Rollup merge of #139774 - compiler-errors:supertrait-alias, r=lcnr

Fix replacing supertrait aliases in `ReplaceProjectionWith`

The new solver has a procedure called `predicates_for_object_candidate`, which elaborates the super-bounds and item-bounds that are required to hold for a dyn trait to implement something via a built-in object impl.

In that procedure, there is a folder called `ReplaceProjectionWith` which is responsible for replacing projections that reference `Self`, so that we don't encounter cycles when we then go on to normalize those projections in the process of proving these super-bounds.

That folder had a few problems: Firstly, it wasn't actually checking that this was a super bound originating from `Self`. Secondly, it only accounted for a *single* projection type def id, but trait objects can have multiple (i.e. `trait Foo<A, B>: Bar<A, Assoc = A> + Bar<B, Assoc = B>`).

To fix the first, it's simple enough to just add an equality check for the self ty. To fix the second, I implemented a matching step that's very similar to the `projection_may_match` check we have for upcasting, since on top of having multiple choices, we need to deal with both non-structural matches and ambiguity.

This probably lacks a bit of documentation, but I think it works pretty well.

Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/171

r? lcnr

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -292,7 +292,7 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
                 inspect::ProbeStep::NestedProbe(ref probe) => {
                     match probe.kind {
                         // These never assemble candidates for the goal we're trying to solve.
-                        inspect::ProbeKind::UpcastProjectionCompatibility
+                        inspect::ProbeKind::ProjectionCompatibility
                         | inspect::ProbeKind::ShadowedEnvProbing => continue,
 
                         inspect::ProbeKind::NormalizedSelfTyAssembly
@@ -314,8 +314,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         }
 
         match probe.kind {
-            inspect::ProbeKind::UpcastProjectionCompatibility
-            | inspect::ProbeKind::ShadowedEnvProbing => bug!(),
+            inspect::ProbeKind::ProjectionCompatibility
+            | inspect::ProbeKind::ShadowedEnvProbing => {
+                bug!()
+            }
 
             inspect::ProbeKind::NormalizedSelfTyAssembly | inspect::ProbeKind::UnsizeAssembly => {}
 

compiler/rustc_trait_selection/src/solve/select.rs

@@ -177,7 +177,7 @@ fn to_selection<'tcx>(
         },
         ProbeKind::NormalizedSelfTyAssembly
         | ProbeKind::UnsizeAssembly
-        | ProbeKind::UpcastProjectionCompatibility
+        | ProbeKind::ProjectionCompatibility
         | ProbeKind::OpaqueTypeStorageLookup { result: _ }
         | ProbeKind::Root { result: _ }
         | ProbeKind::ShadowedEnvProbing