safe transmute: revise safety analysis

Migrate to a simplified safety analysis that does not use visibility.

Closes rust-lang/project-safe-transmute#15

compiler/rustc_hir/src/lang_items.rs

@@ -167,7 +167,7 @@ language_item_table! {
 
     // language items relating to transmutability
     TransmuteOpts,           sym::transmute_opts,      transmute_opts,             Target::Struct,         GenericRequirement::Exact(0);
-    TransmuteTrait,          sym::transmute_trait,     transmute_trait,            Target::Trait,          GenericRequirement::Exact(3);
+    TransmuteTrait,          sym::transmute_trait,     transmute_trait,            Target::Trait,          GenericRequirement::Exact(2);
 
     Add,                     sym::add,                 add_trait,                  Target::Trait,          GenericRequirement::Exact(1);
     Sub,                     sym::sub,                 sub_trait,                  Target::Trait,          GenericRequirement::Exact(1);

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -874,15 +874,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn is_transmutable(
         &self,
         src_and_dst: rustc_transmute::Types<'tcx>,
-        scope: Ty<'tcx>,
         assume: rustc_transmute::Assume,
     ) -> Result<Certainty, NoSolution> {
         use rustc_transmute::Answer;
         // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             ObligationCause::dummy(),
             src_and_dst,
-            scope,
             assume,
         ) {
             Answer::Yes => Ok(Certainty::Yes),

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -543,14 +543,13 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         let args = ecx.tcx().erase_regions(goal.predicate.trait_ref.args);
 
         let Some(assume) =
-            rustc_transmute::Assume::from_const(ecx.tcx(), goal.param_env, args.const_at(3))
+            rustc_transmute::Assume::from_const(ecx.tcx(), goal.param_env, args.const_at(2))
         else {
             return Err(NoSolution);
         };
 
         let certainty = ecx.is_transmutable(
             rustc_transmute::Types { dst: args.type_at(0), src: args.type_at(1) },
-            args.type_at(2),
             assume,
         )?;
         ecx.evaluate_added_goals_and_make_canonical_response(certainty)

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

@@ -2970,11 +2970,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             dst: trait_ref.args.type_at(0),
             src: trait_ref.args.type_at(1),
         };
-        let scope = trait_ref.args.type_at(2);
         let Some(assume) = rustc_transmute::Assume::from_const(
             self.infcx.tcx,
             obligation.param_env,
-            trait_ref.args.const_at(3),
+            trait_ref.args.const_at(2),
         ) else {
             self.dcx().span_delayed_bug(
                 span,
@@ -2986,15 +2985,12 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
             obligation.cause,
             src_and_dst,
-            scope,
             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}` in the defining scope of `{scope}`"
-                );
+                let err_msg = format!("`{src}` cannot be safely transmuted into `{dst}`");
                 let safe_transmute_explanation = match reason {
                     rustc_transmute::Reason::SrcIsUnspecified => {
                         format!("`{src}` does not have a well-specified layout")
@@ -3008,9 +3004,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         format!("At least one value of `{src}` isn't a bit-valid value of `{dst}`")
                     }
 
-                    rustc_transmute::Reason::DstIsPrivate => format!(
-                        "`{dst}` is or contains a type or field that is not visible in that scope"
-                    ),
+                    rustc_transmute::Reason::DstMayHaveSafetyInvariants => {
+                        format!("`{dst}` may carry safety invariants")
+                    }
                     rustc_transmute::Reason::DstIsTooBig => {
                         format!("The size of `{src}` is smaller than the size of `{dst}`")
                     }

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

@@ -310,16 +310,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     .collect(),
                 Condition::IfTransmutable { src, dst } => {
                     let trait_def_id = obligation.predicate.def_id();
-                    let scope = predicate.trait_ref.args.type_at(2);
-                    let assume_const = predicate.trait_ref.args.const_at(3);
+                    let assume_const = predicate.trait_ref.args.const_at(2);
                     let make_obl = |from_ty, to_ty| {
                         let trait_ref1 = ty::TraitRef::new(
                             tcx,
                             trait_def_id,
                             [
                                 ty::GenericArg::from(to_ty),
                                 ty::GenericArg::from(from_ty),
-                                ty::GenericArg::from(scope),
                                 ty::GenericArg::from(assume_const),
                             ],
                         );
@@ -355,7 +353,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let Some(assume) = rustc_transmute::Assume::from_const(
             self.infcx.tcx,
             obligation.param_env,
-            predicate.trait_ref.args.const_at(3),
+            predicate.trait_ref.args.const_at(2),
         ) else {
             return Err(Unimplemented);
         };
@@ -367,7 +365,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let maybe_transmutable = transmute_env.is_transmutable(
             obligation.cause.clone(),
             rustc_transmute::Types { dst, src },
-            predicate.trait_ref.args.type_at(2),
             assume,
         );
 

