Unrolled build for rust-lang#129032

Rollup merge of rust-lang#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: rust-lang#99571

r? `@compiler-errors`

cc `@scottmcm,` `@Lokathor`

Author: rust-timer

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)
             }

library/core/src/mem/transmutability.rs

@@ -19,7 +19,7 @@ fn main() -> () {
         StorageLive(_2);
         StorageLive(_3);
         _3 = ();
-        _2 = transmute::<(), Void>(move _3) -> bb4;
+        _2 = std::intrinsics::transmute::<(), Void>(move _3) -> bb4;
     }
 
     bb1: {

tests/mir-opt/issue_72181_1.main.built.after.mir

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<std::cmp::Ordering, i8>(move _2) -> [return: bb1, unwind unreachable];
+-         _0 = std::intrinsics::transmute::<std::cmp::Ordering, i8>(move _2) -> [return: bb1, unwind unreachable];
 +         _0 = move _2 as i8 (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_inhabited.LowerIntrinsics.panic-abort.diff

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<std::cmp::Ordering, i8>(move _2) -> [return: bb1, unwind unreachable];
+-         _0 = std::intrinsics::transmute::<std::cmp::Ordering, i8>(move _2) -> [return: bb1, unwind unreachable];
 +         _0 = move _2 as i8 (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_inhabited.LowerIntrinsics.panic-unwind.diff

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<&T, *const T>(move _2) -> [return: bb1, unwind unreachable];
+-         _0 = std::intrinsics::transmute::<&T, *const T>(move _2) -> [return: bb1, unwind unreachable];
 +         _0 = move _2 as *const T (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_ref_dst.LowerIntrinsics.panic-abort.diff

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<&T, *const T>(move _2) -> [return: bb1, unwind unreachable];
+-         _0 = std::intrinsics::transmute::<&T, *const T>(move _2) -> [return: bb1, unwind unreachable];
 +         _0 = move _2 as *const T (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_ref_dst.LowerIntrinsics.panic-unwind.diff

@@ -11,7 +11,7 @@
 
       bb0: {
           StorageLive(_1);
--         _1 = transmute::<usize, Box<Never>>(const 1_usize) -> [return: bb1, unwind unreachable];
+-         _1 = std::intrinsics::transmute::<usize, Box<Never>>(const 1_usize) -> [return: bb1, unwind unreachable];
 +         _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_to_box_uninhabited.LowerIntrinsics.panic-abort.diff

@@ -11,7 +11,7 @@
 
       bb0: {
           StorageLive(_1);
--         _1 = transmute::<usize, Box<Never>>(const 1_usize) -> [return: bb1, unwind unreachable];
+-         _1 = std::intrinsics::transmute::<usize, Box<Never>>(const 1_usize) -> [return: bb1, unwind unreachable];
 +         _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
 +         goto -> bb1;
       }

tests/mir-opt/lower_intrinsics.transmute_to_box_uninhabited.LowerIntrinsics.panic-unwind.diff

@@ -10,7 +10,7 @@
 
       bb0: {
           StorageLive(_1);
--         _1 = transmute::<usize, &mut Never>(const 1_usize) -> [return: bb1, unwind unreachable];
+-         _1 = std::intrinsics::transmute::<usize, &mut Never>(const 1_usize) -> [return: bb1, unwind unreachable];
 +         _1 = const 1_usize as &mut Never (Transmute);
 +         goto -> bb1;
       }