Add a flag to build profiles to enable overflow checking

[larsbergstrom]

Would it be possible to add a flag to the build profiles:
http://doc.crates.io/manifest.html#the-profile-sections

That allows us to turn overflow checking on? (that is, enable -Z force-overflow-checks=on)

While we probably wouldn't ship Servo this way, being able to turn it on in our CI systems would catch a bunch of bugs without requiring us to spin up a parallel set of debug-only builders.

[alexcrichton]

There's definitely an interesting question of this I think of where the configuration should go. For something like this it may make sense actually in a few locations. Profiles aren't always the best place because there's only one global compilation profile (chosen by the root crate), which overrides local configuration. So for a use case of "I want overflow checks always enabled in this crate" it's not quite appropriate, but for a CI system it may be appropriate.

That being said Cargo profiles aren't great for CI systems right now as it involves editing Cargo.toml where it's easier to just pass a CLI flag, so there's also that to consider.

Overall, though, I'm very wary about stabilizing any usage of -Z options in Cargo as they should likely become full-fledged -C options first.

[llogiq]

Perhaps something like overflower could help?

[froydnj]

Now that rust-lang/rust#33134 is stabilizing -C overflow-checks, we can add this, correct? I know in rust-lang/rfcs#1535, @nrc expressed a preference that forthcoming Cargo support for arbitrary flags would be used instead of adding another boolean to profiles...how close is that arbitrary flags support @alexcrichton ?

[alexcrichton]

@froydnj let's stick to booleans for now, unforunately arbitrary flags may take awhile :(