Cargo should not consider the build script as stale with every modification

[tomaka]

When a package has a build script and you modify any file in your project, everything gets recompiled.

I'm currently working on a library named glium which is starting be have a moderate size and which uses build scripts only for code generation.

Modifying the code of an example or a test and running "cargo build" recompiles everything, which takes 3-4 minutes on my machine. Due to this, it's very annoying to write tests or examples.

Suggestion

Build scripts should output cargo:depends=/path/to/file just like they output rustc-flags. Only modifications in the files which they depend upon, plus the build script itself and its modules, trigger a recompilation.

[alexcrichton]

This seems like a pretty great use of the metadata output to me, I like the idea!

I would add one caveat that if cargo:depends is not printed, then the behavior is the same as it is today. That means that if you go out of your way to print dependencies then you get better incremental times, but if you don't do anything we at least won't do anything wrong!

[tomaka]

What if a build script has no dependency? Should it print cargo:depends= with nothing behind, or should it print cargo:no-deps?

[alexcrichton]

I'd be down with either, but the latter seems like a better idea

[marcusklaas]

Is this something that could be done by someone with little knowledge of Cargo's internals?

I'd like to work on this, but it would be nice to have a sense of the complexity involved first..

[alexcrichton]

@marcusklaas yeah I think this should be relatively easy. You'd want to add parsing code for this key being printed out first. Next you'd want to serialize it out to a custom location (probably something in the relevant fingerprint directory).

You'd then update the build script fingerprint routine to take this file into account.

[marcusklaas]

Thanks for the pointers. I'll give it a shot!