Comments describe how things are, never how they used to be

Code comments describe the present state of the code only — what it does and why it is shaped that way. Never write comments that contrast with a previous implementation ("rather than X", "no longer uses Y", "previously Z"), justify a review change, or explain what something replaced.

Why: Marco interrupted a review round on PR 1091 twice to delete exactly such comments (an export-macro comment explaining what it replaced, a CMake comment noting dependencies that were removed). The history of how code came to be belongs in commit messages and PR bodies, which are permanently attached to the change; in the source it rots into noise the moment the old state is forgotten.

How to apply: Whenever writing or editing comments in any source file (C++, CMake, SQL, scripts) — especially when applying review feedback or refactoring, where the temptation to narrate the change is strongest. Before committing, re-read added comments and strip any clause whose subject is the old code rather than the current code.