Rollup merge of #121681 - jswrenn:nix-visibility-analysis, r=compiler-errors

Safe Transmute: Revise safety analysis

This PR migrates `BikeshedIntrinsicFrom` to a simplified safety analysis (described [here](https://github.com/rust-lang/project-safe-transmute/issues/15)) that does not rely on analyzing the visibility of types and fields.

The revised analysis treats primitive types as safe, and user-defined types as potentially carrying safety invariants. If Rust gains explicit (un)safe fields, this PR is structured so that it will be fairly easy to thread support for those annotations into the analysis.

Notably, this PR removes the `Context` type parameter from `BikeshedIntrinsicFrom`. Most of the files changed by this PR are just UI tests tweaked to accommodate the removed parameter.

r? `@compiler-errors`

4 files changed, 8 insertions, 18 deletions

diff --git a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs b/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
index ed428bb8e66..df4dcaff1e7 100644
--- a/compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs
+++ b/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),
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
index eba6ba3f7b0..80198ba39f9 100644
--- a/compiler/rustc_trait_selection/src/solve/trait_goals.rs
+++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs
@@ -549,14 +549,13 @@ 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)
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index d9e49f1eb0a..3275a4f3527 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -2960,11 +2960,10 @@ 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,
@@ -2976,15 +2975,12 @@ 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")
@@ -2998,9 +2994,9 @@ 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}`")
                     }
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index b8733bab27b..70f6b240ab7 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -310,8 +310,7 @@ 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,
@@ -319,7 +318,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             [
                                 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,
         );