TypeVisitable doesn't require Clone.

`TypeFoldable` does, because it involves the production of new values.
But `TypeVisitable` only involves the visiting of values.

Author: nnethercote

compiler/rustc_type_ir/src/fold.rs

@@ -73,7 +73,7 @@ type Never = std::convert::Infallible;
 /// which means in practice almost every foldable type needs to also be
 /// visitable. (However, there are some types that are visitable without being
 /// foldable.)
-pub trait TypeFoldable<I: Interner>: TypeVisitable<I> {
+pub trait TypeFoldable<I: Interner>: TypeVisitable<I> + Clone {
     /// The entry point for folding. To fold a value `t` with a folder `f`
     /// call: `t.try_fold_with(f)`.
     ///

compiler/rustc_type_ir/src/visit.rs

@@ -59,7 +59,7 @@ use crate::{self as ty, Interner, TypeFlags};
 ///
 /// To implement this conveniently, use the derive macro located in
 /// `rustc_macros`.
-pub trait TypeVisitable<I: Interner>: fmt::Debug + Clone {
+pub trait TypeVisitable<I: Interner>: fmt::Debug {
     /// The entry point for visiting. To visit a value `t` with a visitor `v`
     /// call: `t.visit_with(v)`.
     ///