From 99aad72af55e9cfd77adeeca0f9b403758201d42 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 15 Aug 2024 09:46:21 +1000
Subject: [PATCH] Add a comment explaining the return type of `Ty::kind`.

---
 compiler/rustc_middle/src/ty/sty.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 0a277fea49c..d60bfb9faa1 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/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