Rework upcasting

compiler/rustc_middle/src/traits/solve/inspect.rs

@@ -73,12 +73,15 @@ pub struct GoalCandidate<'tcx> {
 pub enum CandidateKind<'tcx> {
     /// Probe entered when normalizing the self ty during candidate assembly
     NormalizedSelfTyAssembly,
-    DynUpcastingAssembly,
     /// A normal candidate for proving a goal
-    Candidate {
-        name: String,
-        result: QueryResult<'tcx>,
-    },
+    Candidate { name: String, result: QueryResult<'tcx> },
+    /// Used in the probe that wraps normalizing the non-self type for the unsize
+    /// trait, which is also structurally matched on.
+    UnsizeAssembly,
+    /// During upcasting from some source object to target object type, used to
+    /// do a probe to find out what projection type(s) may be used to prove that
+    /// the source type upholds all of the target type's object bounds.
+    UpcastProbe,
 }
 impl Debug for GoalCandidate<'_> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -100,8 +100,11 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             CandidateKind::NormalizedSelfTyAssembly => {
                 writeln!(self.f, "NORMALIZING SELF TY FOR ASSEMBLY:")
             }
-            CandidateKind::DynUpcastingAssembly => {
-                writeln!(self.f, "ASSEMBLING CANDIDATES FOR DYN UPCASTING:")
+            CandidateKind::UnsizeAssembly => {
+                writeln!(self.f, "ASSEMBLING CANDIDATES FOR UNSIZING:")
+            }
+            CandidateKind::UpcastProbe => {
+                writeln!(self.f, "PROBING FOR PROJECTION COMPATIBILITY FOR UPCASTING:")
             }
             CandidateKind::Candidate { name, result } => {
                 writeln!(self.f, "CANDIDATE {name}: {result:?}")

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2734,8 +2734,9 @@ forward_display_to_print! {
     // HACK(eddyb) these are exhaustive instead of generic,
     // because `for<'tcx>` isn't possible yet.
     ty::PolyExistentialPredicate<'tcx>,
+    ty::PolyExistentialProjection<'tcx>,
+    ty::PolyExistentialTraitRef<'tcx>,
     ty::Binder<'tcx, ty::TraitRef<'tcx>>,
-    ty::Binder<'tcx, ty::ExistentialTraitRef<'tcx>>,
     ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>>,
     ty::Binder<'tcx, TraitRefPrintOnlyTraitName<'tcx>>,
     ty::Binder<'tcx, ty::FnSig<'tcx>>,