23e75cdac42a6)

[rust-timer]

Finished benchmarking commit (2d9299b): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.3%	[0.1%, 0.4%]	16
Improvements ✅ (primary)	-0.4%	[-1.2%, -0.1%]	77
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.1%]	9
All ❌✅ (primary)	-0.4%	[-1.2%, -0.1%]	77

Max RSS (memory usage)

Results (primary 1.6%, 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)	1.6%	[1.6%, 1.6%]	1
Regressions ❌ (secondary)	3.9%	[2.5%, 5.2%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.6%	[1.6%, 1.6%]	1

Cycles

Results (primary -2.4%, secondary 4.9%)

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)	4.9%	[2.1%, 7.8%]	2
Improvements ✅ (primary)	-2.4%	[-2.8%, -2.1%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.4%	[-2.8%, -2.1%]	4

Binary size

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

Bootstrap: 467.629s -> 467.986s (0.08%)
Artifact size: 376.97 MiB -> 376.92 MiB (-0.01%)

[cjgillot]

While keeping original scanned:
@bors try @rust-timer queue

[rust-bors]

⌛ Trying commit effc509 with merge af77cf0…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: af77cf0 (af77cf03e48d4f51f9a9ad3e985759a19ead7043, parent: e1b9081e699065badfc1a9419ec9566e5c8615c4)

[rust-timer]

Finished benchmarking commit (af77cf0): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@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%	[-1.5%, -0.1%]	79
Improvements ✅ (secondary)	-0.3%	[-0.6%, -0.1%]	13
All ❌✅ (primary)	-0.5%	[-1.5%, -0.1%]	79

Max RSS (memory usage)

Results (primary 1.7%, secondary -4.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[1.2%, 2.2%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.2%	[-4.2%, -4.2%]	1
All ❌✅ (primary)	1.7%	[1.2%, 2.2%]	2

Cycles

Results (primary -3.1%)

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%	[-3.1%, -3.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.1%	[-3.1%, -3.1%]	2

Binary size

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

Bootstrap: 468.82s -> 468.751s (-0.01%)
Artifact size: 376.96 MiB -> 376.89 MiB (-0.02%)

[rustbot]

r? @davidtwco

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