Simplify the implementation of iterators over slices of ZSTs

[scottmcm]

Currently, slice iterators over ZSTs store end = start.wrapping_byte_add(len).

That's slightly convenient for is_empty, but kinda annoying for pretty much everything else -- see bugs like #42789, for example.

This PR instead changes it to just end = ptr::invalid(len) instead.

That's easier to think about (IMHO, at least) as well as easier to represent.

next is still to big to get inlined into the mir-opt/pre-codegen/ tests, but if I bump the inline threshold to force it to show the whole thing, this implementation is also less MIR:

> git diff --numstat
241     370     tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.mir
255     329     tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.mir
184     216     tests/mir-opt/pre-codegen/slice_iter.slice_iter_mut_next_back.PreCodegen.after.mir
182     254     tests/mir-opt/pre-codegen/slice_iter.slice_iter_next.PreCodegen.after.mir

(That's ≈70 lines less for Iter::next, for example.)

r? @ghost

Built atop #111282, so draft until that lands.

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit b3389f2eb53d001e94ff67b177cb3bc41f7bbc71 with merge 84b202a37f44bc97a1f2bec83a95928739984869...

[bors]

☀️ Try build successful - checks-actions
Build commit: 84b202a37f44bc97a1f2bec83a95928739984869 (84b202a37f44bc97a1f2bec83a95928739984869)

[rust-timer]

Finished benchmarking commit (84b202a37f44bc97a1f2bec83a95928739984869): comparison URL.

Overall result: ❌✅ regressions and improvements - 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 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)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

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)	-	-	0
Improvements ✅ (primary)	-4.4%	[-5.2%, -3.6%]	2
Improvements ✅ (secondary)	-2.7%	[-2.9%, -2.3%]	3
All ❌✅ (primary)	-4.4%	[-5.2%, -3.6%]	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)	3.4%	[3.4%, 3.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-1.3%, -0.0%]	66
Improvements ✅ (secondary)	-0.3%	[-0.6%, -0.0%]	8
All ❌✅ (primary)	-0.5%	[-1.3%, 0.3%]	67

Bootstrap: 658.646s -> 657.694s (-0.14%)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[scottmcm]

r? @the8472

[the8472]

Interesting, some debug binary size wins. I guess debug doesn't even eliminate the IS_ZST branches?

@bors r+

[bors]

📌 Commit 15aa7fa has been approved by the8472

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 50. This pull request will be tested once the tree is reopened.

[scottmcm]

I guess debug doesn't even eliminate the IS_ZST branches?

Looks like it doesn't, yeah: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=5c4cd54a7b07a07ccecea6a7a092c57e

  br i1 true, label %bb1, label %bb2, !dbg !149

And wrapping_byte_sub is more complicated than it seems https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b45460e41f9c20b10c49223f49be0f4d as it needs some extra stuff like

  %_12.i = alloca %"core::ptr::metadata::PtrComponents<u32>", align 8
  %_11.i = alloca %"core::ptr::metadata::PtrRepr<u32>", align 8
  %_10.i = alloca %"core::ptr::metadata::PtrRepr<u32>", align 8

that isn't there in just the addr+invalid version.

[bors]

⌛ Testing commit 15aa7fa with merge 41c9748da663a483561273aa0ae71d47dba10c61...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[scottmcm]

@bors retry

[scottmcm]

Well, https://www.githubstatus.com/incidents/nf7s6933tnn8 says things are supposed to be working, and #111475 hit a legit problem, not an infrastructure problem, so let's give another one a shot 🤞

@bors p=101 (not important, just getting through the tree, feel free to interrupt)

[bors]

⌛ Testing commit 15aa7fa with merge 5b24e12...

[bors]

☀️ Test successful - checks-actions
Approved by: the8472
Pushing 5b24e12 to master...

[rust-timer]

Finished benchmarking commit (5b24e12): 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)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-6.3%	[-8.7%, -4.0%]	2
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.8%]	2
All ❌✅ (primary)	-3.5%	[-8.7%, 2.3%]	3

Cycles

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

Binary size

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)	-0.5%	[-1.1%, -0.0%]	67
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.0%]	8
All ❌✅ (primary)	-0.5%	[-1.1%, -0.0%]	67

Bootstrap: 659.726s -> 661.63s (0.29%)