Arg splat experiment - HIR FnSig impl

[teor2345]

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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

I've split it from #153697 to make reviewing easier, see that PR for more details.

The PR is the HIR-level FnSig implementation of the feature (next PR will be typecheck):

• Add splat to HIR FnSig
  • which changes legacy mangling scheme function hashes due to the extra bytes
• splat mismatch support, including function pretty-printing
• various FIXME comments

Once this PR merges, I'll rebase #153697.

There aren't any extra tests in this PR, because it doesn't have any externally visible functionality. The visible functionality and tests are in the rest of #153697.

[rustbot]

HIR ty lowering was modified

cc @fmease

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

miri is developed in its own repository. If possible, consider making this change to rust-lang/miri instead.

cc @rust-lang/miri

[rustbot]

r? @dingxiangfei2009

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• Owners of files modified in this PR: compiler, types
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[teor2345]

These attributes don't seem to be necessary, but should they be kept anyway?

Suggested change

	_marker: PhantomData<fn() -> I>,
	#[type_visitable(ignore)]
	#[type_foldable(identity)]
	_marker: PhantomData<fn() -> I>,

View changes since the review

[rust-bors]

☔ The latest upstream changes (presumably #157863) made this pull request unmergeable. Please resolve the merge conflicts.