coverage: Simplify the coverageinfo query

[Zalathar]

The coverageinfo query walks through a mir::Body's statements to find the total number of coverage counter IDs and coverage expression IDs that have been used, as this information is needed by coverage codegen.

This PR makes 3 nice simplifications to that query:

• Extract a common iterator over coverage statements, shared by both coverage-related queries
• Simplify the query's visitor from two passes to just one pass
• Explicitly track the highest seen IDs in the visitor, and only convert to a count right at the end

I also updated some related comments. Some had been invalidated by these changes, while others had already been invalidated by previous coverage changes.

[rustbot]

r? @cjgillot

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Zalathar]

The two-pass structure seems to have served some purpose prior to #113428, when we couldn't know for sure whether an operand was a counter ID or expression ID just by looking at it.

That distinction is now preserved explicitly by the Operand enum, so I see no reason to have two passes when just one would suffice.

[Zalathar]

@rustbot label +A-code-coverage

[cjgillot]

@bors r+

[bors]

📌 Commit e54204c has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit e54204c with merge 69ec430...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 69ec430 to master...

[rust-timer]

Finished benchmarking commit (69ec430): 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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	4.1%	[1.4%, 6.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.8%, 0.8%]	1

Cycles

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)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 628.971s -> 628.962s (-0.00%)
Artifact size: 318.08 MiB -> 318.09 MiB (0.00%)