Reject #[repr(packed)] on #[pin_v2] types

[Dnreikronos]

Fixes #157011

#[repr(packed)] can store an over-aligned field below its alignment, and the drop glue for a packed type moves that field to a properly aligned spot before dropping it. When the field is structurally pinned, that move pulls it out from under a Pin<&mut _> we already handed out, which breaks the pin invariant. The repro in the issue makes it pretty clear: it prints one address during pinned access and a different one on drop.

So the fix just rejects the combo at the type definition. If a type is both #[pin_v2] and #[repr(packed)], it no longer compiles. The check sits in check_packed in rustc_hir_analysis, so it catches the concrete case and the generic one too (like One<T>, where we don't know T's alignment yet), with no layout or monomorphization needed.

One thing I want to be upfront about: this is necessary but don't solve the problem entirely. Sure, it stops the spelling the issue used, but you can still trigger the same move-on-drop through an explicit &pin mut / ref pin mut projection with no #[pin_v2] anywhere. Leaving that broader case open is intentional, imo the narrow ban is the right call for now, and the leftover stays tracked on the pin ergonomics tracking issue #130494.

This is the direction we landed on with @workingjubilee over on Zulip, lgtm from their side: https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.60pin_v2.60.20is.20unsound.20with.20.60packed.60/with/600522893

Tests live in tests/ui/pin-ergonomics/pin_v2-packed.rs: the three rejected shapes (packed struct, generic packed(4) struct, packed union), plus two controls that still compile fine, #[pin_v2] without packed and packed without #[pin_v2].

Disclosure: AI tooling was used on the code changes, and everything was strictly validated by me before sending to remote.

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[mejrs]

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[mejrs]

This error should be emitted whether or not the attribute can be found. Your best option is to use Option<Span> instead. Take for example

rust/compiler/rustc_hir_analysis/src/check/always_applicable.rs

Lines 408 to 415 in 61d7280

	if tcx.adt_def(adt_def_id).is_pin_project() {
	let pin_v2_span = rustc_hir::find_attr!(tcx, adt_def_id, PinV2(attr) => *attr);
	let adt_name = tcx.item_name(adt_def_id);
	return Err(tcx.dcx().emit_err(crate::errors::PinV2WithoutPinDrop {
	span,
	pin_v2_span,
	adt_name,
	}));

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