Fails to compile on linux aarch64

[antimora]

I have a mac with Apple M1 Pro CPU and I use a docker with ubuntu:20.04 image. When I include numpy = "0.16" as my dependency, it fails to compile with the following error:

   Compiling numpy v0.16.1
error[E0600]: cannot apply unary operator `-` to type `u8`
  --> /home/dev/.cargo/registry/src/github.com-1ecc6299db9ec823/numpy-0.16.1/src/npyffi/flags.rs:74:42
   |
74 | pub const NPY_ALIGNED_STRUCT: npy_char = -128; // 0x80
   |                                          ^^^^ cannot apply unary operator `-`
   |
   = note: unsigned values cannot be negated

For more information about this error, try `rustc --explain E0600`.
error: could not compile `numpy` due to previous error

From the documentation and source code, it appears npy_char, which is c_char, will be u8.

I am not familiar with the artitecture of rust-numpy to offer a solution or PR. Could anyone suggest a quick fix for this issue?

[antimora]

The lib compiles fine with numpy = "0.16.0". So it appears a recent change (5a78a87) introduced the breakage.

@aldanor, could you please look into the error? I am guessing the -128 assignment should have been just 0x80.

[adamreichold]

The lib compiles fine with numpy = "0.16.0". So it appears a recent change (5a78a87) introduced the breakage.

This seems inconsistent as that commit is also part of v0.16.0. Did you mean numpy = "0.15.0"?

[adamreichold]

I am guessing the -128 assignment should have been just 0x80.

This does not work on platforms where c_char = i8 as the compiler errors out due to 0x80 not being representable for the type.

I think explicitly casting should work in both cases. Could you give #296 a try?

[antimora]

@adamreichold the build works but I have a review comment in your PR.