make ConstEvaluatable more strict

[lcnr]

relevant zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/.60ConstEvaluatable.60.20generic.20functions/near/204125452

Let's see how much this impacts. Depending on how this goes this should probably be a future compat warning.

Short explanation: we currently forbid anonymous constants which depend on generic types, e.g. [0; std::mem::size_of::<T>] currently errors.

We previously checked this by evaluating the constant and returned an error if that failed. This however allows things like

const fn foo<T>() -> usize {
    if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
        std::mem::size_of::<T>()
    } else {
        8
    }
}

fn test<T>() {
    let _ = [0; foo::<T>()];
}

which is a backwards compatibility hazard. This also has worrying interactions with mir optimizations (#74491 (comment)) and intrinsics (#74538).

r? @oli-obk @eddyb

[lcnr]

@bors try

[bors]

⌛ Trying commit 617f7ad0b02ca21994b4cf28d14634486163b166 with merge 536076e3df53cc305e0c3c5f38bdc73f6d9f08bb...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 536076e3df53cc305e0c3c5f38bdc73f6d9f08bb (536076e3df53cc305e0c3c5f38bdc73f6d9f08bb)

[lcnr]

It looks like the crater queue is fairly full. @Mark-Simulacrum do we currently have the capacity to do another check run?

I also did some small changes here, so once again @bors try

[bors]

⌛ Trying commit 65395c0710061f8745bdfab3520256a178e92c9d with merge a5913924b14838b364c42230cae014cc2534331d...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: a5913924b14838b364c42230cae014cc2534331d (a5913924b14838b364c42230cae014cc2534331d)

[lcnr]

Well, will go ahead and add this PR into the crate queue. If we don't have the capacity for this we can just remove it again.

@craterbot check

[craterbot]

👌 Experiment pr-74595 created and queued.
🤖 Automatically detected try build a5913924b14838b364c42230cae014cc2534331d
🔍 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-74595 is now running

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

[craterbot]

🚨 Experiment pr-74595 has encountered an error: command Command { std: "/workspace/cargo-home/bin/rustup-toolchain-install-master" "8ad7bc3f428300aee6764f6e23527e19eb235e81" "-c" "cargo", kill_on_drop: false } failed
🛠️ If the error is fixed use the retry command.

🆘 Can someone from the infra team check in on this? @rust-lang/infra
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Mark-Simulacrum]

@craterbot retry seems to have been spurious

[craterbot]

🚨 Error: failed to parse the command

🆘 If you have any trouble with Crater please ping @rust-lang/infra!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[lcnr]

@craterbot retry

[craterbot]

🛠️ Experiment pr-74595 queued again.

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

[lcnr]

This behavior was introduced in #70452 which was merged on the 15.04.2020, meaning that this has been only stable for 2 stable versions. (since 1.43.0)

So hopefully there aren't many uses of this yet

[craterbot]

🚧 Experiment pr-74595 is now running

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

[craterbot]

🚨 Experiment pr-74595 has encountered an error: command Command { std: "/workspace/cargo-home/bin/rustup-toolchain-install-master" "8ad7bc3f428300aee6764f6e23527e19eb235e81" "-c" "cargo", kill_on_drop: false } failed
🛠️ If the error is fixed use the retry command.

🆘 Can someone from the infra team check in on this? @rust-lang/infra
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Mark-Simulacrum]

@craterbot retry

[craterbot]

🛠️ Experiment pr-74595 queued again.

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

[craterbot]

🚧 Experiment pr-74595 is now running

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

[lcnr]

I wasn't able to reproduce this locally, but added a #[warn(const_evaluatable_unchecked)] anyways.

[oli-obk]

r=me with CI passing

[lcnr]

@bors r=oli-obk

[bors]

📌 Commit 4226a17 has been approved by oli-obk

[Aaron1011]

@bors retry

[bors]

⌛ Testing commit 4226a17 with merge 8d92815944e001c9703489b285f04a9eb6567699...

[bors]

💔 Test failed - checks-actions

[lcnr]

How can I reproduce this locally? I don't understand this error

[oli-obk]

The only thing I can come up with that could cause this difference is --target=i586-unknown-linux-gnu. You may be able to do a --pass=check test run of that specific test and check why it does this. I'm fine with just ignoring the test on that platform once we know why it's happening

[lcnr]

@bors r=oli-obk

[bors]

📌 Commit 74e0719 has been approved by oli-obk

[bors]

⌛ Testing commit 74e0719 with merge e2be5f5...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: oli-obk
Pushing e2be5f5 to master...

[Mark-Simulacrum]

This was an improvement of up to 2% on wf-projection-stress. Presumably not an expected result?

[lcnr]

We were afraid of causing a regression here and had similar results 🤔 Probably some inlining changes or something

but not really expected

edit: https://perf.rust-lang.org/compare.html?start=3cfc7fe78eccc754b16981704a098d7bd520e2fd&end=2ce316b2752e1543bc064357871d77fd4896ca27