Higher ranked goal source, do overflow handling less badly

compiler/rustc_middle/src/traits/solve.rs

@@ -273,6 +273,8 @@ pub enum GoalSource {
     /// they are from an impl where-clause. This is necessary due to
     /// backwards compatability, cc trait-system-refactor-initiatitive#70.
     ImplWhereBound,
+    /// Instantiating a higher-ranked goal and re-proving it.
+    InstantiateHigherRanked,
 }
 
 /// Possible ways the given goal can be proven.

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

@@ -127,6 +127,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                         let source = match source {
                             GoalSource::Misc => "misc",
                             GoalSource::ImplWhereBound => "impl where-bound",
+                            GoalSource::InstantiateHigherRanked => "higher-ranked goal",
                         };
                         writeln!(this.f, "ADDED GOAL ({source}): {goal:?}")?
                     }