Nim incremental compilation cache does not rebuild after timestamp‑preserving source edits

Direct answer

The Nim compiler does not automatically add a content‑based hash to its incremental cache; it relies solely on file modification timestamps. If a source file is edited while its timestamp stays the same, the cached object file is considered up‑to‑date and stale code may be reused.

Confirmed facts

• Nim versions up to 2.0 validate cache validity by comparing the source file’s mtime with the cached object’s mtime.
• No default content hash is computed for cache validation.
• Editors or VCS commands that preserve timestamps (e.g., git checkout --preserve-timestamps) can cause this mismatch.

Likely explanation

When the timestamp does not change, the compiler’s timestamp‑only check fails to detect the content change, so it reuses the existing .o file from nimcache. This leads to stale builds unless the cache is cleared or forced to rebuild.

Steps to resolve for this case

1. Check your Nim version with nim -v. If you are on a recent build that includes the experimental flag --incremental:hash, you can enable hash‑based validation.
2. If the flag is available, rebuild with nim c --incremental:hash yourfile.nim and verify that a timestamp‑preserving edit triggers a rebuild.
3. If the flag is not present or you prefer not to use it, delete the nimcache directory (rm -rf nimcache) or invoke a full rebuild (nim c --forceBuild yourfile.nim) after any operation that may preserve timestamps.
4. Optionally configure your editor or VCS hook to touch the file (touch sourcefile.nim) after a timestamp‑preserving change, ensuring the mtime updates and the cache is invalidated.

Ask: Are you able to use the --incremental:hash flag on your Nim toolchain? Knowing this determines whether the hash‑based approach is viable for you.