document & impl 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

Author: jswrenn

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

@@ -103,6 +103,7 @@
 #![feature(strict_provenance_atomic_ptr)]
 #![feature(test)]
 #![feature(trait_upcasting)]
+#![feature(transmutability)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_blocks)]

library/core/tests/lib.rs

@@ -790,3 +790,19 @@ fn const_maybe_uninit_zeroed() {
 
     assert_eq!(unsafe { (*UNINIT.0.cast::<[[u8; SIZE]; 1]>())[0] }, [0u8; SIZE]);
 }
+
+#[cfg(not(bootstrap))]
+#[test]
+fn transmute() {
+    use core::mem::{Assume, BikeshedIntrinsicFrom};
+
+    let src = 42u8; // size = 1
+
+    #[repr(C, align(2))]
+    struct Dst(u8); // size = 2
+
+    let Dst(dst) =
+        unsafe { <Dst as BikeshedIntrinsicFrom<u8, { Assume::SAFETY }>>::transmute(src) };
+
+    assert_eq!(src, dst);
+}

library/core/tests/mem.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;
       }

tests/mir-opt/lower_intrinsics.transmute_to_mut_uninhabited.LowerIntrinsics.panic-abort.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;
       }

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

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

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

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

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

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<(), Never>(move _2) -> unwind unreachable;
+-         _0 = std::intrinsics::transmute::<(), Never>(move _2) -> unwind unreachable;
 +         _0 = move _2 as Never (Transmute);
 +         unreachable;
       }

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

@@ -9,7 +9,7 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1;
--         _0 = transmute::<(), Never>(move _2) -> unwind unreachable;
+-         _0 = std::intrinsics::transmute::<(), Never>(move _2) -> unwind unreachable;
 +         _0 = move _2 as Never (Transmute);
 +         unreachable;
       }

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

@@ -1,12 +1,12 @@
-error[E0277]: expected a `FnOnce(&str)` closure, found `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+error[E0277]: expected a `FnOnce(&str)` closure, found `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
   --> $DIR/coerce-unsafe-to-closure.rs:2:44
    |
 LL |     let x: Option<&[u8]> = Some("foo").map(std::mem::transmute);
    |                                        --- ^^^^^^^^^^^^^^^^^^^ call the function in a closure: `|| unsafe { /* code */ }`
    |                                        |
    |                                        required by a bound introduced by this call
    |
-   = help: the trait `FnOnce(&str)` is not implemented for fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+   = help: the trait `FnOnce(&str)` is not implemented for fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
    = note: unsafe function cannot be called generically without an unsafe block
 note: required by a bound in `Option::<T>::map`
   --> $SRC_DIR/core/src/option.rs:LL:COL

tests/ui/closures/coerce-unsafe-to-closure.stderr

@@ -7,9 +7,9 @@ LL |     let _: unsafe extern "rust-intrinsic" fn(isize) -> usize = std::mem::tr
    |            expected due to this
    |
    = note: expected fn pointer `unsafe extern "rust-intrinsic" fn(isize) -> usize`
-                 found fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}`
+                 found fn item `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
 
-error[E0606]: casting `unsafe extern "rust-intrinsic" fn(_) -> _ {transmute::<_, _>}` as `unsafe extern "rust-intrinsic" fn(isize) -> usize` is invalid
+error[E0606]: casting `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}` as `unsafe extern "rust-intrinsic" fn(isize) -> usize` is invalid
   --> $DIR/reify-intrinsic.rs:11:13
    |
 LL |     let _ = std::mem::transmute as unsafe extern "rust-intrinsic" fn(isize) -> usize;