Add arg splat experiment initial tuple impl

[teor2345]

TODO

This PR is waiting for #157811 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:

• Tracking Issue for argument splatting lang experiment #153629
• https://rust-lang.github.io/rust-project-goals/2026/overloading-for-ffi.html
• https://rust-lang.github.io/rust-project-goals/2025h2/interop-problem-map.html

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]

r? @JohnTitor

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[JohnTitor]

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]

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]

This PR is waiting on #157605 #157699 #157811 to merge, then I'll rebase it.

@rustbot author

[rustbot]

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.

[JonathanBrouwer]

@bors try @rust-timer queue

[rust-bors]

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

[rust-timer]

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%)