compiler/rustc_transmute/src/layout/mod.rs

@@ -29,14 +29,21 @@ impl fmt::Debug for Byte {
     }
 }
 
-pub(crate) trait Def: Debug + Hash + Eq + PartialEq + Copy + Clone {}
+pub(crate) trait Def: Debug + Hash + Eq + PartialEq + Copy + Clone {
+    fn has_safety_invariants(&self) -> bool;
+}
 pub trait Ref: Debug + Hash + Eq + PartialEq + Copy + Clone {
     fn min_align(&self) -> usize;
 
     fn is_mutable(&self) -> bool;
 }
 
-impl Def for ! {}
+impl Def for ! {
+    fn has_safety_invariants(&self) -> bool {
+        unreachable!()
+    }
+}
+
 impl Ref for ! {
     fn min_align(&self) -> usize {
         unreachable!()
@@ -83,5 +90,12 @@ pub mod rustc {
         Primitive,
     }
 
-    impl<'tcx> super::Def for Def<'tcx> {}
+    impl<'tcx> super::Def for Def<'tcx> {
+        fn has_safety_invariants(&self) -> bool {
+            // Rust presently has no notion of 'unsafe fields', so for now we
+            // make the conservative assumption that everything besides
+            // primitive types carry safety invariants.
+            self != &Self::Primitive
+        }
+    }
 }

compiler/rustc_transmute/src/layout/tree.rs

@@ -81,7 +81,8 @@ where
         Self::Seq(vec![Self::uninit(); width_in_bytes])
     }
 
-    /// Remove all `Def` nodes, and all branches of the layout for which `f` produces false.
+    /// Remove all `Def` nodes, and all branches of the layout for which `f`
+    /// produces `true`.
     pub(crate) fn prune<F>(self, f: &F) -> Tree<!, R>
     where
         F: Fn(D) -> bool,
@@ -106,7 +107,7 @@ where
             Self::Byte(b) => Tree::Byte(b),
             Self::Ref(r) => Tree::Ref(r),
             Self::Def(d) => {
-                if !f(d) {
+                if f(d) {
                     Tree::uninhabited()
                 } else {
                     Tree::unit()

compiler/rustc_transmute/src/layout/tree/tests.rs

@@ -2,11 +2,15 @@ use super::Tree;
 
 #[derive(Debug, Hash, Eq, PartialEq, Clone, Copy)]
 pub enum Def {
-    Visible,
-    Invisible,
+    NoSafetyInvariants,
+    HasSafetyInvariants,
 }
 
-impl super::Def for Def {}
+impl super::Def for Def {
+    fn has_safety_invariants(&self) -> bool {
+        self == &Self::HasSafetyInvariants
+    }
+}
 
 mod prune {
     use super::*;
@@ -16,17 +20,22 @@ mod prune {
 
         #[test]
         fn seq_1() {
-            let layout: Tree<Def, !> = Tree::def(Def::Visible).then(Tree::from_bits(0x00));
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::from_bits(0x00));
+            let layout: Tree<Def, !> =
+                Tree::def(Def::NoSafetyInvariants).then(Tree::from_bits(0x00));
+            assert_eq!(
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
+                Tree::from_bits(0x00)
+            );
         }
 
         #[test]
         fn seq_2() {
-            let layout: Tree<Def, !> =
-                Tree::from_bits(0x00).then(Tree::def(Def::Visible)).then(Tree::from_bits(0x01));
+            let layout: Tree<Def, !> = Tree::from_bits(0x00)
+                .then(Tree::def(Def::NoSafetyInvariants))
+                .then(Tree::from_bits(0x01));
 
             assert_eq!(
-                layout.prune(&|d| matches!(d, Def::Visible)),
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
                 Tree::from_bits(0x00).then(Tree::from_bits(0x01))
             );
         }
@@ -37,21 +46,32 @@ mod prune {
 
