Enable writing custom profiles

[jdm]

For Servo, I'm interested in allowing developers to build either debug or opt builds of a native submodule. I can nearly get there by matching on the PROFILE environment variable in the makefile, but there's no way to specify arbitrary custom profiles for Cargo.

[alexcrichton]

Could you elaborate a bit more on what you mean by a custom profile? Are you thinking something like:

[profile.my-profile]
...

and then cargo build --profile my-profile or something like that?

[jdm]

Maybe? I don't really know much about Cargo, and @metajack suggested that custom profiles might be a solution. I just want a way to provide developers with choices about certain aspects of the build beyond all optimized or all debug.

[metajack]

We also need this for skia. The old configure had --enable-skia-debug which would turn on Skia debugging which is extraordinarily expensive (it calls glGetError() after every call). You almost never want that in a debug build, except when you are debugging certain graphics problems.

[metajack]

The only other solution I can think of is to use environment variables that the Makefile.cargo's trigger off of, but that doesn't help when we want to make these kinds of decisions in crates instead of C libraries.

[larsbergstrom]

@alexcrichton mentioned we may be able to use --feature and pass the features down explicitly to subcrates and use that.

[jdm]

Features almost work for this (with a patch to add environment variables that can be accessed in native build commands), but they're not propagated to dependencies at the moment. We're back at no solution right now.

[jdm]

Do profiles propagate? If so, it sounds like my original request would be an acceptable short-term fix while we sort out the future of features.

[alexcrichton]

@jdm not currently. After thinking more I don't think that features are sufficient for this use case, although I believe they should be. I'm going to talk more with @wycats about the design of features soon. Profiles do propagate, but the configuration for a profile always comes from the top-level package.

[alexcrichton]

Ah, when I say "not currently", I actually mean that features do not currently work, not that profiles do not propagate. Words!