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

teor2345 · 2026-03-11T08:23:19Z

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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
#[splat] function parameter check at THIR level
splatted MIR lowering and tupling
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 (900 lines) is tests and test output

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

Further Work

perf: Use 1 bit to track splatting in FnDecl #155852:
- Preserve Refactor FnDecl and FnSig non-type fields into a new wrapper type #155223's perf wins with a minor refactor (and some const / inline fns)

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

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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

Add arg splat experiment initial tuple impl

rust-bors · 2026-04-24T04:34:34Z

☀️ Try build successful (CI)
Build commit: a8930df (a8930df882af409d5db502c7a23b6523079a5cd9, parent: 36ba2c7712052d731a7082d0eba5ed3d9d56c133)

rust-timer · 2026-04-24T05:14:19Z

Finished benchmarking commit (a8930df): 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%	[0.2%, 0.2%]	3
Regressions ❌ (secondary)	0.4%	[0.1%, 1.1%]	12
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	3

Max RSS (memory usage)

Results (primary -0.7%, secondary -2.3%)

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)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.7%]	1
Improvements ✅ (secondary)	-5.7%	[-5.7%, -5.7%]	1
All ❌✅ (primary)	-0.7%	[-0.7%, -0.7%]	1

Cycles

Results (secondary -4.1%)

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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.1%	[-4.9%, -3.3%]	2
All ❌✅ (primary)	-	-	0

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)	0.1%	[0.0%, 0.2%]	84
Regressions ❌ (secondary)	0.2%	[0.0%, 0.7%]	59
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	84

Bootstrap: 490.512s -> 491.535s (0.21%)
Artifact size: 394.34 MiB -> 394.41 MiB (0.02%)

teor2345 · 2026-04-27T06:07:37Z

The perf issues above are resolved by a minor refactor, but I'd like to merge them as separate PRs/commits, just in case we need to go back to the original code for testing (or further refactors).

See the perf analysis here, overall this PR plus the refactor in #155852 results in an 0.24% primary benchmark improvement, with no primary regressions:
#155852 (comment)

teor2345 · 2026-04-28T01:19:30Z

I've done a rough split of the code into separate commits, and dropped the unrelated changes.

oli-obk · 2026-04-28T07:38:34Z

+        // FIXME(splat): only allow MacroCall if the macro creates an argument
+        Allow(Target::MacroCall),


could also just not allow this for now, or add a test that excercises this code path

View changes since the review

oli-obk · 2026-04-28T07:40:37Z

+
+        // Multiple splatted arguments are invalid: we can't know which arguments go in each splat.
+        if splatted_arg_indexes.len() > 1 {
+            self.dcx().emit_err(errors::DuplicateSplattedArgs { spans: splatted_spans.clone() });


I'm somewhat worried that due to these errors being non-blocking for the rest of the compiler, that it's possible to cause weird ICEs, but that's nothing new wrt most of the ast validation checks.

View changes since the review

oli-obk · 2026-04-28T07:46:05Z

-    // Function parameter count, including C variadic `...` if present.
-    fn param_count(&self, def_id: DefId) -> (usize, bool /*c_variadic*/) {
+    // Function parameter count, including C variadic `...` and `#[splat]` if present.
+    fn param_count(&self, def_id: DefId) -> (usize, bool /*c_variadic*/, Option<u16> /*splatted*/) {


This return type should be a struct I think 😆 it's getting a little out of hand, and usually is just passed along to another function anyway, so we could just keep passing these modifiers along together. Also the function name doesn't really represent the return value anymore... maybe param_info?

View changes since the review

oli-obk · 2026-04-28T07:49:55Z

+    /// Which function argument is splatted into multiple arguments in callers, if any?
+    /// Splatting functions with `u16::MAX` arguments is not supported, see `FnSigKind` for
+    /// details.
+    splatted: u16,


far future possibility: make newtype_index allow for types other than u32 to be used for the storage. Then we could just encode a new ArgIdx type that inherently forbids u16::MAX (and maybe a few others at the upper limit to allow for more niche opts)

View changes since the review

also 😭 why do we allow more than 250 arguments on functions

oli-obk · 2026-04-28T07:52:31Z

-    /// Create a new FnDeclKind with no implicit self, no lifetime elision, and no C-style variadic argument.
+    /// Marker index for "no splatted argument".
+    /// Must have the same value as `FnSigKind::NO_SPLATTED_ARG_INDEX` and `rustc_ast::FnDecl::NO_SPLATTED_ARG_INDEX`.
+    const NO_SPLATTED_ARG_INDEX: u16 = u16::MAX;


can these all be linked somehow by depending on them in the argument? Or are there two of them who don't have the others' crate in their dependency list?

