is rerun_file necessary?

Line 13 in 2f0cffd

autocfg::rerun_path(file!());

Hi there,

I'm really curious how rerun_path(file!()); works. I've never seen it works. When do we need it? Thanks!

autocfg::rerun_path outputs cargo:rerun-if-changed=PATH, and here PATH is the current source file name given by file!(). As described in the cargo docs:

rerun-if-changed=PATH is a path to a file or directory which indicates that the build script should be re-run if it changes (detected by a more-recent last-modified timestamp on the file). Normally build scripts are re-run if any file inside the crate root changes, but this can be used to scope changes to just a small set of files. (If this path points to a directory the entire directory will not be traversed for changes -- only changes to the timestamp of the directory itself (which corresponds to some types of changes within the directory, depending on platform) will trigger a rebuild. To request a re-run on any changes within an entire directory, print a line for the directory and another line for everything inside it, recursively.) Note that if the build script itself (or one of its dependencies) changes, then it's rebuilt and rerun unconditionally, so cargo:rerun-if-changed=build.rs is almost always redundant (unless you want to ignore changes in all other files except for `build.rs).

So it's really that final note we're applying here -- the build script's own source is the only thing that needs to trigger rerunning itself, at least for num-traits.

It doesn't really matter when depending on this crate, as none of the crate sources will ever be modified, but it's useful in development to reduce the build time.

Thank you so much for the explanation!

Our project is now using --remap-path-prefix to erase the path info and make the generated binaries deterministic, no matter where the source code is. Without the remap, the output of build.rs is

cargo:rerun-if-changed=/home/ding/.cargo/registry/src/github.com-1ecc6299db9ec823/num-traits-0.2.8/build.rs

After we enable a rustc flag --remap-path-prefix=${HOME}/.cargo=/cargo_home, the output is:

cargo:rerun-if-changed=/cargo_home/registry/src/github.com-1ecc6299db9ec823/num-traits-0.2.8/build.rs

/cargo_home is not exist so it seems that cargo cannot judge if the file has a newer timestamp or not, which makes num-traits compiled every time. We wonder if this is a "feature" or "bug" of cargo? or is there anything we can do to reduce the build time?

Oh, I never thought about --remap-path-prefix, but I suppose it's logical that this would affect file!() too. We should probably change this to a simpler rerun_path("build.rs").

That will be great! Thank you! Could I submit a PR?

Sure, a PR is welcome. We should do this across all num crates, and adjust the example given in autocfg too, but it's up to you if you want to be so ambitious. 🙂

I'm glad to help with all of them :-)