safe transmute: revise safety analysis

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

Closes https://github.com/rust-lang/project-safe-transmute/issues/15

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

@@ -874,7 +874,6 @@ 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;
@@ -882,7 +881,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         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)