Auto merge of #114036 - compiler-errors:upcast-to-fewer-assocs, r=lcnr

Rework upcasting confirmation to support upcasting to fewer projections in target bounds

This PR implements a modified trait upcasting algorithm that is resilient to changes in the number of associated types in the bounds of the source and target trait objects.

It does this by equating each bound of the target trait ref individually against the bounds of the source trait ref, rather than doing them all together by constructing a new trait object.

#### The new way we do trait upcasting confirmation

1. Equate the target trait object's principal trait ref with one of the supertraits of the source trait object's principal.
fdcab310b2/compiler/rustc_trait_selection/src/traits/select/mod.rs (L2509-L2525)

2. Make sure that every auto trait in the *target* trait object is present in the source trait ref's bounds.
fdcab310b2/compiler/rustc_trait_selection/src/traits/select/mod.rs (L2559-L2562)

3. For each projection in the *target* trait object, make sure there is exactly one projection that equates with it in the source trait ref's bound. If there is more than one, bail with ambiguity.
fdcab310b2/compiler/rustc_trait_selection/src/traits/select/mod.rs (L2526-L2557)
    * Since there may be more than one that applies, we probe first to check that there is exactly one, then we equate it outside of a probe once we know that it's unique.

4. Make sure the lifetime of the source trait object outlives the lifetime of the target.

<details>
<summary>Meanwhile, this is how we used to do upcasting:</summary>

1. For each supertrait of the source trait object, take that supertrait, append the source object's projection bounds, and the *target* trait object's auto trait bounds, and make this into a new object type:
d12c6e947c/compiler/rustc_trait_selection/src/traits/select/confirmation.rs (L915-L929)

2. Then equate it with the target trait object:
d12c6e947c/compiler/rustc_trait_selection/src/traits/select/confirmation.rs (L936)

This will be a type mismatch if the target trait object has fewer projection bounds, since we compare the bounds structurally in relate:
d12c6e947c/compiler/rustc_middle/src/ty/relate.rs (L696-L698)

</details>

Fixes #114035
Also fixes #114113, because I added a normalize call in the old solver.

r? types

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>>,