Rollup merge of #129110 - nnethercote:Ty-kind-ret-ty-comment, r=jieyouxu

Add a comment explaining the return type of `Ty::kind`.

At least we'll get a useful comment out of #126069 :)

r? ````@lcnr````

compiler/rustc_middle/src/ty/sty.rs

@@ -970,6 +970,10 @@ impl<'tcx> rustc_type_ir::inherent::Ty<TyCtxt<'tcx>> for Ty<'tcx> {
 
 /// Type utilities
 impl<'tcx> Ty<'tcx> {
+    // It would be nicer if this returned the value instead of a reference,
+    // like how `Predicate::kind` and `Region::kind` do. (It would result in
+    // many fewer subsequent dereferences.) But that gives a small but
+    // noticeable performance hit. See #126069 for details.
     #[inline(always)]
     pub fn kind(self) -> &'tcx TyKind<'tcx> {
         self.0.0