remove blanket-Freeze for PhantomData

[p-avital]

At @RalfJung's request, I ported #114559 over so that a new crater run can be performed, as the last known run is 3 years old.

Here's what @RalfJung said back then about that previous PR.

The exact behavior of types that are only partially covered by UnsafeCell is still rust-lang/unsafe-code-guidelines#236. So we shouldn't rush to the conclusion that PhantomData<UnsafeCell> has no interior mutability. Let's take back that blanket impl. This will need a crater run because it is a breaking change -- but the locations where Freeze makes the difference are few and far between, so the impact is likely to be low. (UnsafeCell is also !Sync and that has the much bigger impact, and is not masked by PhantomData.)

I couldn't find an existing ui test folder that fits, so I created a new one. tidy doesn't seem to like that though, it complains about new files and new folders, which is kind of counterproductive? I had to silence it the hard way.

[rustbot]

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

cc @antoyo, @GuillaumeGomez

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

cc @bjorn3

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 18 candidates

[RalfJung]

Please mark PRs like this as a draft, that avoids sending notifications prematurely.

[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)

[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
fmt: checked 6991 files
tidy check
tidy [ui_tests (tests)]: ui subdirectory `tests/ui/unsafe-cell` exists in the filesystem but is not documented in `tests/ui/README.md`
tidy [ui_tests (tests)]: `tests/ui/README.md` subdirectory listing is out of sync with the filesystem. Please add or remove subdirectory entries (## headers with backtick-wrapped names) to match the actual directories in `tests/ui/`
tidy [ui_tests (tests)]: FAIL
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
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'
---
Running eslint on rustdoc JS files
info: ES-Check: checking 7 files...
info: ✓ ES-Check passed! All files are ES10 compatible.
typechecking javascript files
tidy: The following check failed: ui_tests (tests)
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:1625:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1646:29

--- 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:1646:29
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Tidy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1607:36
   6: <bootstrap::core::build_steps::test::Tidy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:1568:21
   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:1140:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1119:14
  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/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/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/0417c25868d6dfbd1c291dfeae950504faa6f790/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/0417c25868d6dfbd1c291dfeae950504faa6f790/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/0417c25868d6dfbd1c291dfeae950504faa6f790/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/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:39
  local time: Tue Jul  7 14:48:57 UTC 2026
  network time: Tue, 07 Jul 2026 14:48:57 GMT
##[error]Process completed with exit code 1.

[p-avital]

Apologies, I rarely use draft PRs, so I tend to forget about them, I'll be more careful in the future :)

[RalfJung]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: e977a0b (e977a0b5ab976a591981e7263bd67a71a4f79308)
Base parent: f10db29 (f10db292a3733b5c67c8da8c7661195ff4b05774)

[RalfJung]

@craterbot check

[craterbot]

👌 Experiment pr-158914 created and queued.
🤖 Automatically detected try build e977a0b
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-158914 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-158914 is completed!
📊 396 regressed and 9 fixed (1013799 total)
📊 5945 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-158914/retry-regressed-list.txt ↩

[JohnTitor]

r? RalfJung

[rustbot]

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

[RalfJung]

@rustbot author

[rustbot]

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

[rust-bors]

☔ The latest upstream changes (presumably #163472) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.