Give ~struct{ *client } a name.

This lets us refer to it outside the main package, and it also means
that CapList's constraint isn't exposing as much implementation detail.

capability.go

@@ -104,7 +104,12 @@ func (id CapabilityID) GoString() string {
 // A Client is a reference to a Cap'n Proto capability.
 // The zero value is a null capability reference.
 // It is safe to use from multiple goroutines.
-type Client struct {
+type Client ClientKind
+
+// The underlying type of Client. We expose this so that
+// we can use ~ClientKind as a constraint in generics to
+// capture any capability type.
+type ClientKind = struct {
 	*client
 }
 

list.go

@@ -1145,10 +1145,11 @@ func (s StructList[T]) String() string {
 }
 
 func (StructList[T]) DecodeFromPtr(p Ptr) StructList[T] {
-	return StructList[T]{ List: List{}.DecodeFromPtr(p) }
+	return StructList[T]{List: List{}.DecodeFromPtr(p)}
 }
 
-type CapList[T ~struct{*client}] PointerList
+// A list of some Cap'n Proto capability type T.
+type CapList[T ~ClientKind] PointerList
 
 func (c CapList[T]) At(i int) (T, error) {
 	ptr, err := PointerList(c).At(i)