Decision — compd's module layout on entry

Status: accepted 2026-10-11. Amends the source layout (AGENTS.md §4, Rust modules).

Context

AGENTS.md §4 forbids new mod.rs and foo/foo.rs files and says code transplanted from elsewhere is renamed on entry. The compositor's crates arrive written in an older layout: about 570 mod.rs files and about 270 self-named module files (foo/foo.rs, each the file of a module foo::foo).

The decision

• Every dir/mod.rs becomes dir.rs on entry. Rust resolves dir.rs with its children in dir/, so the rename changes no module path. An explicit #[path] resolves against the directory of the file that holds it, so each one in a renamed file is rewritten (for example #[path = "encode/dialog_unavailable.rs"] in graphics).
• Self-named module files stay, as a standing exception. Each is the canonical file of a module foo::foo; removing it renames the module at every caller, which is churn a transplant should not carry. No lint can gate them either way (clippy::self_named_module_files enforces the opposite, mod.rs style).
• The exception shrinks and ends. It covers only the compd crates, and each crate leaves it when it is next substantially edited: its self-named modules are dissolved into their parents in a change of their own. No new self-named module file is added to any crate.
• No mod.rs can come back. Every compd crate carries [lints.clippy] mod_module_files = "deny", as the rest of the workspace does, and clippy runs as a gate.

Consequences

• The transplant stays a near-zero semantic change, so its tests prove the same behaviour they proved before.
• Readers of compd will meet foo::foo paths for a while.