Add arg splat experiment initial tuple impl

[teor2345]

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.

Perf Impact

We expect a 0.1% regression on 5 primary and 0.2% regression on 4 secondary benchmarks in this PR, based on this perf run.

We tried a number of different ways to improve perf. Limiting splat to the 255th or lower argument is a simple hack that gives good perf, and is good enough for an experiment.

This PR series already has significant perf wins in #155223 - 0.3% perf improvement across 45 primary benchmarks. We're spending a small amount of that perf for the new feature in this PR.

Out of Scope for this PR

• Change codegen to de-tuple caller and callee
• Better diagnostics
• Full support for splatted function pointer arguments

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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

[rust-bors]

💔 Test for 0a45074 failed: CI. Failed job:

• try - dist-i586-gnu-i586-i686-musl (web logs, enhanced plaintext logs)

[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.

[teor2345]

@bors try job=dist-i586-gnu-i586-i686-musl

[teor2345]

@bors rollup=iffy reason=small perf decrease,job-sensitive test output

[teor2345]

@bors try job=dist-i586-gnu-i586-i686-musl

[rust-bors]

💔 Test for 6d0547c failed: CI. Failed job:

• try - dist-i586-gnu-i586-i686-musl (web logs, enhanced plaintext logs)

[teor2345]

Ok, I think this is it, but let's try a few more jobs just in case

@bors try job=-musl

[teor2345]

Try builds seem good enough, the failing one has passed, and only a few less similar ones remain. Assuming it was musl causing the output differences, I've made the output a lot more flexible.

@bors r=oli-obk

[rust-bors]

📌 Commit 4d9ae09 has been approved by oli-obk

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 89e3e46 (89e3e46efbddcd01d335ea36c0ad8dc66e15190c)
Base parent: 40557f6 (40557f6225e337d68c8d4f086557ce54135f5dd9)

[teor2345]

The above failure is spurious, it is from an old try job.