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

rustbot has assigned @chenyukang.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 72 candidates
Random selection from 18 candidates

JonathanBrouwer · 2026-04-26T11:14:53Z

Is that spurious?
@bors try jobs=x86_64-mingw-1

Prefer `-1` for `None` try-job: x86_64-mingw-1

rust-bors · 2026-04-26T14:24:07Z

☀️ Try build successful (CI)
Build commit: 031ee25 (031ee2518da74818f784615d980c960239a1711f, parent: 68ffae46b581747074413e4242ab0c6ecc4db4a9)

JonathanBrouwer · 2026-04-26T14:26:24Z

@bors retry

JonathanBrouwer · 2026-04-26T15:07:57Z

@bors p=6
Scheduling this before the next rollup

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

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

View reviewed changes