Rollup merge of #129032 - jswrenn:transmute-method, r=compiler-errors

Document & implement the transmutation modeled by `BikeshedIntrinsicFrom`

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

Tracking Issue: https://github.com/rust-lang/rust/issues/99571

r? `@compiler-errors`

cc `@scottmcm,` `@Lokathor`

1 files changed, 5 insertions, 0 deletions

diff --git a/compiler/rustc_ty_utils/src/instance.rs b/compiler/rustc_ty_utils/src/instance.rs
index c7ed6e6110f..d77c3a277bf 100644
--- a/compiler/rustc_ty_utils/src/instance.rs
+++ b/compiler/rustc_ty_utils/src/instance.rs
@@ -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)
             }