Trusted URIs aren't composable

[sgammon]

Consider the following template:

{template .asset_link kind="trusted_resource_uri" visibility="private"}
  {@param uri: trusted_resource_uri}  /** URI postfix to apply. */
  {@inject? cdn_prefix: trusted_resource_uri}  /** CDN prefix to use, if applicable. */

  {if isNonnull($cdn_prefix)}
    {$cdn_prefix}{$uri}
  {else}
    {$uri}
  {/if}
{/template}

From the type names, it's implied that this would work and skip escaping, because, even though the trusted_resource_uri we're returning isn't a constant itself, per se, the two items it is composed of are trusted constants.

Instead, however, invoking this template returns something like:
https://cdn_prefix.com/some%2Fescaped%2Furl%2Fhere.css

Which obviously doesn't work. In the above example, the CDN prefix is essentially just a display detail, hence why we want to keep it in Soy. Because there isn't a way to compose string types into trusted_resource_uri types (for good reason) from within Soy itself, is there any way to accomplish the above smoothly?

In our case, uri is itself already a trusted_resource_uri when Soy begins the render process. So, composing these two URIs is a rather challenging task, even from pure Java, because we'd have to disassemble/re-assemble the TrustedResourceUriProto, which is also verboten.

[iteriani]

Sorry I dropped this (busy things ahead). This seems like a corner case with a workaround (a soy function), we likely won't get around to something like this until we finish the Soy formatter