From 4b85aa99ad487a5e64eaaee830077d866e8a9e0d Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 27 Apr 2023 16:21:20 +1000
Subject: [PATCH] Add a comment about `TypeFoldable` and slices.

---
 compiler/rustc_type_ir/src/structural_impls.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler/rustc_type_ir/src/structural_impls.rs b/compiler/rustc_type_ir/src/structural_impls.rs
index c513bde6ecf..c9675f93f95 100644
--- a/compiler/rustc_type_ir/src/structural_impls.rs
+++ b/compiler/rustc_type_ir/src/structural_impls.rs
@@ -141,6 +141,10 @@ fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::Break
     }
 }
 
+// `TypeFoldable` isn't impl'd for `&[T]`. It doesn't make sense in the general
+// case, because we can't return a new slice. But note that there are a couple
+// of trivial impls of `TypeFoldable` for specific slice types elsewhere.
+
 impl<I: Interner, T: TypeVisitable<I>> TypeVisitable<I> for &[T] {
     fn visit_with<V: TypeVisitor<I>>(&self, visitor: &mut V) -> ControlFlow<V::BreakTy> {
         self.iter().try_for_each(|t| t.visit_with(visitor))