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Do not promote values with const drop that need to be dropped #89988

Merged
merged 2 commits into from
Oct 19, 2021
Merged

Do not promote values with const drop that need to be dropped #89988

merged 2 commits into from
Oct 19, 2021

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tmiasko
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@tmiasko tmiasko commented Oct 17, 2021 •
edited

Changes from #88558 allowed using ~const Drop in constants by
introducing a new NeedsNonConstDrop qualif.

The new qualif was also used for promotion purposes, and allowed
promotion to happen for values that needs to be dropped but which
do have a const drop impl.

Since for promoted the drop implementation is never executed,
this lead to observable change in behaviour. For example:

struct Panic();

impl const Drop for Panic {
    fn drop(&mut self) {
        panic!();
    }
}

fn main() {
    let _ = &Panic();
}

Restore the use of NeedsDrop qualif during promotion to avoid the issue.

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r? @jackh726

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 17, 2021
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tmiasko commented Oct 17, 2021

cc @fee1-dead @matthewjasper @oli-obk

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oli-obk commented Oct 17, 2021

r? @oli-obk

@rust-highfive rust-highfive assigned oli-obk and unassigned jackh726 Oct 17, 2021
@tmiasko tmiasko force-pushed the unpromote-const-drop branch 2 times, most recently from 11496d4 to f8ea04e Compare October 18, 2021 06:43
@oli-obk
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oli-obk commented Oct 18, 2021

cc @rust-lang/wg-const-eval

discussion that spawned this: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/const.20drop.20and.20promoteds

@bors r+

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bors commented Oct 18, 2021

📌 Commit f8ea04e3a33eeb29438889e162c977a3fd5314d4 has been approved by oli-obk

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 18, 2021
RalfJung
RalfJung reviewed Oct 18, 2021
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compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
}

/// Constant containing an ADT that implements non-const `Drop`.
/// This must be ruled out because we cannot run `Drop` during compile-time.
pub struct NeedsNonConstDrop;
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Why do we have both NeedsDrop and NeedsNonConrstDrop? That seems rather redundant.

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We only want to detect nonconst drops in const items, as const drops can actually get executed just fine.

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So why does check_consts have NeedsDrop then? Promotion is a separate pass after all.

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oli-obk commented Oct 18, 2021

@bors r-

@bors bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 18, 2021
@tmiasko
Do not promote values with const drop that need to be dropped
915a581 
Changes from rust-lang#88558 allowed using `~const Drop` in constants by
introducing a new `NeedsNonConstDrop` qualif.

The new qualif was also used for promotion purposes, and allowed
promotion to happen for values that needs to be dropped but which
do have a const drop impl.

Since for promoted the drop implementation is never executed,
this lead to observable change in behaviour. For example:

```rust

struct Panic();

impl const Drop for Panic {
    fn drop(&mut self) {
        panic!();
    }
}

fn main() {
    let _ = &Panic();
}
```

Restore the use of `NeedsDrop` qualif during promotion to avoid the issue.
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oli-obk commented Oct 18, 2021

@bors r+

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bors commented Oct 18, 2021

📌 Commit 915a581 has been approved by oli-obk

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 18, 2021
RalfJung
RalfJung reviewed Oct 18, 2021
View reviewed changes
compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs
/// This must be ruled out because implicit promotion would remove side-effects
/// that occur as part of dropping that value. N.B., the implicit promotion has
/// to reject const Drop implementations because even if side-effects are ruled
/// out through other means, the execution of the drop could diverge.
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"diverge" means "running into an endless loop", right? Panics seem like a much simpler example to me.

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The sentence was intended to encompass panicking. In the context of rustc we generally use diverge to describe a computation that does not return normally, but I can see where you are coming from.

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Oh I see. Yeah I think that is not standard terminology, at least not in my peer group. ;)

@matthiaskrgr matthiaskrgr mentioned this pull request Oct 19, 2021
Merged
bors added a commit to rust-lang-ci/rust that referenced this pull request Oct 19, 2021
@bors
Auto merge of rust-lang#90037 - matthiaskrgr:rollup-cdfhxtn, r=matthi…
2f22e63 
…askrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#89766 (RustWrapper: adapt for an LLVM API change)
 - rust-lang#89867 (Fix macro_rules! duplication when reexported in the same module)
 - rust-lang#89941 (removing TLS support in x86_64-unknown-none-hermitkernel)
 - rust-lang#89956 (Suggest a case insensitive match name regardless of levenshtein distance)
 - rust-lang#89988 (Do not promote values with const drop that need to be dropped)
 - rust-lang#89997 (Add test for issue rust-lang#84957 - `str.as_bytes()` in a `const` expression)
 - rust-lang#90002 (:arrow_up: rust-analyzer)
 - rust-lang#90034 (Tiny tweak to Iterator::unzip() doc comment example.)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit a0724d7 into rust-lang:master Oct 19, 2021
@rustbot rustbot added this to the 1.58.0 milestone Oct 19, 2021
@tmiasko tmiasko deleted the unpromote-const-drop branch October 19, 2021 08:15
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