In addition to CARGO_TARGET_DIR env var, support --target-dir command line option

[matklad]

@nirbheek you wanted this for messon integration. Could you elaborate on why do you think it is better than an environmet variable?

@rust-lang/cargo it seems that such addition would be E-easy and straightforward? The only complication I can think of are build scripts which invoke cargo recursively for whatever reason, but we can just set the env car for them manually. Should we implement this? If yes, I’ll write some mentoring instructions :)

[alexcrichton]

Seems reasonable to me to implement!

[nirbheek]

Could you elaborate on why do you think it is better than an environment variable?

Speaking generally, in my opinion command-line arguments are more explicit and user-friendly than environment variables. They are easier to change or discover, and are less likely to have unintended side-effects.

[matklad]

So, if anyone wants to implement this, here are some pointers to code:

1. To specify command line argment itself, you should define a helper function over here and than call it for all relevant commands. I think that all commands that call arg_target_triple or arg_jobs would probably want to call arg_target_dir as well

2. The code to calculate target_dir is here

3. I think that probably we can thread value of command line argument to config via this call?

4. The test for target dir is here

[smithsps]

@matklad I would be interested in working on this. What would be the best way to direct questions?

[matklad]

@smithsps awesome!

You can ask the questions right here, and we have an IRC channel as well!

[smithsps]

Do we only need to create a single test in build that accompanies the existing one? Or do we need one with every command we include arg_target_dir?

[matklad]

A single dest would be enogh!

[smithsps]

To check in, I've got a working solution.
But, I'm not sure its a complete architectural fit. Could you check if my approach is correct?

I'll probably pull request shortly, but I still need to double check the correct subcommands and their various description.

[dwijnand]

Fixed by #5393

[alexcrichton]

Indeed, thanks!