Prefer `-1` for `None` by scottmcm · Pull Request #155473 · rust-lang/rust

scottmcm · 2026-04-18T06:03:52Z

Currently we pick "weird" numbers like 1114112 for None::<char>. While that's not wrong, it's kinda unnatural -- a human wouldn't make that choice.

This PR instead picks -1 for thinge like None::<char> -- like clang's WEOF -- and None::<bool> and such.

Any enums with more than one niched value (so not Result nor Option) remain as they were before. Also we continue to use 0 when that's possible -- -1 is only preferred when zero isn't possible.

Inspired when someone in discord posted an example like this https://rust.godbolt.org/z/W94s9qdYW and I thought it was odd that we're currently picking -9223372036854775808 to be the value to store to mark an Option<Vec<_>> as None. (Especially since that needs an 8-byte immediate on x64, and writing -1 is only a 4-byte immediate.)

scottmcm · 2026-04-18T06:04:54Z

            // zero is unavailable because wrapping occurs
            move_end(v)


Note that this arm just kinda gave up before; it never even considered whether putting it next to start might be better.

View changes since the review

scottmcm · 2026-04-18T06:14:19Z

@bors try @rust-timer queue

Prefer `-1` for `None`

scottmcm · 2026-04-18T06:26:30Z

It helps to remember to git add all the changes 🤦

scottmcm · 2026-04-18T08:11:23Z

@bors try @rust-timer queue

Prefer `-1` for `None`

rust-bors · 2026-04-18T10:24:25Z

☀️ Try build successful (CI)
Build commit: ed76e05 (ed76e056cbb36453c64da4dd8ffccac4e093f819, parent: 2f201bccb3a7fb5a85b0fcfcc0a020a946d6d58a)

rust-timer · 2026-04-18T11:36:03Z

Finished benchmarking commit (ed76e05): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.5%	[0.0%, 1.5%]	9
Improvements ✅ (primary)	-0.3%	[-2.9%, -0.2%]	86
Improvements ✅ (secondary)	-0.3%	[-1.8%, -0.0%]	62
All ❌✅ (primary)	-0.3%	[-2.9%, 0.3%]	87

Max RSS (memory usage)

Results (secondary 1.1%)

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)	6.5%	[6.5%, 6.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.9%, 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)	-	-	0
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-2.9%	[-3.0%, -2.9%]	2
Improvements ✅ (secondary)	-6.2%	[-7.6%, -3.4%]	3
All ❌✅ (primary)	-2.9%	[-3.0%, -2.9%]	2

Binary size

Results (primary -0.2%, secondary -0.4%)

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)	0.3%	[0.0%, 0.6%]	2
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.9%	[-2.1%, -0.3%]	3
All ❌✅ (primary)	-0.2%	[-0.6%, -0.1%]	5

Bootstrap: 491.618s -> 493.312s (0.34%)
Artifact size: 394.25 MiB -> 396.31 MiB (0.52%)

scottmcm · 2026-04-18T16:25:15Z

Wow, those results are way better than I expected 😅

Seems pretty clearly a net win to me.
@rustbot label: +perf-regression-triaged

rustbot · 2026-04-18T16:25:21Z

r? @chenyukang

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rust-bors · 2026-04-26T20:30:49Z

☀️ Test successful - CI
Approved by: chenyukang,mati865
Duration: 3h 24m 15s
Pushing d4f7856 to main...

github-actions · 2026-04-26T20:34:04Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing c7fe5e9 (parent) -> d4f7856 (this PR)

Test differences

Show 2 test diffs

2 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard d4f78565699b229227c9e9ca8164efa3d5d7213e --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-x86_64-apple: 2h 29m -> 1h 42m (-31.2%)
x86_64-gnu: 1h 49m -> 2h 18m (+27.1%)
x86_64-gnu-llvm-21: 1h 14m -> 54m 30s (-27.0%)
dist-x86_64-illumos: 1h 24m -> 1h 46m (+26.9%)
x86_64-gnu-llvm-21-3: 1h 27m -> 1h 49m (+24.9%)
dist-arm-linux-musl: 1h 20m -> 1h 38m (+22.6%)
x86_64-gnu-llvm-22-2: 1h 31m -> 1h 12m (-21.5%)
dist-apple-various: 1h 50m -> 1h 33m (-15.1%)
dist-powerpc-linux: 1h 26m -> 1h 13m (-14.4%)
i686-msvc-1: 3h 3m -> 2h 38m (-13.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-04-26T21:11:32Z

