Use a proper probe for shadowing impl

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

@@ -153,6 +153,8 @@ 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,
+    /// Looking for param-env candidates that satisfy the trait ref for a projection.
+    ShadowedEnvProbing,
     /// 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

@@ -118,6 +118,9 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             ProbeKind::TraitCandidate { source, result } => {
                 write!(self.f, "CANDIDATE {source:?}: {result:?}")
             }
+            ProbeKind::ShadowedEnvProbing => {
+                write!(self.f, "PROBING FOR IMPLS SHADOWED BY PARAM-ENV CANDIDATE:")
+            }
         }?;
 
         self.nested(|this| {