Inform build scripts whether cargo is `check`ing or `build`ing

[nrc]

Because in cargo check there may be work that build.rs can skip

Maybe doc too, see #8811

cc @wycats

[alexcrichton]

Sounds like a good idea to me! The only catch that I can think of is that we need to ensure we don't cache the check'd output and use it for the real output, other than that should be relatively easy to do with a new env var!

The only catch that I can think of is that we need to ensure we don't cache the check'd output and use it for the real output

that is what is currently happening, ie.:

1. touch build.rs
2. cargo check
   updates eg. target/debug/build/recompile_self-feb93e46b6e91788/output
3. cargo build
   doesn't build anything(because of 2.), but it should!

tested:
cargo 0.25.0
release: 0.25.0

[wycats]

I think this is an ok thing to expose to build scripts, but it further highlights that a better factoring around build scripts might reduce footguns

This kind of solution will help people who remember to check for the mode, but breaking up build.rs into more hooks, many of which wouldn't semantically run at all during check, would have a bigger effect.

But as long as build.rs is the only game in town, I'm cool with this!

[shepmaster]

I'd expand the specific cases described in the title and first comment to include any subcommand, as I've seen Stack Overflow questions for "only do something for documentation" and "only do something for tests"

• How to access current cargo profile (build, test, bench, doc, …) from the build script (build.rs)
• How to change build.rs behavior based on the current Cargo command?

[kellpossible]

I have a use case where a bunch of heavy work in build.rs script can (and should) be skipped for both the check and test commands.

[jyn514]

This would be helpful for rust-lang/rust#76444

[shlevy]

This would be great. My use case is that I'm wrapping a C++ library that has static globals whose constructors read env vars (:scream:), and I'd like to set those appropriately for testing without impacting the normal build.

FWIW I'd prefer to just run a different script altogether if it exists (e.g. build/tests.rs), but not picky.