closure field capturing: don't depend on alignment of packed fields

[RalfJung]

This fixes the closure field capture part of #115305: field capturing always stops at projections into packed structs, no matter the alignment of the field. This means changing a private field type from u8 to u64 can never change how closures capture fields, which is probably what we want.

Here's an example where, before this PR, changing the type of a private field in a repr(Rust) struct can change the output of a program:

#![allow(dead_code)]

mod m {
    // before patch
    #[derive(Default)]
    pub struct S1(u8);
    // after patch
    #[derive(Default)]
    pub struct S2(u64);
}

struct NoisyDrop;
impl Drop for NoisyDrop {
    fn drop(&mut self) {
        eprintln!("dropped!");
    }
}

#[repr(packed)]
struct MyType {
    field: m::S1, // output changes when this becomes S2
    other_field: NoisyDrop,
    third_field: Vec<()>,
}

fn test(r: MyType) {
    let c = || {
        let _val = std::ptr::addr_of!(r.field);
        let _val = r.third_field;
    };
    drop(c);
    eprintln!("before dropping");
}

fn main() {
    test(MyType {
        field: Default::default(),
        other_field: NoisyDrop,
        third_field: Vec::new(),
    });
}

Of course this is a breaking change for the same reason that doing field capturing in the first place was a breaking change. Packed fields are relatively rare and depending on drop order is relatively rare, so I don't expect this to have much impact, but it's hard to be sure and even a crater run will only tell us so much.

Also see the nomination comment.

Cc @rust-lang/wg-rfc-2229 @ehuss

[rustbot]

r? @petrochenkov

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[RalfJung]

@bors try

[bors]

⌛ Trying commit e17af739d0ffdef2d40b093d9bedfa8f44205707 with merge 6c5b2f071bf9fe3a4b05942f31d42d076171aa61...

[bors]

☀️ Try build successful - checks-actions
Build commit: 6c5b2f071bf9fe3a4b05942f31d42d076171aa61 (6c5b2f071bf9fe3a4b05942f31d42d076171aa61)

[RalfJung]

@craterbot run mode=build-and-test

[craterbot]

👌 Experiment pr-115315 created and queued.
🤖 Automatically detected try build 6c5b2f071bf9fe3a4b05942f31d42d076171aa61
🔍 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-115315 is now running

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

[craterbot]

🎉 Experiment pr-115315 is completed!
📊 155 regressed and 103 fixed (356067 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=build-and-test crates=https://crater-reports.s3.amazonaws.com/pr-115315/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-115315-1 created and queued.
🤖 Automatically detected try build 6c5b2f071bf9fe3a4b05942f31d42d076171aa61
⚠️ Try build based on commit e17af739d0ffdef2d40b093d9bedfa8f44205707, but latest commit is a671127. 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-115315-1 is now running

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

[craterbot]

🎉 Experiment pr-115315-1 is completed!
📊 17 regressed and 11 fixed (155 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]

None of these look like they would be caused by this PR... 🤷

@craterbot run mode=build-and-test crates=https://crater-reports.s3.amazonaws.com/pr-115315-1/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-115315-2 created and queued.
🤖 Automatically detected try build 6c5b2f071bf9fe3a4b05942f31d42d076171aa61
⚠️ Try build based on commit e17af739d0ffdef2d40b093d9bedfa8f44205707, but latest commit is a671127. 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

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[RalfJung]

r=me on the impl

@bors r=oli-obk

[bors]

📌 Commit a671127 has been approved by oli-obk

It is now in the queue for this repository.

[RalfJung]

I added the relnotes label because this is a breaking change that maybe should be announced in the release notes -- not sure of our exact policy here.

[bors]

⌛ Testing commit a671127 with merge 651b2ca...

[rust-log-analyzer]

The job aarch64-gnu failed! Check out the build log: (web) (plain)

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

[RUSTC-TIMING] process_spawning test:true 1.334
[RUSTC-TIMING] env test:true 1.519
[RUSTC-TIMING] stdbenches test:true 2.923
[RUSTC-TIMING] test test:true 14.923
##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
##[group]Clock drift check
  local time: Sat Sep 16 00:01:50 UTC 2023
  local time: Sat Sep 16 00:01:50 UTC 2023
Session terminated, killing shell... ...killed.
##[error]The operation was canceled.
Cleaning up orphan processes

[bors]

💔 Test failed - checks-actions

[RalfJung]

@bors retry The runner has received a shutdown signal (aarch64-gnu)

[bors]

⌛ Testing commit a671127 with merge 790309b...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 790309b to master...

[rust-timer]

Finished benchmarking commit (790309b): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.9%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 632.885s -> 632.342s (-0.09%)
Artifact size: 318.13 MiB -> 318.18 MiB (0.01%)