Accessing DST field of packed struct calculates the wrong field address

[mahkoh]

Execute with Miri:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7e7d31feebbea4c3caceaccff1fa6bac

The problem is that

rust/compiler/rustc_codegen_ssa/src/mir/place.rs

Lines 181 to 182 in 6526e5c

	// Bump the unaligned offset up to the appropriate alignment
	let offset = round_up_const_value_to_alignment(bx, unaligned_offset, unsized_align);

and

rust/compiler/rustc_mir/src/interpret/place.rs

Line 413 in 6526e5c

(base.meta, offset.align_to(align))

do not take the reduced field alignment into account and calculate the offset as-if the standard alignment was in place.

[mahkoh]

The labels are incorrect. Neither does this require nightly, nor is it related to const-eval. This is I-unsound on stable.

@rustbot label I-prioritize
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[rustbot]

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[rustbot]

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