Pass cargo subcommand to build scripts

[shlevy]

Describe the problem you are trying to solve
I'd like to have build scripts behave differently for different subcommands. For example, I'm using an external tool BNFC to generate a parser, and it has the ability to also generate a nice PDF describing the language, I'd like to generate that PDF only when running cargo doc.

Also would be nice to be able to distinguish cargo build and cargo build --tests
Describe the solution you'd like
Some env var passed from cargo to the build script.

[Eh2406]

In the current model, the build script may not even be rerun when switching between those commands. So it may need more changes than just an env var. On the other hand, your use case makes sense and I don't have a good suggestion for you at this time.

[aviramha]

Is it okay if I implement it?

[epage]

See also #4001

[epage]

Closing in favor of #4001. If there is a reason we should keep this open, let us know!