Finished benchmarking commit (d4f7856): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	0.7%	[0.2%, 1.9%]	5
Improvements ✅ (primary)	-0.3%	[-2.7%, -0.1%]	67
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	52
All ❌✅ (primary)	-0.3%	[-2.7%, -0.1%]	67

Max RSS (memory usage)

Results (secondary -1.9%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.5%)

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)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.5%	[-2.5%, -2.5%]	1

Binary size

Results (primary -0.2%, secondary -0.1%)

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)	0.0%	[0.0%, 0.0%]	3
Improvements ✅ (primary)	-0.2%	[-0.6%, -0.0%]	25
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	6
All ❌✅ (primary)	-0.2%	[-0.6%, -0.0%]	25

Bootstrap: 488.641s -> 488.853s (0.04%)
Artifact size: 393.90 MiB -> 393.54 MiB (-0.09%)

Only exclude the #155473 change for 1-byte bool-likes try-job: x86_64-gnu-llvm-22-1 try-job: x86_64-gnu-llvm-21-1

Only exclude the #155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc #155473

…=mati865 Only exclude the rust-lang#155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc rust-lang#155473

…uwer Rollup of 14 pull requests Successful merges: - #155850 (Only exclude the #155473 change for 1-byte bool-likes) - #155923 (Subtree sync for rustc_codegen_cranelift) - #151994 (switch to v0 mangling by default on stable) - #154325 (Tweak irrefutable let else warning output) - #155899 (`dlltool`: Set the working directory to workaround `--temp-prefix` bug) - #155273 (Lock stable_crate_ids once in create_crate_num) - #155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - #155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - #155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - #155768 (compiletest: Overhaul the code for running an incremental test revision) - #155907 (Handle hkl const closures) - #155910 (misc stuff from reading borrowck again :)) - #155913 (Delete the 12 year old fixme) - #155920 (remove review queue triagebot mentions)

…=mati865 Only exclude the rust-lang#155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc rust-lang#155473

…uwer Rollup of 15 pull requests Successful merges: - #155923 (Subtree sync for rustc_codegen_cranelift) - #155930 (Sync from portable simd 2026 04 28) - #155850 (Only exclude the #155473 change for 1-byte bool-likes) - #151994 (switch to v0 mangling by default on stable) - #154325 (Tweak irrefutable let else warning output) - #155273 (Lock stable_crate_ids once in create_crate_num) - #155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - #155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - #155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - #155768 (compiletest: Overhaul the code for running an incremental test revision) - #155907 (Handle hkl const closures) - #155910 (misc stuff from reading borrowck again :)) - #155913 (Delete the 12 year old fixme) - #155920 (remove review queue triagebot mentions) - #155936 (Rename `SharedContext::emit_dyn_lint*` into `emit_lint*`)

Rollup merge of #155850 - scottmcm:tweak_niche_assignment, r=mati865 Only exclude the #155473 change for 1-byte bool-likes That's the thing we handle differently in codegen (see `to_immediate_scalar`) so if the other ones are fine, that helps narrow it down further. cc #155473

…uwer Rollup of 15 pull requests Successful merges: - rust-lang/rust#155923 (Subtree sync for rustc_codegen_cranelift) - rust-lang/rust#155930 (Sync from portable simd 2026 04 28) - rust-lang/rust#155850 (Only exclude the rust-lang/rust#155473 change for 1-byte bool-likes) - rust-lang/rust#151994 (switch to v0 mangling by default on stable) - rust-lang/rust#154325 (Tweak irrefutable let else warning output) - rust-lang/rust#155273 (Lock stable_crate_ids once in create_crate_num) - rust-lang/rust#155361 (Document that CFI diverges from Rust wrt. ABI-compatibility rules) - rust-lang/rust#155692 (disable naked-dead-code-elimination test if no RET mnemonic is available) - rust-lang/rust#155747 (Update documentation for `wasm32-wali-linux-musl` after integrating n…) - rust-lang/rust#155768 (compiletest: Overhaul the code for running an incremental test revision) - rust-lang/rust#155907 (Handle hkl const closures) - rust-lang/rust#155910 (misc stuff from reading borrowck again :)) - rust-lang/rust#155913 (Delete the 12 year old fixme) - rust-lang/rust#155920 (remove review queue triagebot mentions) - rust-lang/rust#155936 (Rename `SharedContext::emit_dyn_lint*` into `emit_lint*`)

apiraino · 2026-05-29T10:31:20Z

We are observing some breakage in a number of crates (see linked issues).
There is no layout guarantees so this is technically not a breaking change. This said, should we give some visibility about this in the release notes?

scottmcm · 2026-05-29T22:03:47Z

@apiraino Agreed, a compatibility note sounds like a good idea.

