match lowering: simplify empty candidate selection

[Nadrieril]

In match lowering, match_simplified_candidates is tasked with removing candidates that are fully matched and linking them up properly. The code that does that was needlessly complicated; this PR simplifies it.

The overall change isn't big but I split it up into tiny commits to convince myself that I was correctly preserving behavior. The test changes are all due to the first commit. Let me know if you'd prefer me to split up the PR to make reviewing easier.

r? @matthewjasper

[Nadrieril]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4a9cff7 with merge 1d1bfb7...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1d1bfb7 (1d1bfb7926f9f269a965c3582f73041e9c563b3e)

[rust-timer]

Finished benchmarking commit (1d1bfb7): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -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)	8.0%	[8.0%, 8.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.3%	[-1.5%, 8.0%]	2

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)	4.2%	[3.9%, 4.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 637.219s -> 636.695s (-0.08%)
Artifact size: 306.29 MiB -> 306.35 MiB (0.02%)

[matthewjasper]

One nit, otherwise lgtm

[matthewjasper]

@bors r+

[bors]

📌 Commit a45d892 has been approved by matthewjasper

It is now in the queue for this repository.

[bors]

⌛ Testing commit a45d892 with merge 096598d...

[bors]

☀️ Test successful - checks-actions
Approved by: matthewjasper
Pushing 096598d to master...

[rust-timer]

Finished benchmarking commit (096598d): 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)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	2.4%	[2.2%, 2.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	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: 640.784s -> 640.589s (-0.03%)
Artifact size: 308.61 MiB -> 308.63 MiB (0.01%)