Use probes more aggressively in new solver

compiler/rustc_middle/src/traits/solve.rs

@@ -332,4 +332,9 @@ pub enum CandidateSource {
     /// }
     /// ```
     AliasBound,
+    /// A candidate that is registered only during coherence to represent some
+    /// yet-unknown impl that could be produced downstream without violating orphan
+    /// rules.
+    // FIXME: Merge this with the forced ambiguity candidates, so those don't use `Misc`.
+    CoherenceUnknowable,
 }

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

@@ -141,10 +141,6 @@ pub enum ProbeKind<'tcx> {
     TryNormalizeNonRigid { result: QueryResult<'tcx> },
     /// Probe entered when normalizing the self ty during candidate assembly
     NormalizedSelfTyAssembly,
-    /// Some candidate to prove the current goal.
-    ///
-    /// FIXME: Remove this in favor of always using more strongly typed variants.
-    MiscCandidate { name: &'static str, result: QueryResult<'tcx> },
     /// A candidate for proving a trait or alias-relate goal.
     TraitCandidate { source: CandidateSource, result: QueryResult<'tcx> },
     /// Used in the probe that wraps normalizing the non-self type for the unsize
@@ -154,4 +150,6 @@ pub enum ProbeKind<'tcx> {
     /// 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.
     UpcastProjectionCompatibility,
+    /// Try to unify an opaque type with an existing key in the storage.
+    OpaqueTypeStorageLookup { result: QueryResult<'tcx> },
 }

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

@@ -112,8 +112,8 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             ProbeKind::UpcastProjectionCompatibility => {
                 write!(self.f, "PROBING FOR PROJECTION COMPATIBILITY FOR UPCASTING:")
             }
-            ProbeKind::MiscCandidate { name, result } => {
-                write!(self.f, "CANDIDATE {name}: {result:?}")
+            ProbeKind::OpaqueTypeStorageLookup { result } => {
+                write!(self.f, "PROBING FOR AN EXISTING OPAQUE: {result:?}")
             }
             ProbeKind::TraitCandidate { source, result } => {
                 write!(self.f, "CANDIDATE {source:?}: {result:?}")