Refactor FnDecl and FnSig non-type fields into a new wrapper type by teor2345 · Pull Request #155223 · rust-lang/rust

teor2345 · 2026-04-13T04:45:44Z

Why this Refactor?

This PR is part of an initial cleanup for the arg splat experiment, but it's a useful refactor by itself.

It refactors the non-type fields of FnDecl, FnSig, and FnHeader into a new packed wrapper types, based on this comment in the splat experiment PR:
#153697 (comment)

It also refactors some common FnSig creation settings into their own methods. I did this instead of creating a struct with defaults.

Relationship to `splat` Experiment

I don't think we can use functional struct updates (..default()) to create FnDecl and FnSig, because we need the bit-packing for the splat experiment.

Bit-packing will avoid breaking "type is small" assertions for commonly used types when splat is added.
This PR packs these types:

ExternAbi: enum + unwind variants (38) -> 6 bits
ImplicitSelfKind: enum variants (5) -> 3 bits
lifetime_elision_allowed, safety, c_variadic: bool -> 1 bit

Minor Changes

Fixes some typos, and applies rustfmt to clippy files that got skipped somehow.

rustbot · 2026-04-15T21:15:17Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

HIR ty lowering was modified

cc @fmease

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred in compiler/rustc_sanitizers

cc @rcvalle

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

rustbot · 2026-04-15T21:15:20Z

r? @mati865

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teor2345

This is ready for review now, ~~but I have some questions for reviewers (or compiler experts) about the legacy name mangling hash.~~

I'd also like to check if this PR impacts perf, either positively or negatively. I don't think I have permissions, but let's try anyway:

@bors try @rust-timer queue

View changes since this review

rust-bors · 2026-04-15T21:16:44Z

@teor2345: 🔑 Insufficient privileges: not in try users

nnethercote · 2026-04-16T02:08:02Z

@bors try @rust-timer queue

Refactor FnDecl and FnSig non-type fields into a new wrapper type

rust-bors · 2026-04-16T04:15:34Z

☀️ Try build successful (CI)
Build commit: d647df9 (d647df90c918d4747135501dba51b294ffa85de3, parent: e8e4541ff19649d95afab52fdde2c2eaa6829965)

rust-timer · 2026-04-16T04:56:29Z

Finished benchmarking commit (d647df9): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.7%, -0.1%]	45
Improvements ✅ (secondary)	-0.6%	[-1.4%, -0.2%]	68
All ❌✅ (primary)	-0.3%	[-0.7%, -0.1%]	45

Max RSS (memory usage)

Results (primary -1.8%, secondary 0.5%)

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)	3.1%	[0.9%, 5.3%]	2
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-4.6%	[-4.6%, -4.6%]	1
All ❌✅ (primary)	-1.8%	[-1.8%, -1.8%]	1

Cycles

Results (primary -2.0%, secondary -1.6%)

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)	8.0%	[2.8%, 13.2%]	2
Improvements ✅ (primary)	-2.0%	[-2.3%, -1.7%]	2
Improvements ✅ (secondary)	-3.2%	[-4.3%, -2.1%]	12
All ❌✅ (primary)	-2.0%	[-2.3%, -1.7%]	2

Binary size

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

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)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	63
Improvements ✅ (secondary)	-0.2%	[-1.0%, -0.0%]	56
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	63

Bootstrap: 490.756s -> 487.632s (-0.64%)
Artifact size: 394.16 MiB -> 394.10 MiB (-0.01%)

RalfJung · 2026-04-16T06:07:19Z

                    $($e_name::$variant $( { unwind: $uw } )* => $tok,)*
                }
            }
+            // ALL_VARIANTS.iter().position(|v| v == self), but const


position returns an index, this here returns a bool -- something does not check out.

View changes since the review

Oops, that comment should have been on as_packed, it got left behind when I also needed a const equality check.

Fixed in a70f37b (this PR)

mati865

Thanks.

