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

teor2345 · 2026-03-11T08:23:19Z

TODO

This PR is waiting on #157605 to merge, then I'll rebase it.

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.

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

teor2345 · 2026-06-05T11:59:21Z

@rustbot ready

I've rebased and cherry-picked @Ajay-singh1's bug fix and extra tests from #157434.

RalfJung · 2026-06-05T13:46:56Z

+    // FIXME(splat): should these fields be private, or merged into an Option<u16>?
+    /// Is any function argument splatted?
+    pub is_splatted: bool,
+
+    /// The index of the splatted function argument in `inputs`, only valid if `is_splatted` is true.
+    /// e.g. in `fn overload(a: u8, #[splat] b: (f32, usize))` the index is 1, and it can be called
+    /// as `overload(a, 1.0, 2)`.
+    pub splatted_index: u16,


Do you immediately need reflection support for this? If not, seems better to leave this for later and make the PR a bit smaller.

View changes since the review

I don't think so, happy to split the PR.

Would it be easier as 3 PRs?

feature gate only

splat attribute, typecheck, and MIR

reflection

Isn't the feature gate just a single line?

But reflection is sufficiently new that it might be worth separating. OTOH you have @oli-obk as reviewer who is intimately familiar with reflection so for him it's probably fine. I should have checked the reviewer before commenting, sorry.

Isn't the feature gate just a single line?

About 200 lines when you add attribute parsing and tests. But I get your point.

But reflection is sufficiently new that it might be worth separating. OTOH you have @oli-obk as reviewer who is intimately familiar with reflection so for him it's probably fine.

I'll split it on Monday if I don't hear back from Oli, then other people might be able to review the other parts. Maybe there's more splits that make sense, I'll have a think about it.

About 200 lines when you add attribute parsing and tests. But I get your point.

Ah that's more than just the feature gate then. :)

Separating the syntactic parts out also can make sense yeah, I've seen this for other new features in the past.

Reflection is sufficiently simple that doing it together with adding the field to function signature makes sense

Alright, I'll split it into two PRs:

attribute parsing and syntax (Arg splat experiment - syntax impl #157605)

type checking, lowering, and reflection (this PR, which I'll rebase after the first one merges)

It seems like most conflicting PRs either touch one or the other, so hopefully that will reduce the rebases.

fixup! Add splat builtin attribute & feature gate

fixup! Add AST validation for #[splat]

fixup! Add HIR FnDecl for #[splat]

fixup! Impl HIR typeck for #[splat]

fixup! Impl TypeInfo for splat

rustbot · 2026-06-08T10:45:43Z

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.

teor2345 · 2026-06-08T11:38:16Z

This PR is waiting on #157605 to merge, then I'll rebase it.

@rustbot author

rust-bors · 2026-06-08T20:17:55Z

☔ The latest upstream changes (presumably #157616) made this pull request unmergeable. Please resolve the merge conflicts.

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

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Comment thread compiler/rustc_hir/src/target.rs

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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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RalfJung reviewed Jun 5, 2026

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teor2345 and others added 17 commits June 8, 2026 12:21

Add splat builtin attribute & feature gate

d382a58

Add AST validation for #[splat]

81f0518

Add HIR FnDecl for #[splat]

2c3ac3d

Impl HIR typeck for #[splat]

04ea823

Impl MIR lowering for #[splat]

9e45365

Impl TypeInfo for splat

f79d629

Stop allowing #[splat] on macro calls, for now

ae1efcb

fixup! Add splat builtin attribute & feature gate

Fix a #[splat] AST validation method comment

48bb4f9

fixup! Add AST validation for #[splat]

Make the param_info return type into a struct, and rename the function

12c2c29

fixup! Add HIR FnDecl for #[splat]

Extract splatted typeck into a fn and cleanup error handling

634088a

fixup! Impl HIR typeck for #[splat]

Add splat to libcore's TypeInfo

7508f31

fixup! Impl TypeInfo for splat

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

4cad17b

Add UI tests for #[splat] unhappy path and generics cases

d362262

Minor test cleanups

798d155

Remove a leftover direct call to fn_ctxt.label_generic_mismatches

879982c

Fix another splat-related ICE revealed by new tests

da26f01

Avoid panics during argument mismatch diagnostics due to splat

6575d0d

teor2345 mentioned this pull request Jun 8, 2026

Arg splat experiment - syntax impl #157605

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teor2345 commented Mar 11, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

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TODO

Description

Further Work

Out of Scope for this PR

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JohnTitor commented Mar 11, 2026

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teor2345 commented Mar 11, 2026 •

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