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

rustbot · 2026-05-05T10:41:23Z

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

teor2345

This should be ready for another review.

I've rebased this on the latest main, trying to minimise the number of rebase changes to earlier commits. The rebase was tricky, so there are some fixes which should be squashed into earlier commits, and some code might not quite be in the nicest/clearest commit.

The last 5 commits are revisions from the latest reviews, the detailed commit message says which commit they revise.

View changes since this review

teor2345 · 2026-05-04T03:08:45Z

+    /// 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,


It might make the compiler faster to limit it to 256 arguments, I wonder if that would break any code.

At least we test for u16::MAX arguments. It used to panic, see #88577.

teor2345 · 2026-05-04T03:24:30Z

+        splatted: Option<u16>,
+        args_len: usize,
+    ) -> Result<Self, SplattedArgIndexError> {
+        if let Some(splatted_arg_index) = splatted {


Yeah, I wasn't sure about this either.

I felt like having the option was clearer in callers, and tried to keep the sentinel value as an internal implementation detail which never appears outside the low-level bit-packing code. That way, providing u16::MAX as the splatted argument is easy to error on.

But I don't mind dropping the option, it's unlikely this case will be encountered in practice.
What do you think?

teor2345 · 2026-05-05T03:20:01Z

+                    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!(


I'm not sure if it's reachable, and I don't know how to create a test for this. It seems like other errors happen first, but there might be a way to trigger it. I left a FIXME in the code.

I made a bunch of related fixes and tests while trying to test this:

fixed some invalid method splatted self argument handling which resulted in an ICE (there's a test for it now)

cleaned up some unnecessary diagnostic function arguments

some error messages were outdated (splat is supported on any argument) so I updated them

use the argument span in some error messages, rather than using the argument number

add tests for generics that might be tuples, and for unresolvable/ambiguous types

teor2345 · 2026-05-07T22:30:41Z

@rustbot ready

teor2345 · 2026-05-11T22:01:49Z

Just some rust-analyzer refactors, so I needed to move splat FIXME comments into the new refactored code.

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 · 2026-05-13T01:44:06Z

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.

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

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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 has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 11, 2026

teor2345 force-pushed the fn-arg-splat-experiment branch from 6b05355 to 881fb4b Compare May 11, 2026 22:00

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teor2345 added 11 commits May 13, 2026 11:33

Add splat builtin attribute & feature gate

5c03d8c

Add AST validation for #[splat]

b1b2907

Add HIR FnDecl for #[splat]

0d9e34a

Impl HIR typeck for #[splat]

cbd4ea8

Impl MIR lowering for #[splat]

3e52f90

Impl TypeInfo for splat

69e9a95

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

5d6888e

fixup! Add splat builtin attribute & feature gate

Fix a #[splat] AST validation method comment

8a1dfe1

fixup! Add AST validation for #[splat]

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

ede9033

fixup! Add HIR FnDecl for #[splat]

Extract splatted typeck into a fn and cleanup error handling

0d3a8d7

fixup! Impl HIR typeck for #[splat]

Add splat to libcore's TypeInfo

0e4aed3

fixup! Impl TypeInfo for splat

teor2345 force-pushed the fn-arg-splat-experiment branch from 881fb4b to 0e4aed3 Compare May 13, 2026 01:44

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