Auto merge of #122385 - lcnr:analyze-obligations-for-infer, r=compiler-errors

`obligations_for_self_ty`: use `ProofTreeVisitor` for nested goals

As always, dealing with proof trees continues to be a hacked together mess. After this PR and #124380 the only remaining blocker for core is https://github.com/rust-lang/trait-system-refactor-initiative/issues/90. There is also a `ProofTreeVisitor` issue causing an ICE when compiling `alloc` which I will handle in a separate PR. This issue likely affects coherence diagnostics more generally.

The core idea is to extend the proof tree visitor to support visiting nested candidates without using a `probe`. We then simply recurse into nested candidates if they are the only potentially applicable candidate for a given goal and check whether the self type matches the expected one.

For that to work, we need to improve `CanonicalState` to also handle unconstrained inference variables created inside of the trait solver. This is done by extending the `var_values` of `CanoncalState` with each fresh inference variables. Furthermore, we also store the state of all inference variables at the end of each probe. When recursing into `InspectCandidates` we then unify the values of all these states.

r? `@compiler-errors`

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -30,7 +30,7 @@ use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
 use rustc_span::symbol::sym;
-use rustc_span::DUMMY_SP;
+use rustc_span::{Span, DUMMY_SP};
 use std::fmt::Debug;
 use std::ops::ControlFlow;
 
@@ -1022,10 +1022,14 @@ struct AmbiguityCausesVisitor<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
+    fn span(&self) -> Span {
+        DUMMY_SP
+    }
+
     fn visit_goal(&mut self, goal: &InspectGoal<'_, 'tcx>) {
         let infcx = goal.infcx();
         for cand in goal.candidates() {
-            cand.visit_nested(self);
+            cand.visit_nested_in_probe(self);
         }
         // When searching for intercrate ambiguity causes, we only need to look
         // at ambiguous goals, as for others the coherence unknowable candidate
@@ -1157,5 +1161,5 @@ fn search_ambiguity_causes<'tcx>(
     goal: Goal<'tcx, ty::Predicate<'tcx>>,
     causes: &mut FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
 ) {
-    infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor { causes });
+    infcx.probe(|_| infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor { causes }));
 }