Add TypeParam interface

...and implement it for basic pointer types, and generated types where
appropriate.

This is the first step on generics support.

capability.go

@@ -21,6 +21,15 @@ type Interface struct {
 	cap CapabilityID
 }
 
+// i.EncodeAsPtr is equivalent to i.ToPtr(); for implementing TypeParam.
+// The segment argument is ignored.
+func (i Interface) EncodeAsPtr(*Segment) Ptr { return i.ToPtr() }
+
+// DecodeFromPtr(p) is equivalent to p.Interface(); for implementing TypeParam.
+func (Interface) DecodeFromPtr(p Ptr) Interface { return p.Interface() }
+
+var _ TypeParam[Interface] = Interface{}
+
 // NewInterface creates a new interface pointer.
 //
 // No allocation is performed in the given segment: it is used purely
@@ -572,6 +581,18 @@ func (c Client) Release() {
 	h.Shutdown()
 }
 
+func (c Client) EncodeAsPtr(seg *Segment) Ptr {
+	capId := seg.Message().AddCap(c)
+	return NewInterface(seg, capId).ToPtr()
+}
+
+func (Client) DecodeFromPtr(p Ptr) Client {
+	return p.Interface().Client()
+}
+
+var _ TypeParam[Client] = Client{}
+
+
 // isResolve reports whether ch has been resolved.
 // The caller must be holding onto ch.mu.
 func (ch *clientHook) isResolved() bool {

capnpc-go/templates/baseStructFuncs

@@ -21,3 +21,6 @@ func (s {{.Node.Name}}) String() string {
 }
 {{end}}
 
+func ({{.Node.Name}}) DecodeFromPtr(p {{.G.Capnp}}.Ptr) {{.Node.Name}} {
+	return {{.Node.Name}}{ Struct: {{.G.Capnp}}.Struct{}.DecodeFromPtr(p) }
+}

capnpc-go/templates/interfaceClient

@@ -26,3 +26,7 @@ func (c {{$.Node.Name}}) AddRef() {{$.Node.Name}} {
 		Client: c.Client.AddRef(),
 	}
 }
+
+func ({{$.Node.Name}}) DecodeFromPtr(p {{.G.Capnp}}.Ptr) {{$.Node.Name}} {
+	return {{$.Node.Name}}{ Client: {{.G.Capnp}}.Client{}.DecodeFromPtr(p) }
+}