librustc: Stop trying to make invalid slice with vec in statics.

[luqmana]

Fixes #5917 by not trying to treat &[T] as a slice since it already is one.

[alexcrichton]

If you've still got a build of this lying around, does this fix #7617? I'll also test it once it gets merged though.

[luqmana]

@alexcrichton no, I just tested and it doesn't.

[luqmana]

Though, looking at that bug there seems to be some sort of weird interaction between the COMMANDS static and iterators. Replacing the for loop with a simple while loop works fine. When done with the for loop it's going past the end, hence the garbage values it passes to str::repeat. The odd thing though is that COMMANDS.len() gives the right value. Though I'm not very familiar with the iterators.

[alexcrichton]

Weird... Sounds like it needs some more investigation regardless, thanks for checking though!