Implement selective optimization levels on a per-dependency basis.

[pcwalton]

Inside the dependencies.foo section, you may now specify a minimum
optimization level to be applied to this dependency. If multiple such
optimization levels are supplied, Cargo chooses the highest one.

This will probably need to go through an RFC before landing.

Addresses #1359.

r? @alexcrichton -- would be interested to hear design thoughts

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[alexcrichton]

Thanks for the PR @pcwalton! I talked about something like this in the past with @wycats and the conclusion we've reached is that the specification here is probably best done at the top level, not in the dependencies themselves. In general it seems best if you have complete control over how your dependencies are built rather than having a transitive dependency deciding how it's compiled. One example of this is that the logic for "pick the highest level" isn't always correct as there may be some dependencies you want to force to a lower optimization level.

This issue about configuring dependencies from the top-level definitely comes up quite a bit (#1359, #629, #942), and it's probably best to enable solving all of these in basically one fell swoop. Along those lines I think that we'll want to implement something along the lines of:

[dependencies-config]
foo = { profile = "release" }
"bar:0.1.4" = { override = "path/to/override" }
"crates.io/foo" = { crate-type = [ "..." ] }

This allows you to:

• At the top level have control over how each dependency is compiled
• Reference existing profiles to leverage all the other options in profiles
• Specify any arbitrary package in the dependency graph unambiguously

Some of these details I think still need to be bike-shedded and this may warrant an RFC as well as it's a somewhat substantial change.

[pcwalton]

OK, I'll work on this ASAP. I am blocked on this so there is no other option.

[bors]

☔ The latest upstream changes (presumably #1848) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

This has been inactive for awhile (and I think Servo is working for now), so I'm going to close this in favor of issue like #942 which should provide a nice vector for us to implement this.