Auto merge of #80454 - JulianKnodt:ob_forest_op, r=matthewjasper

Skip Ty w/o infer ty/const in trait select

Remove some allocations & also add `skip_current_subtree` to skip subtrees with no inferred items.

r? `@eddyb` since marked in the FIXME

compiler/rustc_middle/src/ty/walk.rs

@@ -13,7 +13,7 @@ type TypeWalkerStack<'tcx> = SmallVec<[GenericArg<'tcx>; 8]>;
 pub struct TypeWalker<'tcx> {
     stack: TypeWalkerStack<'tcx>,
     last_subtree: usize,
-    visited: SsoHashSet<GenericArg<'tcx>>,
+    pub visited: SsoHashSet<GenericArg<'tcx>>,
 }
 
 /// An iterator for walking the type tree.