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document & impl the transmutation modeled by BikeshedIntrinsicFrom

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Documents that `BikeshedIntrinsicFrom` models transmute-via-union,
which is slightly more expressive than the transmute-via-cast
implemented by `transmute_copy`. Additionally, we provide an
implementation of transmute-via-union as a method on the
`BikeshedIntrinsicFrom` trait with additional documentation on
the boundary between trait invariants and caller obligations.

Whether or not transmute-via-union is the right kind of transmute
to model remains up for discussion [1]. Regardless, it seems wise
to document the present behavior.

[1] https://rust-lang.zulipchat.com/#narrow/stream/216762-project-safe-transmute/topic/What.20'kind'.20of.20transmute.20to.20model.3F/near/426331967
This commit is contained in:
Jack Wrenn 2024-08-12 22:52:59 +00:00
parent 4d5b3b1962
commit 2540070fd4
17 changed files with 372 additions and 38 deletions
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5
compiler/rustc_ty_utils/src/instance.rs
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@ -363,6 +363,11 @@ fn resolve_associated_item<'tcx>(
tcx.item_name(trait_item_id)
),
}
} else if tcx.is_lang_item(trait_ref.def_id, LangItem::TransmuteTrait) {
let name = tcx.item_name(trait_item_id);
assert_eq!(name, sym::transmute);
let args = tcx.erase_regions(rcvr_args);
Some(ty::Instance::new(trait_item_id, args))
} else {
Instance::try_resolve_item_for_coroutine(tcx, trait_item_id, trait_id, rcvr_args)
}