Add arg splat experiment initial tuple impl by teor2345 · Pull Request #153697 · rust-lang/rust

teor2345 · 2026-03-11T08:23:19Z

Description

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This PR is part of the argument splatting lang experiment, and FFI overloading / C++ interop project goals:

Example code using existing unstable features:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=f42a3754a63a3d9365670e57257053d5

Discussion of implementation strategy:

#t-lang > On overloading @ 💬

The PR is the initial implementation of the feature:

splat incomplete feature gate
#[splat] attribute on function arguments
Splatted function argument TypeInfo
#[splat] function parameter check at THIR level
splatted MIR lowering (as tupled arguments)
feature gate and UI tests for item type filtering, non-splattable arguments, splattable tuples, generics, and the "overloading at home" example
- about half the diff (1100 lines) is tests and test output

Once this PR merges, we can add further functionality, then test it out in interop tools.

Perf Impact

We expect a 0.1% regression on 5 primary and 0.2% regression on 4 secondary benchmarks in this PR, based on this perf run.

We tried a number of different ways to improve perf. Limiting splat to the 255th or lower argument is a simple hack that gives good perf, and is good enough for an experiment.

This PR series already has significant perf wins in #155223 - 0.3% perf improvement across 45 primary benchmarks. We're spending a small amount of that perf for the new feature in this PR.

Out of Scope for this PR

Change codegen to de-tuple caller and callee
Better diagnostics
Full support for splatted function pointer arguments

rustbot · 2026-03-11T08:23:24Z

r? @JohnTitor

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JohnTitor · 2026-03-11T10:19:07Z

It should be better for someone on https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/On.20overloading/with/573924937 to review this, @oli-obk could you take over?

oli-obk · 2026-03-12T06:45:05Z

Let's wait for the ongoing discussion on Zulip to figure out whether we need to have a proc macro, an AST manipulating attribute (like define_opaque), or just a normal attribute

JonathanBrouwer · 2026-06-18T09:38:53Z

@bors try @rust-timer queue

rust-bors · 2026-06-18T11:48:57Z

☀️ Try build successful (CI)
Build commit: 2bd9dab (2bd9dab3bd10849e2abc3a5cc97c04adf523e7da)
Base parent: 9639eef (9639eefc07db282b25e9c83cdfebb10688ed5114)

rust-timer · 2026-06-18T12:39:14Z

Finished benchmarking commit (2bd9dab): comparison URL.

Overall result: ❌ regressions - 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)	0.2%	[0.1%, 0.2%]	14
Regressions ❌ (secondary)	0.3%	[0.2%, 0.4%]	26
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.1%, 0.2%]	14

Max RSS (memory usage)

Results (primary 4.1%, secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.1%	[2.1%, 7.4%]	3
Regressions ❌ (secondary)	3.0%	[2.4%, 3.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.1%	[2.1%, 7.4%]	3

Cycles

Results (secondary 5.6%)

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)	5.6%	[3.1%, 7.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, 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)	0.1%	[0.0%, 0.2%]	80
Regressions ❌ (secondary)	0.1%	[0.0%, 0.7%]	62
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	80

Bootstrap: 482.613s -> 483.026s (0.09%)
Artifact size: 390.75 MiB -> 390.78 MiB (0.01%)

teor2345 · 2026-06-23T06:46:44Z

Perf Update

We expect a 0.1% regression on 5 primary and 0.2% regression on 4 secondary benchmarks in this PR, based on this perf run.

We tried a number of different ways to improve perf. Limiting splat to the 255th or lower argument is a simple hack that gives good perf, and is good enough for an experiment.

This PR series already has significant perf wins in #155223 - 0.3% perf improvement across 45 primary benchmarks. We're spending a small amount of that perf for the new feature in this PR.

@rustbot ready

teor2345 · 2026-06-23T06:48:06Z

Umm?

@rustbot reviewer

And remove redundant const fn

rustbot · 2026-06-23T07:51:09Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

oli-obk · 2026-06-23T07:35:56Z

+                        None => {
+                            if !tuple_arguments.is_splatted() {
+                                // FIXME(splat): when the arg is splatted, adjust its index
+                                use_splat_fallback = true;


how does this case happen?

View changes since the review

does it only happen in error cases?

I expanded these comments to clarify it's only for better error handling

oli-obk · 2026-06-23T07:46:25Z

+
+                    let tupled_args = Expr {
+                        ty: Ty::new_tup_from_iter(tcx, tupled_arg_tys),
+                        temp_scope_id: method.temp_scope_id,


except for this line, everything leading up to it is 100% equivalent to the function call logic, pull everything into a method and add a flag/enum to make the differences clear (and pass in the receiver as an option, then you can just chain all the pieces together and collect them)

View changes since the review

wait actually even this is equivalent, just loading the same id via different means. so yea, it should just be the receiver that's the difference?

