Make ZeroVec miri-clean with stacked borrows

[Manishearth]

#6723

WIP demo showing miri-cleanliness of zerovec, using pointer metadata APIs.

Currently it is not possible to fix VarZeroCow

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[sffc]

/gemini review

[Copilot]

Pull Request Overview

This PR updates the ZeroVec library to be Miri-clean with stacked borrows by adopting the stable pointer metadata APIs instead of unsafe transmute operations. The changes eliminate undefined behavior when converting between different pointer types by properly preserving pointer metadata.

• Replaces unsafe transmute operations with stable ptr::metadata and ptr::from_raw_parts APIs
• Removes ManuallyDrop wrappers in favor of direct Box::into_raw operations
• Enables the ptr_metadata feature flag across affected components

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
utils/zerovec/src/ule/mod.rs	Updates to_boxed_var_ule to use pointer metadata APIs
utils/zerovec/src/ule/encode.rs	Updates encode_varule_to_box to use pointer metadata APIs
utils/zerovec/src/lib.rs	Enables ptr_metadata feature flag
utils/zerovec/src/cow.rs	Updates pointer conversion logic in new_owned
utils/zerovec/derive/src/varule.rs	Replaces transmute with pointer metadata APIs
components/*/src/lib.rs	Enables ptr_metadata feature flag in various components

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[Copilot]

This should be #![feature(ptr_metadata)] to enable the feature, not #![cfg(ptr_metadata)] which is a conditional compilation attribute.

Suggested change

	#![cfg(ptr_metadata)]
	#![feature(ptr_metadata)]

[Copilot]

The commented-out line and the current cast through *mut () suggest uncertainty about the correct approach. This should be clarified with a proper explanation or the commented code should be removed.

Suggested change

	// disallowed?
	// let raw_u8: *mut u8 = raw_v as *mut [u8] as *mut u8;
	let raw_u8: *mut u8 = raw_v as *mut () as *mut u8;
	// SAFETY: `V: VarULE` guarantees that the memory layout of `V` is a valid byte slice,
	// and `Box<V>` is heap-allocated, so it is safe to treat its pointer as a pointer to bytes.
	let raw_u8: *mut u8 = raw_v as *mut u8;

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[sffc]

/gemini review

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