Rename splat to avoid stable name collisions

[teor2345]

TODO

• rename the attribute to rustc_splat
  • add the rustc_internal (?) feature to the UI tests
• keep using the splat symbol for the feature name

Tracking issue: #153629

This PR renames #[splat] to #[arg_splat], in a mostly automated way (using fastmod).

The rename commands I used were:

fastmod --fixed-strings '#[splat]' '#[arg_splat]'
fastmod --fixed-strings '#![feature(splat' '#![feature(arg_splat'
# All the remaining commands are no-ops on beta
fastmod --fixed-strings 'multiple `splat` attributes' 'multiple `arg_splat` attributes'
# Legacy symbol demangling
fastmod --fixed-strings '$splat$' '$arg_splat$'
# Revert some changes that aren't in rustc_demangle yet
fastmod --fixed-strings 'main::Type<fn(#[arg_splat]' 'main::Type<fn(#[splat]'
fastmod --fixed-strings 'main::Type<*const fn(#[arg_splat]' 'main::Type<*const fn(#[splat]'

Part of #159428

Backport Advice

Cherry-pick the first commit to the beta branch, then run the fastmod commands above.
I've tested this locally, and it works, see my splat-rename-beta branch.

Remaining Work

rustc_demangle needs an update to the attribute name, a release, and then a version bump in rust-lang/rust. I'll do that separately. It doesn't need to be backported, because the last version bump was after beta branched.

There's also some external code that needs updates, the full list is in the ticket: #159428 (comment)

Can someone with write access also update the description for the F-splat GitHub label 🙂

@rustbot label +beta-nominated +A-attributes +A-macros +A-resolve +C-bug +F-splat +P-high +T-compiler

[rustbot]

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

This PR changes rustc_public

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

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

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

cc @rust-lang/rustfmt

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[teor2345]

No P-high for me!

@rustbot label +beta-nominated +A-attributes +A-macros +A-resolve +C-bug +F-splat +T-compiler

[jieyouxu]

FWIW we only assign P-* for issues not PRs; I'm happy to review the rename.
r? me

[apiraino]

I will "de-nominate" from backport as now it's a bit too early. Feel free to nominate again when the PR is reviewed and approved :)

@rustbot label -beta-nominated

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- test::system_tests stdout ----
2026-07-24T07:59:22.994885Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.023898Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.066637Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.178743Z  WARN rustfmt_nightly::test: Default value false used explicitly for wrap_comments
2026-07-24T07:59:23.300221Z  WARN rustfmt_nightly::test: Default value [] used explicitly for skip_macro_invocations
2026-07-24T07:59:23.314555Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.412058Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.461992Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_imports
2026-07-24T07:59:23.467056Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.488181Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
2026-07-24T07:59:23.560018Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_imports
2026-07-24T07:59:23.568307Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.679622Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
Warning: the `fn_args_layout` option is deprecated. Use `fn_params_layout`. instead
2026-07-24T07:59:23.708299Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
2026-07-24T07:59:23.715227Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_modules
Warning: the `merge_imports` option is deprecated. Use `imports_granularity="Crate"` instead

thread '<unnamed>' (54641) panicked at src/tools/rustfmt/src/test/mod.rs:842:40:
called `Result::unwrap()` on an `Err` value: ModuleResolutionError(ModuleResolutionError { module: "ZYXW", kind: NotFound { file: "tests/source/reorder_modules/ZYXW.rs" } })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

thread 'test::system_tests' (54640) panicked at src/tools/rustfmt/src/test/mod.rs:82:10:
Failed to join a test thread: Any { .. }
---- test::system_tests stdout end ----
---- test::idempotence_tests stdout ----
2026-07-24T07:59:22.944331Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:22.978387Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.058591Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.198770Z  WARN rustfmt_nightly::test: Default value false used explicitly for wrap_comments
2026-07-24T07:59:23.354421Z  WARN rustfmt_nightly::test: Default value [] used explicitly for skip_macro_invocations
2026-07-24T07:59:23.366876Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.520452Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.547348Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.598288Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_imports
2026-07-24T07:59:23.606702Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.613247Z  WARN rustfmt_nightly::test: Default value 0 used explicitly for struct_field_align_threshold
2026-07-24T07:59:23.617609Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
2026-07-24T07:59:23.617615Z  WARN rustfmt_nightly::test: Default value 4 used explicitly for tab_spaces
2026-07-24T07:59:23.617620Z  WARN rustfmt_nightly::test: Default value SameLineWhere used explicitly for brace_style
2026-07-24T07:59:23.617624Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_modules
2026-07-24T07:59:23.617629Z  WARN rustfmt_nightly::test: Default value false used explicitly for hard_tabs
2026-07-24T07:59:23.636048Z  WARN rustfmt_nightly::test: Default value false used explicitly for wrap_comments
2026-07-24T07:59:23.641961Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
2026-07-24T07:59:23.744255Z  WARN rustfmt_nightly::test: Default value false used explicitly for wrap_comments
2026-07-24T07:59:23.752239Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_imports
2026-07-24T07:59:23.758744Z  WARN rustfmt_nightly::test: Default value 2015 used explicitly for style_edition
2026-07-24T07:59:23.887035Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
Warning: the `fn_args_layout` option is deprecated. Use `fn_params_layout`. instead
2026-07-24T07:59:23.912767Z  WARN rustfmt_nightly::test: Default value 100 used explicitly for max_width
2026-07-24T07:59:23.916047Z  WARN rustfmt_nightly::modules: Couldn't find file for mod foo;`
2026-07-24T07:59:23.919729Z  WARN rustfmt_nightly::test: Default value true used explicitly for reorder_modules
Warning: the `merge_imports` option is deprecated. Use `imports_granularity="Crate"` instead

thread '<unnamed>' (54612) panicked at src/tools/rustfmt/src/test/mod.rs:842:40:
called `Result::unwrap()` on an `Err` value: ModuleResolutionError(ModuleResolutionError { module: "ZYXW", kind: NotFound { file: "tests/target/reorder_modules/ZYXW.rs" } })

thread 'test::idempotence_tests' (54611) panicked at src/tools/rustfmt/src/test/mod.rs:82:10:
Failed to join a test thread: Any { .. }
---- test::idempotence_tests stdout end ----

[rust-bors]

☔ The latest upstream changes (presumably #159814) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[ytmimi]

Please restore src/tools/rustfmt/tests/source/reorder_modules/ZYXW/mod.rs, src/tools/rustfmt/tests/source/reorder_modules/ZYXW/mod.rs, src/tools/rustfmt/tests/target/reorder_modules/ABCD/mod.rs, src/tools/rustfmt/tests/target/reorder_modules/ZYXW/mod.rs

Removing these files is causing rustfmt test failures:

  thread '<unnamed>' (54612) panicked at src/tools/rustfmt/src/test/mod.rs:842:40:
  called `Result::unwrap()` on an `Err` value: ModuleResolutionError(ModuleResolutionError { module: "ZYXW", kind: NotFound { file: "tests/target/reorder_modules/ZYXW.rs" }

View changes since this review

[rustbot]

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