Add support for splatted function pointers by teor2345 · Pull Request #159643 · rust-lang/rust

teor2345 · 2026-07-21T08:42:42Z

Tracking issue: #153629

This PR adds support for splatted function pointers.

Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication.

Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers?

Close#158603

teor2345 · 2026-07-21T08:43:58Z

@rustbot label +C-bug

rustbot · 2026-07-23T04:23:58Z

r? @petrochenkov

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petrochenkov · 2026-07-23T12:16:36Z

@rustbot reroll

folkertdev

This PR got quite big and seemingly fixes a bunch of different things, is there any chance you can split it up? At least the refactoring into using the custom enums seems like it could be split out, just stubbing out the branches for FnPtr and errors. If possible a separate PR would be nice (just assign me as a reviewer to preserve context), otherwise separate commits would also work.

It's just really tough to review like this.

View changes since this review

folkertdev · 2026-07-25T12:15:39Z

+
+fn main() {
+ // FIXME(rustfmt): the attribute gets deleted by rustfmt
+ #[rustfmt::skip]


Might be better to put the #[rustfmt::skip] on the function to reduce noise?

Happy to do that. It's better for readability, but slightly worse for maintenance because now every line is unformatted, not just the splatted type lines.
Ultimately we'll fix this in rustfmt if function argument attributes become a thing.

folkertdev · 2026-07-25T12:17:46Z

We usually just use assert_eq!, is there any particular reason to deviate from that? Your generic<T: Tuple + Debug>(#[splat] a: T) could use format! and return a String?

No particular reason, it will be much more readable to use assert_eq! across all the splat UI tests. The generic case can just return the argument.
A codegen bug could swap the tupled argument and return value, or overwrite the return value, but the multi-argument case will catch that. Or it might be worth having a test that swaps the order of the arguments in the return value. I'll have a think about it.

folkertdev · 2026-07-25T12:25:04Z

- receiver: Option<&'tcx hir::Expr<'tcx>>,
+ has_receiver: bool,
+ // This is the method receiver, or the function path
+ receiver_or_func: &'tcx hir::Expr<'tcx>,


this still feels kind of wrong, although I don't have any obviously better suggestions.

To make the states clearer, it could be:
enumSplattedFunc{FnDefReceiver(Expr),/* NoFnDefReceiver, edit: can't be known at call site */FnPtrExpression(Expr),}

rustbot · 2026-07-25T12:30:55Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

teor2345

This PR got quite big and seemingly fixes a bunch of different things, is there any chance you can split it up? At least the refactoring into using the custom enums seems like it could be split out, just stubbing out the branches for FnPtr and errors. If possible a separate PR would be nice (just assign me as a reviewer to preserve context), otherwise separate commits would also work.

That's good feedback. I ended up squashing the whole thing to make it easier to rebase and rewrite, but I didn't split it into commits again.

I think it can be split into something like:

inline the splatted_callee function
refactor using custom enums (with stubs)
support for splatted FnPtrs

In a separate PR:

update tests to use assert_eq! (and a single rustfmt::skip)
- edit: for FnPtr specific tests, I made this change in the commit where I added FnPtr support
remove resolved FIXMEs in splat UI tests

~~If you'd like some of those commits in different PRs, or in a different order, please let me know.~~

View changes since this review

teor2345 · 2026-07-27T07:57:09Z

- receiver: Option<&'tcx hir::Expr<'tcx>>,
+ has_receiver: bool,
+ // This is the method receiver, or the function path
+ receiver_or_func: &'tcx hir::Expr<'tcx>,


To make the states clearer, it could be:
enumSplattedFunc{FnDefReceiver(Expr),/* NoFnDefReceiver, edit: can't be known at call site */FnPtrExpression(Expr),}

teor2345 · 2026-07-27T08:00:07Z

+
+fn main() {
+ // FIXME(rustfmt): the attribute gets deleted by rustfmt
+ #[rustfmt::skip]


Happy to do that. It's better for readability, but slightly worse for maintenance because now every line is unformatted, not just the splatted type lines.
Ultimately we'll fix this in rustfmt if function argument attributes become a thing.

teor2345 · 2026-07-27T08:04:45Z

No particular reason, it will be much more readable to use assert_eq! across all the splat UI tests. The generic case can just return the argument.
A codegen bug could swap the tupled argument and return value, or overwrite the return value, but the multi-argument case will catch that. Or it might be worth having a test that swaps the order of the arguments in the return value. I'll have a think about it.

teor2345 · 2026-07-28T06:07:31Z

I split into these commits:

inline the splatted_callee function
refactor using custom enums (with stubs)
support for splatted FnPtrs
- including test cleanups for FnPtr tests

And I opened PR #160049 with the unrelated test cleanups.

@rustbot ready

rust-bors · 2026-08-06T13:29:59Z

📌 Commit f88563c has been approved by folkertdev

It is now in the queue for this repository.

Add support for splatted function pointers Tracking issue: rust-lang#153629 This PR adds support for splatted function pointers. Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication. Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers? Closerust-lang#158603

Rollup of 14 pull requests Successful merges: - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160634 (miri subtree update) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160504 (cleanup borrowck, improve c-variadic handling) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160636 (derive(Diagnostic): link to proper docs)

Add support for splatted function pointers Tracking issue: rust-lang#153629 This PR adds support for splatted function pointers. Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication. Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers? Closerust-lang#158603

…uwer Rollup of 21 pull requests Successful merges: - #159784 (Hint that memchr returns an in-bounds index) - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160634 (miri subtree update) - #157641 (Do not promote extern statics) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #160103 (Add regression test for GAT bound mismatched type error) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - #160504 (cleanup borrowck, improve c-variadic handling) - #160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160636 (derive(Diagnostic): link to proper docs) - #160644 (Clean up some manual debug impls) - #160649 (move naked function ui tests)

