Cargo should be able to conditionally build targets

[asomers]

Describe the problem you are trying to solve
Some targets, especially [[test]] targets, don't make sense to run on every platform. Cargo should allow you to conditionally enable them in the manifest. On Nix this merely causes an annoyance; some tests must have their entire bodies #[cfg()]ed out. But on libc the problem is more severe; the s390x and sparc64 targets crash in the test harness before executing any tests. There's no way to fix that problem from within test code itself; a Cargo solution is required.

Describe the solution you'd like
It would be great if the [[test]] section could be conditionally enabled, like this:

[[test]]
name = "test-something"
only-on = 'cfg(target_os = "multics")'

or this:

[[test.'cfg(target_os = "multics")']]
name = "test-something"

Notes
Here's a PR that shows s390x and sparc64 failing early in the test harness.
rust-lang/libc#1235

[andreeaflorescu]

We also have a similar problem and I think it is due to the fact that cargo downloads and compiles all the dependencies even though they're not used on the platform that we're building on. It looks to me that cargo is ignoring the targets & profiles specified in Cargo.toml. Is this the expected behaviour?
Should the dev dependencies be downloaded & compiled for a release build?

[tasogare3710]

I also having similar same problem.
However, in order to solve this, I think that concept like configuration of apache ivy which can be freely customized is necessary.

cargo profile may be replaced in this configuration concept?

[epage]

Looks like #9208 is a duplicate of this but it has more discussions, so I'm closing this in favor of that. If there is something I overlooked, let us know!