Don't require Drop for [PhantomData<T>; N] where N and T are generic, if T requires Drop

[oli-obk]

fixes #115403
fixes #115410

This was accidentally regressed in #114134, because it was accidentally stabilized in #102204 (cc @rust-lang/lang, seems like an innocent stabilization, considering this PR is more of a bugfix than a feature).

While we have a whole month to beta backport this change before the regression hits stable, I'd still prefer not to go through an FCP on this PR (which fixes a regression), if T-lang wants an FCP, I can can open an issue about the change itself.

[rustbot]

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[oli-obk]

cc @fee1-dead

[fee1-dead]

cc @lcnr you assigned yourself on my PR

[lcnr]

r? @lcnr

[lcnr]

this breaks

struct NeedsDrop;

impl Drop for NeedsDrop {
    fn drop(&mut self) {
        println!("dropping");
    }
}

struct Foo<const N: usize>([NeedsDrop; N]);

struct Bar(Foo<0>);

const fn should_not_drop(x: Bar) {}

fn main() {
    should_not_drop(Bar(Foo([])));
}

[lcnr]

something someething missing negation 😁

Suggested change

	/// HACK: in order to mistakenly assume that `[PhantomData<T>; N]` requires drop glue
	/// HACK: in order to not mistakenly assume that `[PhantomData<T>; N]` requires drop glue

[lcnr]

r=me after adding the test for the above regression

[oli-obk]

huh I had that test locally... I fail at git

[oli-obk]

@bors r=lcnr

[bors]

📌 Commit 320bb81 has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit 320bb81 with merge 1e746d7...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 1e746d7 to master...

[rust-timer]

Finished benchmarking commit (1e746d7): 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)

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

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)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 628.819s -> 627.948s (-0.14%)
Artifact size: 318.09 MiB -> 318.11 MiB (0.00%)

[Mark-Simulacrum]

note: this was also picked up in crater (https://crater-reports.s3.amazonaws.com/beta-1.73-1.2/beta-2023-09-10/reg/construe-0.0.3/log.txt), looks like it's already beta-nominated though.

[apiraino]

Beta backport accepted as per compiler team on Zulip

@rustbot label +beta-accepted