Add support for splatted function pointers Tracking issue: rust-lang#153629 This PR adds support for splatted function pointers. Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication. Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers? Closerust-lang#158603

…uwer Rollup of 20 pull requests Successful merges: - #159784 (Hint that memchr returns an in-bounds index) - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160634 (miri subtree update) - #157641 (Do not promote extern statics) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #160103 (Add regression test for GAT bound mismatched type error) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - #160504 (cleanup borrowck, improve c-variadic handling) - #160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160636 (derive(Diagnostic): link to proper docs) - #160644 (Clean up some manual debug impls)

Add support for splatted function pointers Tracking issue: rust-lang#153629 This PR adds support for splatted function pointers. Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication. Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers? Closerust-lang#158603

…uwer Rollup of 28 pull requests Successful merges: - #159784 (Hint that memchr returns an in-bounds index) - #160673 (Improve `canonical_param_env_cache`) - #150885 (Revive L4Re target) - #159643 (Add support for splatted function pointers) - #160433 (delegation: add support for wrapping of the return value with `From::from`) - #160530 (refactor handling of target features in Session) - #160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - #160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - #160683 (Add regression test for unknown feaeture name reported with other errors) - #157641 (Do not promote extern statics) - #158904 (Fix FutureDropPoll shim for by-move async closures) - #159816 (added note/help about iterator invalidation when mutating a collection inside a for loop) - #160103 (Add regression test for GAT bound mismatched type error) - #160335 (dlopen offload) - #160445 (codegen: classify localized MSVC linker progress as linker_info) - #160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - #160504 (cleanup borrowck, improve c-variadic handling) - #160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - #160587 (Add regression test for associated type outlives bound at call site) - #160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - #160633 (delegation: fix determining wrong `FnKind` when delegation is inside const arg) - #160636 (derive(Diagnostic): link to proper docs) - #160644 (Clean up some manual debug impls) - #160649 (move naked function ui tests) - #160672 (Improve `MaybeLiveLocals`) - #160693 (Add branch config for perf. unrolling in bors) - #160696 (rustc_codegen_llvm: handle sm_101* features being an alias) - #160706 (renovate: clarify that vulnerability PRs are opened automatically)

Rollup merge of #159643 - teor2345:splat-fn-ptr, r=folkertdev Add support for splatted function pointers Tracking issue: #153629 This PR adds support for splatted function pointers. Currently splatted function pointers ICE due to unpopulated side-tables. This PR fixes the ICE by populating the side-table correctly, and fixes the MIR lowering code. It also refactors the surrounding code to reduce code duplication. Generic function pointers also work. I'm sure if there's something extra we need to do to support all generic function pointers? Close#158603

…uwer Rollup of 28 pull requests Successful merges: - rust-lang/rust#159784 (Hint that memchr returns an in-bounds index) - rust-lang/rust#160673 (Improve `canonical_param_env_cache`) - rust-lang/rust#150885 (Revive L4Re target) - rust-lang/rust#159643 (Add support for splatted function pointers) - rust-lang/rust#160433 (delegation: add support for wrapping of the return value with `From::from`) - rust-lang/rust#160530 (refactor handling of target features in Session) - rust-lang/rust#160606 (bootstrap: Store and use an explicit CheckKind in `check::Rustc`) - rust-lang/rust#160628 (fix ICE in `suggest_add_reference_to_arg` for non-callable items) - rust-lang/rust#160683 (Add regression test for unknown feaeture name reported with other errors) - rust-lang/rust#157641 (Do not promote extern statics) - rust-lang/rust#158904 (Fix FutureDropPoll shim for by-move async closures) - rust-lang/rust#159816 (added note/help about iterator invalidation when mutating a collection inside a for loop) - rust-lang/rust#160103 (Add regression test for GAT bound mismatched type error) - rust-lang/rust#160335 (dlopen offload) - rust-lang/rust#160445 (codegen: classify localized MSVC linker progress as linker_info) - rust-lang/rust#160499 (rustc_resolve: move diagnostic attribute linting to attr parsing) - rust-lang/rust#160504 (cleanup borrowck, improve c-variadic handling) - rust-lang/rust#160577 (expand: Feature gate AST-based attribute macros on expressions and statements) - rust-lang/rust#160587 (Add regression test for associated type outlives bound at call site) - rust-lang/rust#160625 (platform-support/netbsd.md: No longer mention 8.x, due to EoL.) - rust-lang/rust#160633 (delegation: fix determining wrong `FnKind` when delegation is inside const arg) - rust-lang/rust#160636 (derive(Diagnostic): link to proper docs) - rust-lang/rust#160644 (Clean up some manual debug impls) - rust-lang/rust#160649 (move naked function ui tests) - rust-lang/rust#160672 (Improve `MaybeLiveLocals`) - rust-lang/rust#160693 (Add branch config for perf. unrolling in bors) - rust-lang/rust#160696 (rustc_codegen_llvm: handle sm_101* features being an alias) - rust-lang/rust#160706 (renovate: clarify that vulnerability PRs are opened automatically)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 21, 2026

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rustbot added F-splat `#![feature(splat)]` https://github.com/rust-lang/rust/issues/153629 I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ labels Jul 21, 2026

rustbot added the C-bug Category: This is a bug. label Jul 21, 2026

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teor2345 mentioned this pull request Jul 28, 2026
Use assert_eq! in splat codegen tests #160049
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Add overloading experiment blog post rust-lang/blog.rust-lang.org#1907
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This was referenced Aug 18, 2026
Nightly support for function overloading in FFI bindings rust-lang/goals#643
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C++/Rust Interop Problem Space Mapping rust-lang/goals#388
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Add more splat fn type tests #161711
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