Use FieldIdx in various things related to aggregates

[scottmcm]

Shrank AggregateKind by 8 bytes on x64, since the active field of a union is tracked as an Option<FieldIdx> instead of Option<usize>.

Part 3/? of rust-lang/compiler-team#606

IndexSlice was added in #109787

[rustbot]

r? @lcnr

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred to the CTFE / Miri engine

cc @rust-lang/miri

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit b5b6def with merge 1ed4ca37934bc0bad8719018caa6d02f6f508a43...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1ed4ca37934bc0bad8719018caa6d02f6f508a43 (1ed4ca37934bc0bad8719018caa6d02f6f508a43)

[rust-timer]

Finished benchmarking commit (1ed4ca37934bc0bad8719018caa6d02f6f508a43): comparison URL.

Overall result: ❌✅ regressions and improvements - 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.3%, 0.5%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-1.3%, -0.6%]	3
All ❌✅ (primary)	-	-	0

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
Regressions ❌ (secondary)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-3.3%	[-3.3%, -3.3%]	1
Improvements ✅ (secondary)	-2.0%	[-3.1%, -1.0%]	9
All ❌✅ (primary)	-3.3%	[-3.3%, -3.3%]	1

Cycles

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

[Nilstrieb]

regressions are small and I know that at least some of these benchmarks have been noisy, so this seems perf neutral to me

[oli-obk]

r? @oli-obk @bors r+

[bors]

📌 Commit b5b6def has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit b5b6def with merge a93bcdc...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing a93bcdc to master...

[rust-timer]

Finished benchmarking commit (a93bcdc): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.3%, 1.2%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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
Regressions ❌ (secondary)	1.6%	[1.1%, 2.6%]	3
Improvements ✅ (primary)	-3.4%	[-3.4%, -3.4%]	1
Improvements ✅ (secondary)	-1.8%	[-3.2%, -0.7%]	6
All ❌✅ (primary)	-3.4%	[-3.4%, -3.4%]	1

Cycles

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

[Mark-Simulacrum]

Regressions limited to secondary benchmarks and I believe these couple benchmarks are prone to sporadic noise, so marking as triaged.