D505 - The declared surface is what guests import

Status: decided Date: 2026-09-03

Every library a guest imports is declared, whether or not anything in it is implemented, in the crate where its implementation will live. A library that declares names and implements none is listed in SERVES_NOTHING with its reason, and status counts those names separately.

Why: a declared surface that is nearly the implemented set makes coverage look finished by construction. Declaring what guests ask for makes the declared/implemented ratio measure something, and a guest reaching an unimplemented interface is named and counted rather than answered by a placeholder it might store as a handle.

Rejected:

• Declaring only what is implemented: the coverage figure can only ever look complete.
• Implementing part of a library to shorten SERVES_NOTHING: writes the abstraction before its caller.
• Placing declarations wherever is convenient now: the implementation would later have to move.