View changes since the review

oli-obk · 2026-04-28T09:09:33Z

+        };
+
+        // If the arguments should be wrapped in a tuple (ex: closures, splats), unwrap them here
+        if let Some(first_tupled_arg_index) = first_tupled_arg_index {


Pull this condition's body out into a separate function

View changes since the review

oli-obk · 2026-04-28T09:11:02Z

+
+                    let type_errors = ocx.try_evaluate_obligations();
+                    if type_errors.is_empty() {
+                        assert_matches!(new_tupled_type.kind(), ty::Tuple(_));


This assert seems odd since you just created the tuple with this kind a few lines above, did you mean to check calee_tuple_type?

View changes since the review

oli-obk · 2026-04-28T09:11:56Z

+
+            // Keep the type variable if the argument is splatted, so we can force it to be a tuple later.
+            let tuple_type = if fn_sig_kind.splatted().is_some() {
+                let calee_tuple_type = self.try_structurally_resolve_type(


typo calee is missing an l

View changes since the review

oli-obk · 2026-04-28T09:14:21Z

+                    let ocx_error =
+                        ocx.eq(&origin, self.param_env, calee_tuple_type, new_tupled_type);
+                    if let Err(ocx_error) = ocx_error {
+                        struct_span_code_err!(


is this error reachable? if so, please add a test specifically for it

View changes since the review

oli-obk · 2026-04-28T09:21:45Z

                sym::variadic => {
                    self.write_scalar(Scalar::from_bool(fn_sig_kind.c_variadic()), &field_place)?;
                }
+                sym::splat => {


Also add it to libcore in the same commit

View changes since the review

rust-bors · 2026-04-28T10:58:09Z

☔ The latest upstream changes (presumably #155329) made this pull request unmergeable. Please resolve the merge conflicts.

RalfJung · 2026-04-28T11:47:25Z

        };

        // Split the rust-call tupled arguments off.
+        // FIXME(splat): un-tuple splatted arguments in codegen, for performance


Why "for performance"? If this follows the RustCall approach, the ABI requires untupling for correctness. I am confused by this comment.

View changes since the review

Oh I see, I think I got confused by "splatted MIR lowering and tupling". This doesn't actually do anything on the ABI level yet, it passes things as structs.

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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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

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

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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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rust-bors Bot pushed a commit that referenced this pull request Apr 24, 2026

Auto merge of #153697 - teor2345:fn-arg-splat-experiment, r=<try>

a8930df

Add arg splat experiment initial tuple impl

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teor2345 force-pushed the fn-arg-splat-experiment branch from 7e8fef2 to d83e6d9 Compare April 26, 2026 22:23

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teor2345 force-pushed the fn-arg-splat-experiment branch from d4229d6 to 482f842 Compare April 26, 2026 23:44

teor2345 mentioned this pull request Apr 27, 2026

perf: Use 1 bit to track splatting in FnDecl #155852

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teor2345 added 8 commits April 28, 2026 10:51

Add splat builtin attribute & feature gate

108b87f

Add AST validation for #[splat]

f6ad19f

Add HIR FnDecl for #[splat]

9eac82b

Impl HIR typeck for #[splat]

571f4fc

Impl MIR lowering for #[splat]

ce1b06e

Impl TypeInfo for splat

5fcfb4e

Add FIXME comments for splat in tools

a28d527

Fix splat ICEs in arg mismatch checks

84cab65

teor2345 force-pushed the fn-arg-splat-experiment branch from 482f842 to 84cab65 Compare April 28, 2026 01:19

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teor2345 mentioned this pull request Apr 29, 2026

Add a missing GenericTypeVisitable, and avoid having interner traits for FnSigKind and Abi #155562

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		// FIXME(splat): only allow MacroCall if the macro creates an argument
		Allow(Target::MacroCall),

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Further Work

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rust-timer commented Apr 24, 2026

Overall result: ❌ regressions - please read:

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