Compute transmutability from rustc_target::abi::Layout

In its first step of computing transmutability, `rustc_transmutability`
constructs a byte-level representation of type layout (`Tree`). Previously, this
representation was computed for ADTs by inspecting the ADT definition and
performing our own layout computations. This process was error-prone, verbose,
and limited our ability to analyze many types (particularly default-repr types).

In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This
helps ensure that layout optimizations are reflected our analyses, and increases
the kinds of types we can now analyze, including:
- default repr ADTs
- transparent unions
- `UnsafeCell`-containing types

Overall, this PR expands the expressvity of `rustc_transmutability` to be much
closer to the transmutability analysis performed by miri. Future PRs will work
to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`,
coroutines, etc.).

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -40,7 +40,7 @@ pub struct ImplCandidate<'tcx> {
 
 enum GetSafeTransmuteErrorAndReason {
     Silent,
-    Error { err_msg: String, safe_transmute_explanation: String },
+    Error { err_msg: String, safe_transmute_explanation: Option<String> },
 }
 
 struct UnsatisfiedConst(pub bool);

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -557,7 +557,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                                 GetSafeTransmuteErrorAndReason::Error {
                                     err_msg,
                                     safe_transmute_explanation,
-                                } => (err_msg, Some(safe_transmute_explanation)),
+                                } => (err_msg, safe_transmute_explanation),
                             }
                         } else {
                             (err_msg, None)
@@ -3061,15 +3061,16 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             return GetSafeTransmuteErrorAndReason::Silent;
         };
 
+        let dst = trait_ref.args.type_at(0);
+        let src = trait_ref.args.type_at(1);
+        let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
+
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             obligation.cause,
             src_and_dst,
             assume,
         ) {
             Answer::No(reason) => {
-                let dst = trait_ref.args.type_at(0);
-                let src = trait_ref.args.type_at(1);
-                let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
                 let safe_transmute_explanation = match reason {
                     rustc_transmute::Reason::SrcIsNotYetSupported => {
                         format!("analyzing the transmutability of `{src}` is not yet supported.")
@@ -3083,6 +3084,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         format!("at least one value of `{src}` isn't a bit-valid value of `{dst}`")
                     }
 
+                    rustc_transmute::Reason::DstUninhabited => {
+                        format!("`{dst}` is uninhabited")
+                    }
+
                     rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
                         format!("`{dst}` may carry safety invariants")
                     }
@@ -3128,14 +3133,23 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         format!("`{dst}` has an unknown layout")
                     }
                 };
-                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation }
+                GetSafeTransmuteErrorAndReason::Error {
+                    err_msg,
+                    safe_transmute_explanation: Some(safe_transmute_explanation),
+                }
             }
             // Should never get a Yes at this point! We already ran it before, and did not get a Yes.
             Answer::Yes => span_bug!(
                 span,
                 "Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
             ),
-            other => span_bug!(span, "Unsupported rustc_transmute::Answer variant: `{other:?}`"),
+            // Reached when a different obligation (namely `Freeze`) causes the
+            // transmutability analysis to fail. In this case, silence the
+            // transmutability error message in favor of that more specific
+            // error.
+            Answer::If(_) => {
+                GetSafeTransmuteErrorAndReason::Error { err_msg, safe_transmute_explanation: None }
+            }
         }
     }
 

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -314,12 +314,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     .flat_map(|cond| flatten_answer_tree(tcx, obligation, predicate, cond))
                     .collect(),
                 Condition::IfTransmutable { src, dst } => {
-                    let trait_def_id = obligation.predicate.def_id();
+                    let transmute_trait = obligation.predicate.def_id();
                     let assume_const = predicate.trait_ref.args.const_at(2);
-                    let make_obl = |from_ty, to_ty| {
-                        let trait_ref1 = ty::TraitRef::new(
+                    let make_transmute_obl = |from_ty, to_ty| {
+                        let trait_ref = ty::TraitRef::new(
                             tcx,
-                            trait_def_id,
+                            transmute_trait,
                             [
                                 ty::GenericArg::from(to_ty),
                                 ty::GenericArg::from(from_ty),
@@ -331,17 +331,45 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             obligation.cause.clone(),
                             obligation.recursion_depth + 1,
                             obligation.param_env,
-                            trait_ref1,
+                            trait_ref,
                         )
                     };
 
+                    let make_freeze_obl = |ty| {
+                        let trait_ref = ty::TraitRef::new(
+                            tcx,
+                            tcx.lang_items().freeze_trait().unwrap(),
+                            [ty::GenericArg::from(ty)],
+                        );
+                        Obligation::with_depth(
+                            tcx,
+                            obligation.cause.clone(),
+                            obligation.recursion_depth + 1,
+                            obligation.param_env,
+                            trait_ref,
+                        )
+                    };
+
+                    let mut obls = vec![];
+
+                    // If the source is a shared reference, it must be `Freeze`;
+                    // otherwise, transmuting could lead to data races.
+                    if src.mutability == Mutability::Not {
+                        obls.extend([make_freeze_obl(src.ty), make_freeze_obl(dst.ty)])
+                    }
+
                     // If Dst is mutable, check bidirectionally.
                     // For example, transmuting bool -> u8 is OK as long as you can't update that u8
                     // to be > 1, because you could later transmute the u8 back to a bool and get UB.
                     match dst.mutability {
-                        Mutability::Not => vec![make_obl(src.ty, dst.ty)],
-                        Mutability::Mut => vec![make_obl(src.ty, dst.ty), make_obl(dst.ty, src.ty)],
+                        Mutability::Not => obls.push(make_transmute_obl(src.ty, dst.ty)),
+                        Mutability::Mut => obls.extend([
+                            make_transmute_obl(src.ty, dst.ty),
+                            make_transmute_obl(dst.ty, src.ty),
+                        ]),
                     }
+
+                    obls
                 }
             }
         }

compiler/rustc_transmute/src/layout/dfa.rs

@@ -35,6 +35,7 @@ impl<R> Transitions<R>
 where
     R: Ref,
 {
+    #[allow(dead_code)]
     fn insert(&mut self, transition: Transition<R>, state: State) {
         match transition {
             Transition::Byte(b) => {
@@ -82,6 +83,7 @@ impl<R> Dfa<R>
 where
     R: Ref,
 {
+    #[allow(dead_code)]
     pub(crate) fn unit() -> Self {
         let transitions: Map<State, Transitions<R>> = Map::default();
         let start = State::new();

compiler/rustc_transmute/src/layout/nfa.rs

@@ -160,6 +160,7 @@ where
         Self { transitions, start, accepting }
     }
 
+    #[allow(dead_code)]
     pub(crate) fn edges_from(&self, start: State) -> Option<&Map<Transition<R>, Set<State>>> {
         self.transitions.get(&start)
     }