@bors r+

View changes since this review

rust-bors · 2026-04-18T11:07:33Z

📌 Commit a70f37b has been approved by mati865

It is now in the queue for this repository.

rust-bors · 2026-04-19T03:11:36Z

☀️ Test successful - CI
Approved by: mati865
Duration: 3h 24m 47s
Pushing 6f109d8 to main...

github-actions · 2026-04-19T03:14:52Z

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 0febdba (parent) -> 6f109d8 (this PR)

Test differences

Show 91 test diffs

91 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 6f109d8a2da2fe8d0fbfc52178300c033737b218 --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

x86_64-msvc-1: 2h 2m -> 2h 43m (+33.3%)
x86_64-gnu-tools: 48m 51s -> 1h 1m (+25.5%)
x86_64-gnu-llvm-21-1: 1h 15m -> 57m 1s (-24.7%)
dist-android: 22m 39s -> 28m (+23.6%)
dist-powerpc64le-linux-gnu: 1h 16m -> 1h 35m (+23.5%)
dist-x86_64-illumos: 1h 48m -> 1h 24m (-22.1%)
dist-i586-gnu-i586-i686-musl: 1h 34m -> 1h 15m (-19.8%)
dist-i686-linux: 1h 49m -> 1h 27m (-19.7%)
aarch64-apple: 2h 27m -> 2h 56m (+19.1%)
dist-aarch64-msvc: 1h 37m -> 1h 56m (+19.0%)

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 · 2026-04-19T03:52:31Z

Finished benchmarking commit (6f109d8): 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
Improvements ✅ (primary)	-0.3%	[-0.7%, -0.2%]	33
Improvements ✅ (secondary)	-0.6%	[-1.2%, -0.1%]	47
All ❌✅ (primary)	-0.3%	[-0.7%, -0.2%]	33

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (primary -3.6%, secondary -3.8%)

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)	-3.6%	[-4.4%, -2.8%]	2
Improvements ✅ (secondary)	-3.8%	[-6.3%, -2.0%]	13
All ❌✅ (primary)	-3.6%	[-4.4%, -2.8%]	2

Binary size

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

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)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	46
Improvements ✅ (secondary)	-0.2%	[-1.0%, -0.0%]	44
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	46

Bootstrap: 493.472s -> 491.328s (-0.43%)
Artifact size: 394.42 MiB -> 394.27 MiB (-0.04%)

bjorn3 reviewed Apr 13, 2026

View reviewed changes

Comment thread compiler/rustc_codegen_cranelift/src/value_and_place.rs Outdated

teor2345 changed the title ~~Refactor FnDef fields into a new wrapper type~~ Refactor FnDecl and FnSig non-type fields into a new wrapper type Apr 15, 2026

teor2345 force-pushed the fndef-refactor branch from 6150d1f to 27a7d65 Compare April 15, 2026 07:13

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teor2345 force-pushed the fndef-refactor branch from 27a7d65 to 18f1956 Compare April 15, 2026 19:16

teor2345 added 3 commits April 16, 2026 07:08

Fix typos near the refactor

88adf77

Format missed clippy lint source files

0a742de

Refactor FnDecl and FnSig flags into packed structs

dafb6bb

teor2345 force-pushed the fndef-refactor branch from 18f1956 to dafb6bb Compare April 15, 2026 21:08

teor2345 marked this pull request as ready for review April 15, 2026 21:15

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 15, 2026

rustbot assigned mati865 Apr 15, 2026

teor2345 commented Apr 15, 2026

View reviewed changes

Comment thread tests/ui/symbol-names/basic.legacy.stderr

Comment thread compiler/rustc_codegen_cranelift/src/value_and_place.rs Outdated

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rust-bors Bot pushed a commit that referenced this pull request Apr 16, 2026

Auto merge of #155223 - teor2345:fndef-refactor, r=<try>

d647df9

Refactor FnDecl and FnSig non-type fields into a new wrapper type

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 16, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 16, 2026

teor2345 mentioned this pull request Apr 16, 2026

Add arg splat experiment initial tuple impl #153697

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RalfJung reviewed Apr 16, 2026

View reviewed changes

fixup! Refactor FnDecl and FnSig flags into packed structs

a70f37b

mati865 approved these changes Apr 18, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 18, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Apr 19, 2026

rust-bors Bot merged commit 6f109d8 into rust-lang:main Apr 19, 2026
12 checks passed

rustbot added this to the 1.97.0 milestone Apr 19, 2026

teor2345 mentioned this pull request Apr 21, 2026

Tracking Issue for argument splatting lang experiment #153629

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Why this Refactor?

Relationship to splat Experiment

Minor Changes

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rust-timer commented Apr 16, 2026

Overall result: ✅ improvements - no action needed

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Test differences

Job duration changes

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rust-timer commented Apr 19, 2026

Overall result: ✅ improvements - no action needed

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