Force recompile when migrations changed

[dmarcuse]

The migrate! macro is very handy right now, but it would be nice if it would also handle changes to the migrations - currently, the compiler will just ignore changes to these files when determining whether to use the cached build or not. Emitting const _: &[u8] = include_bytes!(...) for each migration should solve this.

[raftario]

The current behaviour was chosen to be consistent with the query_file! family. Emitting include_*! would also not solve the problem of cached builds with newly created migrations if I'm not mistaken.

[dmarcuse]

Hmm, that is true, and I didn't realize that query_file also worked this way. I still think it would be worth having as an option, maybe as a feature flag.

For now I've just been using a build script that walks the migrations directory and emits cargo:rerun-if-changed=... for each file, which works well enough.

[abonander]

For modifying existing files, this should have the same solution as here: #663 (comment)

However, picking up new files is another issue entirely, yeah.

[abonander]

By the way, you don't need to manually walk the migrations/ directory, it looks like cargo:rerun-if-changed=migrations should suffice: https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorerun-if-changedpath

If the path points to a directory, it will scan the entire directory for any modifications.

[abonander]

A new API in proc_macro just recently hit nightly which would fix this: proc_macro::tracked_path::path().