fail_ and fail_bounds_check lang items don't belong to libcore

[farcaller]

Both fail_ and fail_bounds_check are platform-dependent and the default implementation in libcore makes it impossible to override those.

I can expect fail to be infinite loop or bkpt instruction, it must not pull in error strings and chunks of fmt that are never doing to be displayed.

At least it's reasonable to cfg-guard the implementations, so cross-tool users could build libcore without them.

[farcaller]

After discussing this on IRC: the problem is that the code as is doesn't allow error strings and source paths to be optimised away, and that, alongside with fmt chunks, uses significant amount of space.

[bgamari]

I opened the above pull request as a temporary solution since this issue affects Zinc quite badly. Some of the machines we target have as little as 16kB of RAM; as it stands the moment the user inadvertently incorporates an unoptimizable division into their code they find they have nearly filled a quarter of their device.

[steveklabnik]

Since this bug was filed, fail was changed to painc. There is no panic_ lang item, I don't know if that was also changed. There is a panic_bounds_check, but it calls out to panic_fmt, which is a "weak lang item", meaning that while libcore declares it, it does not define it. A no_std yes core crate which uses panics must define panic_fmt.

As such, I believe this bug can be closed. Let me know if I've missed something!