rerun-if-changed always trigger rebuild on non-exist file

[bozaro]

Original cause: bozaro/lz4-rs#21

Root cause:

• I use skeptic for file README.md
• Skeptic generate some code and tell to cargo:

cargo:rerun-if-changed=README.md
cargo:rerun-if-changed=README.md.skt.md

• File README.md.skt.md is not presend in my project and cargo always rerun build.

I would expect that the restart will only happen if README.md.skt.md file appears.

Unfortunatelly, adding check for file exists in Skeptic is incorrect, because it should rerun if file appears at some time.

[alexcrichton]

Thanks for the report! This is expected behavior, though, for Cargo. Cargo needs to rerun a build script when a file is deleted, so if it's not present Cargo doesn't know whether it ws deleted or just didn't exist before.

[bozaro]

I do not understand how you can correctly handle incremental compilation, when the appearance of a file can affect the state of the artifact.
For example, I can't tell cargo: rerun if the list of files changed. Something like:

cargo:rerun-wildcard=*.md

[alexcrichton]

Yeah in general Cargo doesn't (and nor do most build systems) have the ability to get rerun when a file appears. Typically this works out as some other file indicate that said file appeared, but right now it's not possible to do so with Cargo.

[TheButlah]

Is this truly intended behavior? If cargo knows which directory to watch for changes via println!("cargo:rerun-if-changed=path_to_nonexistent_file"), it should be able to do (pseudocode):

enum ChangeHistory {
  Exists(Mtime),
  NoFile
}

for (path, history) in tracked_changes {
  match history {
    Exists(mt) => rerun_build_if_mtime_changed(path, mt)
    NoFile => rerun_build_if_file_exists(path),
  }
}