Rollup merge of #128201 - compiler-errors:closure-clone, r=oli-obk

Implement `Copy`/`Clone` for async closures

We can do so in the same cases that regular closures do.

For the purposes of cloning, coroutine-closures are actually precisely the same as regular closures, specifically in the aspect that `Clone` impls care about which is the upvars. The only difference b/w coroutine-closures and regular closures is the type that they *return*, but this type has not been *created* yet, so we don't really have a problem.

IDK why I didn't add this impl initially -- I went back and forth a bit on the internal representation for coroutine-closures before settling on a design which largely models regular closures. Previous (not published) iterations of coroutine-closures used to be represented as a special (read: cursed) kind of coroutine, which would probably suffer from the pitfalls that coroutines have that oli mentioned below in https://github.com/rust-lang/rust/pull/128201#issuecomment-2251230274.

r? oli-obk

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2262,8 +2262,21 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 }
             }
 
-            // FIXME(async_closures): These are never clone, for now.
-            ty::CoroutineClosure(_, _) => None,
+            ty::CoroutineClosure(_, args) => {
+                // (*) binder moved here
+                let ty = self.infcx.shallow_resolve(args.as_coroutine_closure().tupled_upvars_ty());
+                if let ty::Infer(ty::TyVar(_)) = ty.kind() {
+                    // Not yet resolved.
+                    Ambiguous
+                } else {
+                    Where(
+                        obligation
+                            .predicate
+                            .rebind(args.as_coroutine_closure().upvar_tys().to_vec()),
+                    )
+                }
+            }
+
             // `Copy` and `Clone` are automatically implemented for an anonymous adt
             // if all of its fields are `Copy` and `Clone`
             ty::Adt(adt, args) if adt.is_anonymous() => {