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
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

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]

rustbot · 2026-06-05T11:58:28Z

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 · 2026-06-05T11:59:21Z

@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

type checking, lowering, and reflection

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

fixup! Impl TypeInfo for splat

rust-log-analyzer · 2026-06-05T18:01:05Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [ui] tests/ui/specialization/specialization-translate-projections-with-params.rs ... ok
test [ui] tests/ui/specialization/trait-specialization-default-methods-55380.rs ... ok
test [ui] tests/ui/specialization/specialization-translate-projections.rs ... ok
test [ui] tests/ui/specialization/transmute-specialization.rs ... ok
test [ui] tests/ui/splat/splat-assoc-fn-tuple-simple.rs ... ok
test [ui] tests/ui/splat/splat-cannot-resolve.rs ... ok
test [ui] tests/ui/splat/splat-fn-ptr-tuple.rs ... ok
test [ui] tests/ui/splat/splat-fn-tuple-generic-fail.rs ... ok
test [ui] tests/ui/splat/splat-fn-tuple-generic.rs ... ok
test [ui] tests/ui/splat/splat-fn-tuple-simple.rs ... ok
test [ui] tests/ui/splat/splat-generics-inside-tuple.rs ... ok
test [ui] tests/ui/splat/splat-invalid-trait-impl.rs ... ok
test [ui] tests/ui/splat/splat-generics-everywhere.rs ... ok
test [ui] tests/ui/splat/splat-maybe-tuple.rs ... ok
test [ui] tests/ui/splat/splat-invalid.rs ... ok
test [ui] tests/ui/splat/splat-non-fn-arg.rs ... ok
test [ui] tests/ui/splat/splat-non-tuple.rs ... ok
test [ui] tests/ui/splat/splat-method-tuple-simple.rs ... ok
test [ui] tests/ui/stability-attribute/accidental-stable-in-unstable.rs ... ok
test [ui] tests/ui/splat/splat-overload-at-home-fail.rs ... FAILED
test [ui] tests/ui/splat/splat-trait-tuple.rs ... ok
test [ui] tests/ui/splat/splat-overload-at-home.rs ... ok
test [ui] tests/ui/stability-attribute/check-stability-issue-138319.rs ... ok
test [ui] tests/ui/stability-attribute/const-stability-attribute-implies-missing.rs ... ok
test [ui] tests/ui/stability-attribute/allowed-through-unstable.rs ... ok
test [ui] tests/ui/stability-attribute/const-stability-attribute-implies-using-stable.rs ... ok
test [ui] tests/ui/stability-attribute/allow-unstable-reexport.rs ... ok
---
test [ui] tests/ui/zero-sized/zero-sized-btreemap-insert.rs ... ok

failures:

---- [ui] tests/ui/splat/splat-overload-at-home-fail.rs stdout ----

error: Error: expected failure status (Some(1)) but received status Some(101).
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/splat/splat-overload-at-home-fail.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/splat/splat-overload-at-home-fail" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0308]: mismatched types
##[error]  --> /checkout/tests/ui/splat/splat-overload-at-home-fail.rs:35:16
   |
LL |     foo.method(42f32);
   |         ------ ^^^^^ expected `i32`, found `f32`
   |         |
   |         arguments to this method are incorrect
   |
note: method defined here
  --> /checkout/tests/ui/splat/splat-overload-at-home-fail.rs:26:8
   |
LL |     fn method<T: MethodArgs>(&self, #[splat] args: T) {
   |        ^^^^^^                       ----------------
help: change the type of the numeric literal from `f32` to `i32`
   |
LL -     foo.method(42f32);
LL +     foo.method(42i32);
   |


