Unary negation not properly typechecked

[msullivan]

fn main() {
    -"foo";
}

produces

rustc: Constants.cpp:1747: static llvm::Constant* llvm::ConstantExpr::getNeg(llvm::Constant*, bool, bool): Assertion `C->getType()->isIntOrIntVectorTy() && "Cannot NEG a nonintegral value!"' failed.