lint against repeated repr attributes

[ettolrach]

fixes #156029 by adding a lint against repeated repr attributes.

todo for the draft before marking as ready:

• are we certain that we will ban this in the future? if so, a new issue should be made to use the @future_incompatible = FutureIncompatibleInfo part of the declare_lint! macro.
• should it be warn or deny?
• add tests
• remove all other todos (mostly explanations and doc. if anyone else is reading this right now, i like writing doc! so that's not an issue for me)

r? scottmcm

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

fmt: checked 6867 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [style (compiler)]: /checkout/compiler/rustc_lint_defs/src/builtin.rs:279: TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME
tidy [style (compiler)]: /checkout/compiler/rustc_lint_defs/src/builtin.rs:283: TODO is used for tasks that should be done before merging a PR; If you want to leave a message in the codebase use FIXME
tidy [style (compiler)]: FAIL
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (26.1.1)
linting python files
---
info: ES-Check: there were no ES version matching errors!  🎉
typechecking javascript files
tidy: The following check failed: style (compiler)
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy --root-path=/checkout --cargo-path=/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo --output-dir=/checkout/obj/build --concurrency=4 --npm-path=/node/bin/yarn --ci=true --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1618:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1417:29

--- BACKTRACE vvv
   0: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   1: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   2: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   3: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   4: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1417:29
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Tidy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1595:36
   6: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1339:21
   7: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
   8: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:232:18
   9: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1138:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1117:14
  11: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  12: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  13: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:206:18
  16: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:287:21
  17: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
  18: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:544:19
  19: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:45
  local time: Wed May 27 20:04:17 UTC 2026
  network time: Wed, 27 May 2026 20:04:17 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[scottmcm]

Wearing my lang hat but not speaking as team consensus (since we've not talked about it):

I think you should start with the "obviously good" version, and we can do future PRs later to expand it or to update severity. Concretely, thus, I'd say it should be a warning (because if it's inert it doesn't need to be deny) when you have a clearly-unnecessary one that wasn't emitted from a macro, or something like that.

(Our bar in the compiler for deny-by-default is pretty high. My personal heuristic is whether it's worth running your unit tests despite the warning, and an inert extra attribute doesn't need to block that.)

We can then look at impacts and ratchet it up over time or over an edition, but those decisions can be made later and don't need to block you making a useful lint in the meantime.

---

I'm a bad reviewer for lint stuff code-wise, though, so let's try
r? JonathanBrouwer

[traviscross]

+1 to what @scottmcm said on how best to approach this.