thread 'rustc' (154883) panicked at compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs:1704:17:
assertion `left == right` failed
  left: 1
 right: 2
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed::<usize, usize>
   4: <rustc_hir_typeck::fn_ctxt::FnCtxt>::label_fn_like
   5: <rustc_hir_typeck::fn_ctxt::checks::FnCallDiagCtxt>::check_single_incompatible
   6: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_argument_types
   7: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_kind
   8: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
   9: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation
  10: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_block
  11: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_kind
  12: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  13: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_expr_with_expectation
  14: <rustc_hir_typeck::fn_ctxt::FnCtxt>::check_return_or_body_tail
  15: rustc_hir_typeck::check::check_fn
  16: rustc_hir_typeck::typeck_with_inspect
      [... omitted 2 frames ...]
  17: <rustc_middle::query::plumbing::TyCtxtEnsureOk>::typeck::<rustc_span::def_id::LocalDefId>
  18: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis::check_crate::{closure#1}>::{closure#0}
  19: std::panicking::catch_unwind::<(), core::panic::unwind_safe::AssertUnwindSafe<rustc_data_structures::sync::parallel::par_for_each_in<&rustc_span::def_id::LocalDefId, &[rustc_span::def_id::LocalDefId], <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners<rustc_hir_analysis::check_crate::{closure#1}>::{closure#0}>::{closure#0}::{closure#1}::{closure#0}>>
  20: rustc_data_structures::sync::parallel::par_for_each_in::<&rustc_span::def_id::LocalDefId, &[rustc_span::def_id::LocalDefId], <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners<rustc_hir_analysis::check_crate::{closure#1}>::{closure#0}>
  21: rustc_hir_analysis::check_crate
  22: rustc_interface::passes::analysis
      [... omitted 2 frames ...]
  23: std::panicking::catch_unwind::<core::option::Option<rustc_interface::queries::Linker>, core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}>>
  24: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>
  25: <rustc_middle::ty::context::TyCtxt>::create_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}>
  26: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  27: std::panicking::catch_unwind::<(), core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}>>
  28: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  29: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
---
note: please make sure that you have updated to the latest nightly

note: rustc 1.98.0-nightly (7c18df87f 2026-06-05) running on aarch64-unknown-linux-gnu

note: compiler flags: -Z threads=1 -Z simulate-remapped-rust-src-base=/rustc/FAKE_PREFIX -Z translate-remapped-path-to-local-path=no -Z ignore-directory-in-diagnostics-source-blocks=/cargo -Z ignore-directory-in-diagnostics-source-blocks=/checkout/vendor -C codegen-units=1 -Z ui-testing -Z deduplicate-diagnostics=no -Z write-long-types-to-disk=no -C strip=debuginfo -C prefer-dynamic -C rpath -C debuginfo=0

query stack during panic:
#0 [typeck_root] type-checking `main`
#1 [analysis] running analysis passes on crate `splat_overload_at_home_fail`
end of query stack
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.
------------------------------------------

---- [ui] tests/ui/splat/splat-overload-at-home-fail.rs stdout end ----

failures:
    [ui] tests/ui/splat/splat-overload-at-home-fail.rs

test result: FAILED. 20882 passed; 1 failed; 392 ignored; 0 measured; 20 filtered out; finished in 616.60s

Some tests failed in compiletest suite=ui mode=ui host=aarch64-unknown-linux-gnu target=aarch64-unknown-linux-gnu
Bootstrap failed while executing `--stage 2 test --skip tidy --skip compiler --skip src`

rust-bors · 2026-06-05T22:43:48Z

☔ The latest upstream changes (presumably #157504) 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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Comment thread compiler/rustc_builtin_macros/src/splat.rs Outdated

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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

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

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

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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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This was referenced Jun 3, 2026

Fix ICE in label_fn_like for splatted args and add UI tests teor2345/rust#1

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Fix ICE in label_fn_like for splatted args and add UI tests #157434

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teor2345 added 10 commits June 5, 2026 13:47

Add splat builtin attribute & feature gate

268d92e

Add AST validation for #[splat]

8eb2d70

Add HIR FnDecl for #[splat]

98aac94

Impl HIR typeck for #[splat]

22bc57c

Impl MIR lowering for #[splat]

d200b37

Impl TypeInfo for splat

52457e0

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

72e9c36

fixup! Add splat builtin attribute & feature gate

Fix a #[splat] AST validation method comment

9924670

fixup! Add AST validation for #[splat]

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

2bd125b

fixup! Add HIR FnDecl for #[splat]

Extract splatted typeck into a fn and cleanup error handling

b4eabf2

fixup! Impl HIR typeck for #[splat]

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teor2345 and others added 4 commits June 5, 2026 17:35

Add splat to libcore's TypeInfo

a95c68c

fixup! Impl TypeInfo for splat

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

0026933

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

0f4f78e

Minor test cleanups

bff7f20

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teor2345 added 3 commits June 5, 2026 19:09

Remove a leftover direct call to fn_ctxt.label_generic_mismatches

739be5d

Fix another splat-related ICE revealed by new tests

262bc6c

Avoid panics during argument mismatch diagnostics due to splat

a4b5d0b

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