BrianIAm · 2026-06-08T15:19:04Z

Just a small note: 1114112 isn't an arbitrary "weird" number, it's 0x110000, the first value above the Unicode codespace (valid scalar values are 0x0000..=0x10FFFF), which makes it a natural niche for char specifically. It's the tightest possible niche that still lies outside the valid range.

That said, -1 is arguably more ergonomic in practice (smaller immediate on x86-64, matches WEOF convention, easier to spot in a debugger), so the change may still be the right call.

If the value does stay as -1, it might be worth a comment explaining why 0x110000 was the previous choice, so the next reader doesn't have to work it out. And if it stays as 0x110000, a comment like // 0x110000 - one past the end of valid Unicode scalar values would prevent the "weird number" confusion from recurring.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 18, 2026

scottmcm commented Apr 18, 2026

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rust-bors Bot pushed a commit that referenced this pull request Apr 18, 2026

Auto merge of #155473 - scottmcm:tweak_niche_assignment, r=<try>

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Prefer `-1` for `None`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 18, 2026

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scottmcm mentioned this pull request Apr 18, 2026

Use range attributes to pick smaller branch assembly phrasings llvm/llvm-project#192772

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rust-bors Bot pushed a commit that referenced this pull request Apr 18, 2026

Auto merge of #155473 - scottmcm:tweak_niche_assignment, r=<try>

ed76e05

Prefer `-1` for `None`

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Apr 18, 2026

scottmcm marked this pull request as ready for review April 18, 2026 16:25

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 18, 2026

rustbot assigned chenyukang Apr 18, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Apr 18, 2026

rust-bors Bot merged commit d4f7856 into rust-lang:main Apr 26, 2026
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rustbot added this to the 1.97.0 milestone Apr 26, 2026

moulins mentioned this pull request Apr 26, 2026

Remove redundant information in rustc_abi::Variants #151742

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scottmcm mentioned this pull request Apr 27, 2026

Only exclude the #155473 change for 1-byte bool-likes #155850

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rust-bors Bot pushed a commit that referenced this pull request Apr 27, 2026

Auto merge of #155850 - scottmcm:tweak_niche_assignment, r=<try>

be43df9

Only exclude the #155473 change for 1-byte bool-likes try-job: x86_64-gnu-llvm-22-1 try-job: x86_64-gnu-llvm-21-1

aureliar8 mentioned this pull request Apr 28, 2026

Memory layout change for #[repr(C)] struct which contains Option<T> #155934

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JonathanBrouwer mentioned this pull request Apr 28, 2026

Rollup of 14 pull requests #155937

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JonathanBrouwer mentioned this pull request Apr 28, 2026

Rollup of 15 pull requests #155941

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RalfJung mentioned this pull request May 17, 2026

Spurious test failure (SIGSEGV) in rust-analyzer hir-def test #156460

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ehuss mentioned this pull request May 17, 2026

Disable rust-analyzer tests on LLVM 21 #156655

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theemathas mentioned this pull request May 29, 2026

Behavior change in mem::transmute() in 1.97/beta #157099

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scottmcm added the relnotes Marks issues that should be documented in the release notes of the next release. label May 29, 2026

rustbot mentioned this pull request May 29, 2026

Tracking issue for release notes of #155473: Prefer -1 for None #157128

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theemathas mentioned this pull request Jun 5, 2026

Incoming breakage in Rust 1.97.0 0xflick/pounce#145

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BoxyUwU mentioned this pull request Jun 24, 2026

Draft release notes for 1.97.0 #158353

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rust-timer commented Apr 18, 2026

Overall result: ❌✅ regressions and improvements - please read:

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Test differences

Job duration changes

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rust-timer commented Apr 26, 2026

Overall result: ✅ improvements - no action needed

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