remove blanket-Freeze for PhantomData

[RalfJung]

The exact behavior of types that are only partially covered by UnsafeCell is still up for debate. So we shouldn't rush to the conclusion that PhantomData<UnsafeCell<T>> 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]

r? @TaKO8Ki

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo

[RalfJung]

@bors try

[bors]

⌛ Trying commit 105df08 with merge 8d0c454bea9d9cc2cc7e288388f3b7d1da19c04f...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8d0c454bea9d9cc2cc7e288388f3b7d1da19c04f (8d0c454bea9d9cc2cc7e288388f3b7d1da19c04f)

[RalfJung]

@craterbot check

[craterbot]

👌 Experiment pr-114559 created and queued.
🤖 Automatically detected try build 8d0c454bea9d9cc2cc7e288388f3b7d1da19c04f
⚠️ Try build based on commit 105df08, but latest commit is 344af673f0a63147411f917245695d11be7d26a8. Did you forget to make a new try build?
🔍 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-114559 is now running

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

[Noratrieb]

Regarding the UI test, there's a good chance there are some UnsafeCell tests in the root dir and it would be nice moving those.

[craterbot]

🎉 Experiment pr-114559 is completed!
📊 721 regressed and 2 fixed (343114 total)
📰 Open the full report.

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

[RalfJung]

@craterbot run mode=check-only crates=https://crater-reports.s3.amazonaws.com/pr-114559/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-114559-1 created and queued.
🤖 Automatically detected try build 8d0c454bea9d9cc2cc7e288388f3b7d1da19c04f
⚠️ Try build based on commit 105df08, but latest commit is 5f6edef. Did you forget to make a new try build?
🔍 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

[RalfJung]

Ah never mind, this needs a rerun with the last commit included.
@craterbot abort name=pr-114559-1

@bors try

[craterbot]

🗑️ Experiment pr-114559-1 deleted!

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

[bors]

⌛ Trying commit 5f6edef with merge 4001eed09e19538fd1a219411bb07c1bce2bbf55...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4001eed09e19538fd1a219411bb07c1bce2bbf55 (4001eed09e19538fd1a219411bb07c1bce2bbf55)

[RalfJung]

@craterbot run mode=check-only crates=https://crater-reports.s3.amazonaws.com/pr-114559/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-114559-1 created and queued.
🤖 Automatically detected try build 4001eed09e19538fd1a219411bb07c1bce2bbf55
🔍 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-114559-1 is now running

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

[craterbot]

🎉 Experiment pr-114559-1 is completed!
📊 246 regressed and 0 fixed (721 total)
📰 Open the full report.

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

[RalfJung]

The vast majority of these seem to trace back to 2 root causes:

• code exploiting that types like this in the ff crate have no interior mutability.
• something deep inside the macro soup that is the abi_stable crate, probably related to types like this and then taking references to them in const.

[RalfJung]

So, this seems unlikely right now, we'd need solid motivation for the breaking change.