         #[test]
         fn invisible_def() {
-            let layout: Tree<Def, !> = Tree::def(Def::Invisible);
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::uninhabited());
+            let layout: Tree<Def, !> = Tree::def(Def::HasSafetyInvariants);
+            assert_eq!(
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
+                Tree::uninhabited()
+            );
         }
 
         #[test]
         fn invisible_def_in_seq_len_2() {
-            let layout: Tree<Def, !> = Tree::def(Def::Visible).then(Tree::def(Def::Invisible));
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::uninhabited());
+            let layout: Tree<Def, !> =
+                Tree::def(Def::NoSafetyInvariants).then(Tree::def(Def::HasSafetyInvariants));
+            assert_eq!(
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
+                Tree::uninhabited()
+            );
         }
 
         #[test]
         fn invisible_def_in_seq_len_3() {
-            let layout: Tree<Def, !> =
-                Tree::def(Def::Visible).then(Tree::from_bits(0x00)).then(Tree::def(Def::Invisible));
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::uninhabited());
+            let layout: Tree<Def, !> = Tree::def(Def::NoSafetyInvariants)
+                .then(Tree::from_bits(0x00))
+                .then(Tree::def(Def::HasSafetyInvariants));
+            assert_eq!(
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
+                Tree::uninhabited()
+            );
         }
     }
 
@@ -60,21 +80,26 @@ mod prune {
 
         #[test]
         fn visible_def() {
-            let layout: Tree<Def, !> = Tree::def(Def::Visible);
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::unit());
+            let layout: Tree<Def, !> = Tree::def(Def::NoSafetyInvariants);
+            assert_eq!(layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)), Tree::unit());
         }
 
         #[test]
         fn visible_def_in_seq_len_2() {
-            let layout: Tree<Def, !> = Tree::def(Def::Visible).then(Tree::def(Def::Visible));
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::unit());
+            let layout: Tree<Def, !> =
+                Tree::def(Def::NoSafetyInvariants).then(Tree::def(Def::NoSafetyInvariants));
+            assert_eq!(layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)), Tree::unit());
         }
 
         #[test]
         fn visible_def_in_seq_len_3() {
-            let layout: Tree<Def, !> =
-                Tree::def(Def::Visible).then(Tree::from_bits(0x00)).then(Tree::def(Def::Visible));
-            assert_eq!(layout.prune(&|d| matches!(d, Def::Visible)), Tree::from_bits(0x00));
+            let layout: Tree<Def, !> = Tree::def(Def::NoSafetyInvariants)
+                .then(Tree::from_bits(0x00))
+                .then(Tree::def(Def::NoSafetyInvariants));
+            assert_eq!(
+                layout.prune(&|d| matches!(d, Def::HasSafetyInvariants)),
+                Tree::from_bits(0x00)
+            );
         }
     }
 }

compiler/rustc_transmute/src/lib.rs

@@ -49,8 +49,8 @@ pub enum Reason {
     DstIsUnspecified,
     /// The layout of the destination type is bit-incompatible with the source type.
     DstIsBitIncompatible,
-    /// There aren't any public constructors for `Dst`.
-    DstIsPrivate,
+    /// The destination type may carry safety invariants.
+    DstMayHaveSafetyInvariants,
     /// `Dst` is larger than `Src`, and the excess bytes were not exclusively uninitialized.
     DstIsTooBig,
     /// Src should have a stricter alignment than Dst, but it does not.
@@ -106,13 +106,11 @@ mod rustc {
             &mut self,
             cause: ObligationCause<'tcx>,
             types: Types<'tcx>,
-            scope: Ty<'tcx>,
             assume: crate::Assume,
         ) -> crate::Answer<crate::layout::rustc::Ref<'tcx>> {
             crate::maybe_transmutable::MaybeTransmutableQuery::new(
                 types.src,
                 types.dst,
-                scope,
                 assume,
                 self.infcx.tcx,
             )