Lift T: Sized bounds from some strict_provenance pointer methods

[WaffleLapkin]

This PR removes requirement for T (pointee type) to be Sized to call pointer::{addr, expose_addr, with_addr, map_addr}. These functions don't use T's size, so there is no reason for them to require this. Updated public API:

cc @Gankra, #95228
r? libs-api

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[ghost]

This should probably also change the similar methods on NonNull.

[WaffleLapkin]

@H4x5 good catch! Weirdly enough there is no NonNull::expose_addr, but I've removed the bound from all other methods.

[dtolnay]

Reassigning yaahc's reviews as she has stepped down from the review rotation.

r? rust-lang/libs-api

[Amanieu]

This was previously discussed in the original PR here. Is there any specific use case that allowing this would enable?

Alternatively, @Gankra do you have any objections to removing these bounds since you added them in the first place?

I personally can't see any harm in removing these bounds for addr and expose_addr, but doing so for with_addr and map_addr seem much more questionable since we currently have no existing API for manually modifying the address part of a fat pointer without touching the metadata.

[jedel1043]

@Amanieu The gc crate currently uses kind of a linked list of NonNull<dyn Trace> trait objects to track the managed data. Every pointer to a garbage collected object is tagged with a "root" bit to determine if a pointer is currently on the stack or not. It currently uses as usize, but this PR will enable using addr directly:

https://github.com/Manishearth/rust-gc/blob/a243a107694c636d539856495a2ac0341a616428/gc/src/lib.rs#L119-L121

However, there's no way to easily modify the address of a pointer to a dynamic object, so tagging a NonNull<dyn Trace> pointer as rooted is done using a hack:

https://github.com/Manishearth/rust-gc/blob/a243a107694c636d539856495a2ac0341a616428/gc/src/lib.rs#L1001-L1010

Having map_addr would really help in this case.

[WaffleLapkin]

@Amanieu I can't recall the exact use case, but I wanted some of these methods when working on Arc re-implementation.

but doing so for with_addr and map_addr seem much more questionable since we currently have no existing API for manually modifying the address part of a fat pointer without touching the metadata.

That is actually a shame and exposing at least some option here would be really nice (the perfect solution would be to stabilize byte_offset, but anyway...), for example offsetting a pointer to unsized values is required to fully support Arc-like structures that hold a pointer to (DataStructureMeta, T).

---

In general extending our support of un-Sized values is really valuable IMO since it's currently lacking while it could support a lot of interesting use cases (my opinion, I'm a bit obsessed with DST lol).

[Amanieu]

We already have an (unstable) API for fat pointer manipulation: #81513

I'm tending towards accepting this PR, but I'd like to hear more opinions from the libs-api team on how these APIs interact with each other.

[m-ou-se]

Discussed in the libs-api meeting. No concerns from us.

@bors r+

[bors]

📌 Commit 662f1f2 has been approved by m-ou-se

It is now in the queue for this repository.