Yeah the receiver is optional, and it determines:

the temp_scope_id

the start of the args list (receiver or just function args)

the generated call span

The segment was only used for debugging in a span bug, so I removed it.

teor2345

Thanks for the review, I'll try to make this changes later today

View changes since this review

teor2345 · 2026-06-23T13:53:51Z

I pushed some fixups for each comment, and resolved the comments that seemed trivial or straightforward.
Each fixup commit message has a second line explaining what it fixes, they're roughly in review comment order.

@rustbot ready

oli-obk

r=me with history fixed up

View changes since this review

- const, async, and unsafe functions - const generics, complex types, and where clauses (including impl Tuple)

teor2345 · 2026-06-23T14:48:01Z

Fixups squashed, including an extra fix that deletes a redundant UI test comment and some whitespace:
d9a9b94

@rustbot ready

rustbot assigned JohnTitor Mar 11, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Mar 11, 2026

teor2345 mentioned this pull request Mar 11, 2026

Tracking Issue for argument splatting lang experiment #153629

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oli-obk reviewed Mar 11, 2026

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Comment thread compiler/rustc_builtin_macros/src/splat.rs Outdated

oli-obk assigned oli-obk and unassigned JohnTitor Mar 11, 2026

oli-obk added the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Mar 12, 2026

teor2345 marked this pull request as draft March 13, 2026 06:49

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 13, 2026

teor2345 force-pushed the fn-arg-splat-experiment branch from 89102bf to c784a57 Compare March 16, 2026 07:35

rustbot added the A-attributes Area: Attributes (`#[…]`, `#![…]`) label Mar 16, 2026

programmerjake reviewed Mar 16, 2026

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Comment thread compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs Outdated

programmerjake reviewed Mar 16, 2026

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Comment thread compiler/rustc_hir/src/target.rs

programmerjake reviewed Mar 16, 2026

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Comment thread tests/ui/splat/splat-non-function.rs Outdated

teor2345 force-pushed the fn-arg-splat-experiment branch from c784a57 to 2d9e563 Compare March 20, 2026 01:37

rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. A-query-system Area: The rustc query system (https://rustc-dev-guide.rust-lang.org/query.html) labels Mar 20, 2026

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teor2345 mentioned this pull request Mar 20, 2026

Fix typos and markdown errors #154121

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rustbot added the T-clippy Relevant to the Clippy team. label Mar 20, 2026

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teor2345 mentioned this pull request Jun 22, 2026

perf: store splat arg index in a single byte #158251

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Limit splat AST and FnDecl to 255 args

e4bb0e1

And remove redundant const fn

oli-obk requested changes Jun 23, 2026

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teor2345 commented Jun 23, 2026

View reviewed changes

Comment thread compiler/rustc_hir/src/hir.rs

Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

Comment thread compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs Outdated

Comment thread compiler/rustc_type_ir/src/ty_kind.rs Outdated

oli-obk approved these changes Jun 23, 2026

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teor2345 and others added 11 commits June 23, 2026 16:42

Impl HIR FnSig for #[splat]

ade9fa3

Impl HIR typeck for #[splat]

cfd9e33

Remove a leftover direct call to fn_ctxt.label_generic_mismatches

ad77eda

Fix ICE in label_fn_like when splatted arg index is out of bounds

ac47723

Impl MIR lowering side-tables for #[splat]

c29f2db

Impl actual MIR lowering for #[splat]

4cb4e0c

Impl TypeInfo for splat

6c7c69c

Add UI tests for #[splat] unhappy path, generics, fn colors

c3593c1

- const, async, and unsafe functions - const generics, complex types, and where clauses (including impl Tuple)

Add failing test for #[splat] with dyn AsRef<T> where T: Tuple

28b3d02

Add splat UI tests for 255 splatted arg limit

b23bbed

UI test arg diffs greater than u8::MAX

9b44c23

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Description

Perf Impact

Out of Scope for this PR

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JohnTitor commented Mar 11, 2026

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JonathanBrouwer commented Jun 18, 2026

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rust-bors Bot commented Jun 18, 2026

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rust-timer commented Jun 18, 2026

Overall result: ❌ regressions - please read:

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teor2345 commented Jun 23, 2026

Perf Update

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rustbot commented Jun 23, 2026

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